Theory of Evolution

The theory of evolution is often misunderstood, misinterpreted, or misused with unclear intentions. However, this set of facts and laws has only one intention: to explain how nature works.

  1. The beginning of the theory of evolution
  2. The Darwin / Wallace theory of evolution
  3. Debate around Darwin’s theory
  4. The principles of genetics
  5. The synthetic theory
  6. Molecular biology and genetics
  7. Some scientific questions debated around the theory of evolution

7.1. Selectionism versus neutralism

7.2. Punctuationism versus gradualism

7.3. Notion of species

7.4. Importance of natural selection in evolution

7.5. More we know, more debates

  1. Philosophical reflection and theory of evolution

8.1. Theory of evolution and evolutionism

8.2. Evolution and purpose

  1. Bibliography

1. The beginning of the theory of evolution

During the 18th century, a group of researchers, who were called naturalists, managed to gather a large amount of information about the fauna and flora in many different areas of our planet. One problem posed by the accumulation of such a remarkable volume of information was its organization. The classification of living beings was carried out, at first, by means of extensive descriptions of the morphology and origin of the different individuals found. These types of descriptions were not a real help to achieve classifications that were sufficiently univocal [ Velázquez 2007 : 131-142].

The system devised and developed by Linnaeus (1707-1778) represented an important improvement in the organization of available information. It consisted of proposing a series of rules to assign to all known living beings a gender and species label. This classification, the first edition of which was published in 1735, was called Sistema Naturae . Logically, at that time, it was the morphological properties of the different living beings that made it possible to assign gender and species to a specific individual. Although it is not without arbitrariness, the work carried out by Linnaeus greatly simplified the task of classifying animals and plants. In general, the arborescent structure that it developed is still valid today, despite the changes experienced by biology since then.

For Linnaeus the identified species constituted groups of well differentiated beings and without any relation of origin. The kinship criterion, as we have indicated, was merely morphological. This so-called fixist perspective considered that each of the species was created as it was, and its individuals did not undergo changes over time.

However, the accumulation of data provided by naturalists, and the advances experienced in their organization, led to the adoption of other approaches quite different from the fixist. Soon the idea that some species came from others and that, therefore, it was necessary to achieve a classification that reflected the affinities between the different living beings from other perspectives: it was necessary to achieve what was called a natural classification .

Buffon (1707-1788) already questioned Linnaean fixism but, properly speaking, the first to propose a hypothesis about the way in which some species could come from others was the Frenchman Jean Baptiste de Monet, a Lamarck knight, known simply as Lamarck (1744-1829). In his Zoological Philosophy , written in 1809, he presented a systematic description of the evolution of living things.

For Lamarck, species come from one another, from the simplest to the most complex. The organs of each species would develop as a consequence of the reaction and adaptation to the environment. The changes would therefore be gradual and would occur over long periods of time. Lamarck thought that fixity was absurd because animals could not have survived, without evolving, the changing climatic conditions that in some periods of time were very aggressive.

The originality of Lamarck’s proposal consists in defending that changes occur through adaptation to the environment. Certain organs are strengthened with the use that the animal makes of them conditioned by the environment and, on the other hand, other organs atrophy and end up being eliminated due to disuse. Lamarck considered that these modifications in the diverse organs are transmitted by inheritance to the descendants. The latter is what has been called “inheritance of acquired characters.” Actually the idea that Lamarck was advocating was a version of “the function creates the organ.” An important consequence of the Lamarckian proposal was that the transformation of organisms had to be necessary, gradual, ascending and continuous. That is, from worms, for example, over time we would have men again.

It can be said, therefore, that Lamarck was the first to formulate an evolutionary hypothesis in the strict sense, although at that time the word evolution was reserved to the development of the embryo, and his proposal was called transformist. Unlike Darwin’s proposal, the subject of Lamarckian evolution is the individual: it is the individual who experiences the transformation by adaptive use or disuse and this transformation is the one that is later transmitted to their offspring.

Lamarck’s proposal, although it garnered many adhesions and seemed to explain in a natural way the increased complexity and diversity observed in nature, also met with opposition from scientists of the stature of Cuvier (1792-1832), professor of anatomy. Comparative, using what Brentano later called the teleological principle [ Brentano 1979 : 244], gave the guidelines for deducing some animal forms from others of the same animal. These guidelines have been further developed by modern paleontology.

Certainly, in living beings, in particular in higher animals, slight modifications of some organs can be observed as a consequence of their use and, above all, it is easier to verify the atrophy of those organs that are not used. This does not allow to affirm that the function creates the organ, rather it could be said that the functionality of the organ can be enhanced by its use. What science has forcefully rejected so far is the inheritance of acquired characters. Neither experimental evidence nor any mechanism by which individuals can transmit the supposed improvements acquired in the course of their lives has been found. The principles that govern the transformation of individual characters, which are now commonly accepted by science, were first established by Darwin and Wallace.

2. The Darwin / Wallace theory of evolution

As is well known, Charles R. Darwin (1809-1882) participated as a naturalist in the Beagle expedition across South America and the Pacific in 1831. The journey that began when he was only 22 years old ended five years later. During this period Darwin had time to make many observations, compile information and reflect on the data he was collecting and on some texts such as the one that bears the name of Principles of Geologyof Charles Lyell, where he found good syntheses of evolutionary arguments such as those defended by Lamarck. All of this led him to embrace a transformative perspective on nature. In the years following his journey, Darwin developed his own ideas and collected new data with which to carry out a work in which he wanted to present, in an orderly manner, his vision of nature. Perhaps one of the texts that most influenced the development of his thesis was the book by Thomas R. Malthus (1766-1834) published for the first time in 1798: An Essay on the Principle of Population. In this book Malthus defended the thesis that the struggle for survival was necessary as a consequence of the fact that the population tends to grow following a geometric progression while food does so following an arithmetic progression.

In 1858 Darwin received a package by mail sent from a remote island in the Malay archipelago, present-day Indonesia. The package contained a text that summarized the results of the research carried out by Alfred Russel Wallace (1823-1913). The writing contained an extraordinary exposition of “the theory of evolution by natural selection.” Its expository clarity means that even today this text retains great pedagogical value. Darwin had been developing a theory equivalent to that of that writing for two decades and was about to abandon his project when reading the work. It was precisely Charles Lyell and the botanist Joseph Dalton Hooker who intervened in favor of the interests of their friend Darwin. Wallace’s writing was published in the “Proceedings” of the prestigious Linnean Society, preceded by another contribution by Darwin containing some excerpts from an unpublished 1844 essay and a letter written to the botanist Asa Gray. The writings were published in August 1858, thus saving Darwin’s right to claim the originality of the work that he had been preparing for so long and that had not yet seen the light. It was in the following year, 1859, that Darwin published the results of the work he had done during the preceding years in a book entitled “On the Origin of Species by Means of Natural Selection.” The success of this book allows us to affirm that it was at this time that the “theory of evolution by means of natural selection” was born. The writings were published in August 1858, thus saving Darwin’s right to claim the originality of the work that he had been preparing for so long and that had not yet seen the light. It was in the following year, 1859, that Darwin published the results of the work he had done during the preceding years in a book entitled “On the Origin of Species by Means of Natural Selection.” The success of this book allows us to affirm that it was at this time that the “theory of evolution by means of natural selection” was born. The writings were published in August 1858, thus saving Darwin’s right to claim the originality of the work that he had been preparing for so long and that had not yet seen the light. It was in the following year, 1859, that Darwin published the results of the work he had done during the preceding years in a book entitled “On the Origin of Species by Means of Natural Selection.” The success of this book allows us to affirm that it was at this time that the “theory of evolution by means of natural selection” was born. when Darwin published the results of the work he had done during the preceding years in a book entitled “On the Origin of Species by Means of Natural Selection.” The success of this book allows us to affirm that it was at this time that the “theory of evolution through natural selection” was born. when Darwin published the results of the work he had done during the preceding years in a book entitled “On the Origin of Species by Means of Natural Selection.” The success of this book allows us to affirm that it was at this time that the “theory of evolution through natural selection” was born.

The structure of the theory of evolution by natural selection [ Lewontin 1970 ; Sarkar 2007 ], as Darwin and Wallace put it in their writings, is based on three basic points:

1) The descendants inherit the characters of the parents from generation to generation. Darwin, however, did not know the laws of inheritance that were being worked on precisely in the years when he made his theory known. The laws of inheritance that are scientifically accepted today and that were discovered by Mendel were not known until the beginning of the 20th century. Darwin’s proposed explanations for the inheritance of the characters proved wrong and were soon rejected. These explanations, however, were not part of the content of the “Origin of Species”.

2) In the inheritance process, spontaneous variations occur that are by chance or blind. There is talk of random or blind variations in a double sense. On the one hand, its causes cannot be determined. On the other hand, these variations are not oriented to a better adaptation of the organism to the environment, that is, there is no a priori orientation in them. In the first edition of the “Origin of Species” Darwin explicitly rejected the inheritance of acquired characters advocated by Lamarck. Later, however, he qualified that rejection.

3) There is differentiated reproduction in the individuals of a population. The reason is twofold: either some individuals have higher fertility than others, or they are better adapted to the environment. Better adaptation to the environment will translate into greater survival and, consequently, more offspring.

The impact of the Darwin / Wallace ideas was enormous. Very shortly after the publication of the “Origin of Species”, as early as the 1960s, evolution based on natural selection advocated by Darwin was, in practice, universally accepted. However, very soon the first objections to his proposal began to be raised. The objections from the 60’s were not directed against the fact that there was evolution, that is, that the various species descended from other common and earlier species, but were directed directly against what made their proposal original, that is, That the engine of evolution was random variation and natural selection.

In relation to the development of Darwin’s proposal in the following years, and the criticisms that it has received to this day, it must be said that Darwin paid great attention to the possibility of explaining the development of complex structures on the basis of variations by chance and natural selection as the main cause of such development. In fact, although for Darwin this theory explained many aspects of the evolution of living beings, including the origin of species, this did not go so far as to imply that the evolution of organisms could be explained solely by means of natural selection. Darwin accepted the existence of other mechanisms that cause evolutionary change. Darwin’s reasons then for maintaining his plural view of the causes of evolution were, however,

Darwin personally faced many of the objections that have been raised to this day to his theory of evolution. Their points of view were exposed in successive editions of “Origin of Species” [ Darwin 2002 : 183]. It not only focused on the problem of the origin and the increase in the complexity of living beings, but also, for example, addressed problems such as the scarcity of available fossil record of the supposed living beings that should have existed as a consequence of a gradual evolution as defended in his proposal [ Darwin 2002 : 349].

3. Debate around Darwin’s theory

The weight of the objections put to his theory, together with the ignorance of the laws of genetics led Darwin, after 1859, to downplay the mechanism of natural selection and even to accept the existence of Lamarkian-type mechanisms as an explanation. of transformations in living beings.

One of the main objections to Darwin’s theory in these years was raised by William Thomson (Lord Kelvin). Kelvin shared with Darwin a way of understanding the transmission of hereditary characteristics that led him to conceive the process of evolution by natural selection in an extraordinarily slow way. Not only were the changes that served as matter for natural selection minute and gradual, but to transmit the characters to the offspring without any loss of variation it was necessary for the novelty to appear in two individuals and for them to mate with each other. The probability of things happening in this way was so small that in order to explain the evolution and variety of life on Earth as it is presented to our experience it was necessary that the process had lasted billions of years.

The problem was that the estimated time to land was much shorter. At that time it was thought that the energy we receive from the Sun came exclusively from gravity. You could calculate the approximate mass of the Sun and the energy it emitted. With these assumptions, Kelvin’s calculations predicted a lifetime for the Sun that did not exceed a few hundred million years. Logically, life on earth could not exceed that time, which was much less than the estimated time necessary for the development of life as we know it. Radioactivity, the true source of the energy that we receive from the Sun, was discovered in the decade that began in 1890. These considerations must have had a significant influence on Darwin’s proposals, which, as we have pointed out, played down the importance in successive editions of theOrigin of Species to blind variations and it was given to other mechanisms such as inheritance of acquired characteristics induced by the environment.

Despite admitting a plurality of mechanisms as the engine of evolution, for Darwin there was an evolutionary continuity between all species, including humans. However, Darwin did not defend that human higher faculties were the result of natural selection. It can be said that Wallace was more strict than Darwin in defending the mechanism of natural selection. His panselectionism led him to consider random variations and natural selection as the only force of biological evolution. However, Wallace admitted the influence of another different force, of a “spiritual” nature, when it came to explaining the origin of life, the emergence of the consciousness of animals and, mainly, the higher human faculties such as, for example , your ability to do math or your artistic skills. For Wallace the world of matter was clearly subordinate to that other world of spirit in which natural selection did not fit as an explanation. Wallace was stricter in his defense of natural selection in organic evolution than Darwin, and also more outspoken in his defense of a “spiritual” realm for which natural selection was not an explanation [Sarkar 2007 : 31-32].

Another very important biologist in the 19th century was the German August Weismann (1834-1914). Weismann, also a panselector, completely rejected the possibility admitted by Darwin of the existence of Lamarkian-type mechanisms. The distinction he made between germ cells, isolated from environmental influences, and somatic cells, points to what would later become the general framework of modern evolutionary theory.

Despite the initial success of Darwin’s theory, and the efforts of biologists like Weismann to defend natural selection and to downplay Lamarkism, in the 1990s a period began in which the “blind variation plus selection” mechanism it loses popularity in favor of other Lamarkian-type mechanisms or those that could also be framed within the so-called orthogenesis (evolution with a certain direction). One of the defenders of neo-Lamarckism of these years, Herbert Spencer, was the one who coined the expression “survival of the fittest”, which has often been translated as “survival of the fittest”, and which has helped so little in the correct understanding of the theory proposed by Darwin / Wallace.

The reasons for this regression of the Darwinian proposal are varied. We have already mentioned the serious difficulties arising from Kelvin’s considerations. The probabilistic arguments did not seem to support the theory initially proposed by Darwin. A certain skepticism grew about the possibility that natural selection, by itself, was capable of explaining the appearance of species diversity. This skepticism was fueled by the ignorance of the mechanisms of genetics and, also, by the lack of quantitative experimental data to support the thesis of the “Origin of species”.

On the other hand, already in classical philosophy, arguments had been formulated that were based on the purpose to defend the existence of a superior being on which the world depends. Talking about a mechanism that seemed to steal the finality from nature aroused and continues to provoke the liveliest debates.

The misgivings regarding the new theory of evolution were heightened when what stood out was the continuity between animals and man. Darwin explicitly defended such continuity in a book published in 1871 entitled “The Descent of Man.” The gradual nature of higher human faculties (intelligence and linguistic ability, for example) with respect to animals did openly clash, for example, with the doctrine held by all Christian confessions on the way of being peculiar to the human being. Darwin proposed a selective explanation for certain moral qualities found in man and also, in his own way, in animals: group cooperation, common defense, transmission of knowledge from parents to children, for example. But the difficulties to support the evolution from animals of faculties such as intelligence or human linguistic capacity forced Darwin to resort to the use-inheritance of Lamarkism and other hypotheses that today are completely untenable. What Darwin did not renounce at any time was the continuity between animals and man, which meant reducing human cultural dimensions to pure biology.

Both neolamarckism and orthogenesis served in the last decade of the 19th century as an alternative, or at least as a complement, to the theory of Darwin and Wallace in the way of explaining what, already in those years, was accepted by scientists as a certain and incontrovertible fact: the fact of the evolution or descent of all living beings from common ancestors, including human organic characteristics. What was questioned in these years, or even flatly denied, was the ability of natural selection, by itself, to generate the diversity of species and the degree of complexity achieved by living beings.

The debate would be situated in a new framework with the development experienced by genetics in the early 20th century.

4. The principles of genetics

In the journey that we are making of the ideas that make up the modern theory of evolution, we have examined one of the pillars that support this theory: the ideas set forth in the “Origin of Species” on small variations and natural selection. . The other important pillar are the ideas published in 1866 by the Augustinian monk born in Heinzendorf (then in Austrian territory and currently belonging to the Czech Republic), Gregor Johann Mendel (1822-1884). Although in his work he expounded the fundamental principles of modern genetics, the importance of its content was not recognized until the beginning of the 20th century.

Mendel obtained the principles of inheritance by experimenting with certain pea plants that displayed a series of well-defined characters: flower size and color, seed shape and color, etc. He made crosses between plants with different characters and quantified and interpreted the results obtained in the crossing of several generations of plants. He came to a series of conclusions that were later known as Mendel’s laws and that remain in force today.

Mendel distinguished between character and factor. The characters were the visible properties manifested by the plants: color, shape, etc. The manifestation of the various “characters” depended on a set of independent and discrete “factors” that were present in the plants [ Curtis-Barnes 1996 : 207 ff.].

Mendel’s first law is called the “segregation principle” and establishes the hypothesis that each individual carries pairs of factors for each character, and that the factors of each pair segregate or separate from each other when the gametes are formed ( germ or reproductive cells). In this way, in the offspring, when the paternal and maternal gametes unite, one factor from the new couple is inherited from the father plant and the other from the mother plant. Later, these factors were called genes, the units of heredity, and the varieties that presented these factors or genes were called alleles.

The result of the experiments carried out by Mendel led him to conclude that one of the two factors of the pair is always “dominant” with respect to the other, which is then called recessive. That is, when the dominant and recessive factors were present in the plant, the character presented by the plant was always that of the variant of the dominant factor or allele.

Mendel’s second law is called the “principle of independent transmission.” This principle states that when gametes are formed, the alleles of one gene are segregated independently of the alleles of another gene. Therefore, the possible combinations of the different characters when crossing different plants must also be independent. That is, the color character, for example, was not linked to the size character, but in the reproduction, sizes and colors could be combined independently.

These laws were the interpretation of the distribution of characters that Mendel obtained by experimentally crossing the different pea plants. This interpretation was able to perfectly quantify the results of the proportions of characters obtained in the experiments.

For some, the scheme proposed by Mendel constituted an achievement for biology even more important than Darwin’s proposal itself. It can be said that this scheme introduced Biology in the field of quantification, which constitutes the ideal to which all science aspires that seeks to rely on experimentation.

As we have indicated, Mendel’s work went unnoticed until in 1900 it was simultaneously rediscovered by three botanists. All three recognized Mendel’s proposal as a predecessor to their own works. At the beginning of the century the zoologist William Bateson (1861-1926) emerged as the greatest defender of Mendel’s laws. Bateson starred in a new controversy that pitted him against current Darwinian evolutionists such as Karl Pearson and, especially, the zoologist Walter Frank Raphael Weldon (1860-1906).

Bateson believed that it was more in line with Mendel’s discovery that the variations that gave rise to evolution were discontinuous and not small variations as hypothesized by Darwinian theory. In fact, he did not believe that evolution took place following the scheme presented by Darwin. On the other hand, Pearson and Weldon thought that Mendel’s laws only worked in very exceptional cases. They also rejected the distinction between character and Mendelian factor and formulated a set of laws that omitted this distinction and were based only on external characters presented by individuals. Weldon attempted to construct a statistical theory of evolution that conformed to Darwin’s ideas. The confrontation between Bateson and Weldon ended with Weldon’s death in 1906, but the dispute between the Mendelians and the so-called “biometricians” did not end. The former stressed, contrary to Darwin’s theory, the importance of discontinuity in changes transmitted by inheritance. The latter were faithful to the evolution of the Darwinian type that emphasized the gradualness in the changes of the characters. Mendel therefore contributed to further weaken confidence in Darwinian theses in the early years of the 20th century.

5. The synthetic theory

The wall that separated the positions of Mendelians and biometricians began to crumble after 1918. In this year RA Fisher (1890-1962) was able to show that the laws formulated by the latter could be explained within the framework established by Mendel’s laws. This contribution, together with the work of other authors such as John Burdon Sanderson Haldane (1892-1964), allowed the construction of a theory of natural selection based on the Mendelian model of inheritance. The modern theory of evolution had its beginning in the works of these years, which reached their maturity in the early 1930s.

In the construction of the new theoretical framework, the distinction coined by Wilhelm Johannsen (1857-1927) in 1909 between the notion of genotype and phenotype was very important. The latter is constituted by the set of detectable characteristics in an organism (structural, physiological or behavioral) that are determined by the expression in the living being of the genotype and by its interaction with the environment. This distinction updated the one originally proposed by Mendel between character and factor. The notion of gene, also coined by Johannsen, was then postulated to achieve a theory consistent with experience but, although enough was already known about how genes participated in the inheritance of characters, it was not really known at that time what it was or what the genetic material consisted of.

A key point in the union of the two competing perspectives of these first years of the century consisted in assuming that the development of each living being, from the embryo to adulthood, is like a black box, that is, any Consideration of how genes interact with the organism and its environment. This is certainly a very important simplification, but it made such a synthesis approachable. In the new scheme it was assumed that natural selection could be modeled on the basis of changes that occurred only in the genome. In other words, only modifications in the genome are responsible for evolutionary change, and these modifications are not conditioned in their production by the phenotype or by the environment, but are modifications at random, according to Darwin’s ideas. and Wallace.

In the 1920s, Haldane, Fisher, and Wright were very influential in the development of the Theory of Evolution. Haldane published several articles in which he made a treatment of natural selection from genetics: he analyzed a great variety of genetic models and, also, different ways in which natural selection could occur: weak or intense, constant, cyclical, etc. One of the conclusions he reached was that the process of natural selection acting on blind variations was faster than previously thought. The fear that there would not be enough time for natural selection to lead to major evolutionary modifications did not appear to be justified in light of this work. The theories that competed with natural selection in the early years of the century — those that defended orthogenesis and those of the neo-Lamarka type — received a severe blow with these works. These three authors are considered today as the fathers of population genetics, which remains the foundation for the current theory of evolution.

Haldane focused on studying the consequences for evolution of the various genetic models. Fisher and Wright tried to offer theories of a general nature that explained the history of life on Earth. Both maintained some differences regarding the role of natural selection in evolution. Fisher was in favor of which the best explanation of evolution is provided by natural selection acting on small variations that occur in large populations in which their individuals mate randomly. On the other hand, Wright thought that in the explanation of the changes the small isolated populations were more important in which important fluctuations could take place due precisely to the small number of the individuals that compose them. This hypothesis has later been known as “genetic drift.” The debate between these two positions has been relevant in the development of the modern theory of evolution.

The integration of previous work with the rest of biology was the task of Theodosius Grygorovych Dobzhansky (1900-1975), who managed to unify the empirical results of natural populations with the theoretical models of Haldane, Fisher and Wright. His most important book was published in 1937 and was entitled “Genetics and the Origin of Species.” One of the topics that focused his interest was that of speciation: the appearance of new species from existing ones. This is the problem that appeared in the title of Darwin’s famous book and that, in reality, he did not clarify. Dobzhansky highlighted the importance of geographic isolation as one of the most important causes for the appearance of a new species. This type of speciation was called allopatric speciation. His study and, in general, the study of speciation has been developed later, among others, by Ernst Mayr (1904-2005). Today this type of speciation is not considered more important than sympatric speciation, in which species formation does not require geographic isolation. Julian Huxley (1887-1975) popularized in 1942 the new theoretical framework reached by the authors cited in a book that had a wide circulation and in whose title he called the new theory of evolution the “modern synthesis” [Huxley 1946 ]. Since then this theory has been known as the “synthetic theory of evolution.”

6. Molecular biology and genetics

Another important milestone in shaping the theory of evolution occurred with Watson and Crick’s 1953 design of the double helix model of the DNA molecule . Since the 1940s it was known that DNA molecules (deoxyribonucleic acid) contained genetic information. In 1953 the structure of this information was determined. DNA molecules were found to encode genetic information along linear sequences of 4 nitrogenous bases or nucleotides called Adenine, Cytosine, Guanine, and Thymine. These bases constitute the four letters of an alphabet with which the information that is expressed in the development of the living being is written in the genome.

The distinction between genotype and phenotype was firmly established in this way. The most basic level of the phenotype would be proteins: macromolecules composed of amino acids that constitute the fundamental structural part of various living organisms. The correspondence between the different DNA base sequences with each of the 20 different types of amino acids in existence is known. Specifically, each of the amino acids is encoded by three of the basic letters of the genetic code. Each group of three letters that codes for an amino acid is called a “codon.” Not all DNA is coding. In addition there are amino acids that are associated with different codons. This is why the genetic code is said to be degenerate. At the same time,

Below we briefly expose some of the most important notions that were established by the genetics that developed from the 1950s and that are determining factors in the way Evolution is understood today [ Ayala 2006a : 223 and H.H.].

The DNA is, as mentioned, where the molecule is encoded genetic information. It is a long molecule in the shape of a helix and can be represented as two long molecular filaments coiled up and joined by the bases or nucleotides. There are four types of bases and each strand is attached to the other by the complementary bases of the other.

DNA molecules are packed together with proteins in dense bodies called chromosomes . Each species has a certain number of chromosomes. The human species has specifically 46. In this case, which is of sexual reproduction, 23 chromosomes correspond to the father and the other 23 to the mother. We have 23 pairs of homologous chromosomes.

The gene is the discrete unit of inheritance that was first identified by Mendel. In the current paradigm, each gene corresponds to a morphological characteristic of the organism, for example, the color of some part of the body such as hair or eyes. The gene is a segment of the chromosome that is in a specific place called a locus . Each chromosome can have many thousands of gene loci. The loci are on both homologous chromosomes. Each gene at a particular locus can have variant forms called alleles.. That means that allele genes vary in one or more parts of their nucleotide sequence. Genes are therefore presented in pairs, one on a maternal chromosome and the other on the corresponding paternal or homologous chromosome. The two homologous genes occupy a locus on each of the homologous chromosomes. The existence of alleles is the prerequisite for there to be evolution. It has been proven that there is a great genetic diversity, that is, a great diversity of alleles within different populations. Artificial selection is a sign that there is wide genetic variability in natural populations.

A key notion in the theory of evolution is that of species . In the “modern synthesis” the notion of biological species was characterized by Dobzhansky and by Mayr, for sexually reproducing organisms, as “groups of natural interfertile populations that are reproductively isolated from other groups” [ Ayala 2006b: 258]. This notion is the one that is most widely accepted today, despite its obvious limitations, such as, for example, the fact that it is valid only for groups that reproduce sexually or, also, that its application is not possible for species that already are extinct. The notion is important, among other reasons, because defined in this way, each species constitutes a discrete and independent evolutionary unit (there are no exchanges of genes between different species). Much has been written about the mechanisms that lead to the formation of a species. In all that has been written, the importance of reproductive isolation mechanisms of which several types have been identified is highlighted.

The discoveries of the 1950s in genetics and biochemistry have led to countless studies and research carried out from the new theoretical framework and concrete practical results have already been obtained. These studies have resulted, for example, in the culmination of the Human Genome Project in 2003. During the 13 years that the project lasted, the approximately 20,000-25,000 genes that our DNA possesses were identified and the sequence of the three genes was determined. billion bases that make up DNA. Furthermore, the theory of evolution has been remarkably refined. Currently, for example, taxonomies of living beings can be tackled based on the genetic heritage of each species and not on external morphological aspects that are more arbitrary. Now it is known, among other things, what was not known when the synthetic theory was first formulated: what does genetic material consist of? They are being understood little by little, it is a task for years that has just begun, the very meaning of genetic information, which has to do with its expression in the living organism. All this knowledge has opened many expectations, for example, within medicine and, also, in theoretical biology in general. But, on the other hand, the extraordinary complexity that hides in living beings has also been revealed. Regarding the process of evolution, the advances mentioned have solved old questions, but they have also opened new ones that stand as challenges for science that are even more arduous than the old ones. what does the genetic material consist of. They are being understood little by little, it is a task for years that has just begun, the very meaning of genetic information, which has to do with its expression in the living organism. All this knowledge has opened many expectations, for example, within medicine and, also, in theoretical biology in general. But, on the other hand, the extraordinary complexity that hides in living beings has also been revealed. Regarding the process of evolution, the advances mentioned have solved old questions, but they have also opened new ones that stand as challenges for science that are even more arduous than the old ones. what does the genetic material consist of. They are being understood little by little, it is a task for years that has just begun, the very meaning of genetic information, which has to do with its expression in the living organism. All this knowledge has opened many expectations, for example, within medicine and, also, in theoretical biology in general. But, on the other hand, the extraordinary complexity that hides in living beings has also been revealed. Regarding the process of evolution, the advances mentioned have solved old questions, but they have also opened new ones that stand as challenges for science that are even more arduous than the old ones. which has to do with its expression in the living organism. All this knowledge has opened many expectations, for example, within medicine and, also, in theoretical biology in general. But, on the other hand, the extraordinary complexity that hides in living beings has also been revealed. Regarding the process of evolution, the advances mentioned have solved old questions, but they have also opened new ones that stand as challenges for science that are even more arduous than the old ones. which has to do with its expression in the living organism. All this knowledge has opened many expectations, for example, within medicine and, also, in theoretical biology in general. But, on the other hand, the extraordinary complexity that hides in living beings has also been revealed. Regarding the process of evolution, the advances mentioned have solved old questions, but they have also opened new ones that stand as challenges for science that are even more arduous than the old ones. The extraordinary complexity hidden in living beings has also been revealed. Regarding the process of evolution, the advances mentioned have solved old questions, but they have also opened new ones that stand as challenges for science that are even more arduous than the old ones. The extraordinary complexity hidden in living beings has also been revealed. Regarding the process of evolution, the advances mentioned have solved old questions, but they have also opened new ones that stand as challenges for science that are even more arduous than the old ones.

In short, it can be said that there is a common framework accepted by most scientists in which the ingredients that we have previously described are included, among others. The essential nucleus on which there is common agreement among the entire scientific community could be summarized by saying that “evolution occurs through the action of mechanisms such as natural selection on, primarily, small blind variations that occur at the genetic level” [ Sarkar 2007 : 69]. But within that framework there are issues that continue to be the subject of lively debate. There are also questions already raised at the beginning of the formulation of evolutionary theories but now seen from a new perspective and, therefore, from a better understanding of their complexity.

We have also seen that, from its inception, the theory of evolution that was born and developed from Darwin’s ideas has not been peacefully accepted. Some of the controversies that arose in the final years of the 19th century and their roots have already been mentioned. Outside the scientific field, the controversies have not been minor. One of the movements that has offered the most resistance to Darwin’s ideas has been Creationism. The confrontations with the theory of evolution, which continue to this day, give the possibility of tracing a history of which even a simple outline is beyond the reach of this voice [see the voice Intelligent Design ].

Some of the more important scientific questions that have been controversial in recent years in relation to the theory of evolution are briefly listed below. They are collected here because understanding them also allows for a better understanding of the theory of evolution and its scope. On the other hand, the philosophical debate, to which the last part of this voice is devoted, is not alien to the scientific debate.

7. Some scientific questions debated around the theory of evolution

7.1. Selectionism versus neutralism

In the 1960s, a new technique began to be used, gel electrophoresis, with which the degree of genetic variation of populations could be verified quite precisely. The studies carried out with this technique, and others also simple to carry out in the laboratory, led to determine that the proportion of heterozygous loci, that is, those that present different alleles on homologous chromosomes, ranges between 5 and 20 percent. Taking into account that the technique used detects variations in proteins and that their coding by DNA is degenerate, it was reasonable to think that the degree of genetic variation was even greater than these percentages [ Ayala 2006c : 280].

The degree of variation that resulted from these experiments was much higher than expected. An explanation for this phenomenon was proposed by the Japanese geneticist Motoo Kimura in 1968. For this scientist, and those who like him defend the so-called “neutralist theory”, most genetic differences neither favor nor hinder the survival of organisms, which it leads them to conclude that the fact that they survive or are eliminated from a population is simply a matter of chance. The neutralists say that if the majority of genetic differences were subject to natural selection, the variation would be much smaller and, therefore, natural selection would not have the impact on evolution that synthetic theory suggests.

Another of the phenomena used by the neutralists to defend their proposal is the constancy of the rate of genetic change experienced throughout the generations. Different studies have been carried out that relate the evolutionary history common to various species and the number of differences in the respective DNA sequences. The results suggest that genes can be considered as molecular clocks since the rate of change experienced is relatively constant over long periods of time and, furthermore, these values ​​are similar in different species. If natural selection acted as proposed by synthetic theory, say the neutralists, the rates of change would be more variable as a consequence of the different selective pressures that occur in time and in different species.

This proposal has perhaps been the one that has provoked the roughest debates in the world of biology since the synthetic theory was formulated until today. In fact, in 1969 the neutralists King and Jukes announced, in a somewhat provocative way, the birth of a model of evolution in which drift according to neutral alleles had replaced selection as the evolutionary force.

In fact, the debates around neutralism-selectionism have not stopped yet. Authors such as Ayala or Sarkar think that, currently, this debate is not about determining the success of one of the proposals and the exclusion of the other. Rather, it would be a matter of determining to what extent selection acts and to what extent neutralism is valid and helps to understand evolution. In fact, Kimura himself accepted that natural selection is the determining evolutionary force at the morphological scale and that neutralism at the molecular level presents problems when trying to explain the adaptive differences that are manifested at higher levels of organization. Ayala also affirms that the synthetic theory does not force the rate of evolution to be as irregular as the neutralists suppose. The enormous time intervals over which molecular evolution occurs cause the fluctuations to offset each other, giving the impression that they occur at a constant rate. Mathematical models have even been formulated in which the molecular clock is made compatible with evolution governed by natural selection.

The so-called “almost neutral” theory has also been proposed, in which it is tried to maintain the consistency of the neutralist theses with many data indicating the performance of natural selection. Most biologists still believe in the importance of natural selection as one of the engines of evolution, but this debate is still alive and many are confident that the data that is being obtained from the sequencing projects of different genomes will help determine the weight that the contributions of both proposals have in the general framework of the theory of evolution [ Sarkar 2007 : 65-67].

7.2. Punctuationism versus gradualism

One of the classic distinctions underlying some of the debates around evolution has been that of microevolution versus macroevolution. Current genetics has introduced a mistake in these terms that should be kept in mind. Microevolution can be understood as the evolution that occurs as a consequence of small observable variations within the same species. The macroevolution would be, instead, the one that brings with it great changes such as the diversification of species over long periods of time. The new theoretical framework leads to understand these notions with a different sense, although related. Microevolution is now what we can verify at the biochemical level: a modification in a base pair of a gene, a mutation, would be the most elementary fact of microevolution.

As a consequence of the influence of the gradualist theses defended by Darwinism, it is usually argued that the difference between microevolution and macroevolution is only a matter of time: macroevolution would be nothing other than the accumulation of microevolutionary changes. This assumption has been subject to nuances and discussions even among Darwinists themselves. It is not doubted that microevolutionary changes are at the base of macroevolutionary changes. What is discussed is the reducibility of one to the other, that is, that the explanation of the laws and mechanisms of microevolution leads to a complete explanation of those of macroevolution. In other words, it is questioned or denied that the laws of macroevolution that we can establish are derivable from those established for microevolution. Ayala,Ayala 2006b : 268].

Closely related to the difference between micro and macroevolution is one of the most important debates that arose within the scientific community and that, initially, seemed to break with the very foundations of the theoretical framework of modern synthesis. It is about the confrontation between the gradualism typical of synthetic theory with what has been called punctuationism or saltationism. This name derives from the one received by the theory proposed by Niels Elredge (1943-) and Stephen Jay Gould (1941-2002) in 1972, which the latter called “Point Balance”.

The problem behind the origin of this proposal is the contrast between the gradualism that seems to derive from the synthetic theory and the leaps in the existing fossil record, which are far from being a gradual continuity. Until the punctuated equilibrium saw the light, the most common way, although not the only one, to justify the existence of these holes in the fossil record was the easiest and most direct: not all living beings that have existed have been fossilized, or, We have not yet discovered many of the fossils that will allow us to fill in the existing gaps. Mayr, on the other hand,Mayr 2005 : 212]. The fact is that the accumulation of new fossils did not seem to support the easy solution, and that there was reason to doubt the compatibility of the gradualist orthodoxy with the data provided by paleontology.

Gould and Eldredge argued in their work that the fossil record positively showed that in evolution there were short periods in which evolutionary changes occurred very rapidly, and that these were followed by other long periods of stasis in which the different forms remained stable. . In other words, evolution seemed to leap from one species to another without there being intermediate species. It is not that we did not have the fossils of the intermediate links – missing links – but, simply, those links had not existed.

Initially punctuated equilibrium seemed to some an alternative theory to modern synthesis, it received much criticism and provoked a lively debate. It was soon seen that the punctuationist or saltationist scheme was not a real difficulty in continuing to uphold the principles of the modern Darwinian synthesis. The same authors of the theory explained that the problem is that you are playing with two different time scales. On the one hand, we have to consider the time in which evolution occurs following a model of small and gradual changes and, on the other hand, the time that is relevant to the fossil record called geological time, whose scale is much larger than that of the First. In Gould’s words: ‘What the theory of punctuated equilibrium tries to explain is the macroevolutionary role of species and speciation as expressed in geological time. His statements about speed and stability describe the history of individual species, and his statements about rates and styles of change deal with the tracing of these individual histories in the unfamiliar domain of geologic time, where the duration of a human life is below any possible appreciation, and the entire history of human civilization is to the duration of the primate phylogeny as a blink to a human life »[Gould 2004 : 797].

Many biologists have pointed out that macroevolutionary patterns of stasis and jumps could be produced by models based on microevolution. It also seems to have been shown in recent decades that rapid morphological changes can occur in natural populations. It thus seems to confirm that although saltationism was the predominant pattern of macroevolutionary change, the processes involved remain within the framework of modern synthesis.

Gould himself has defended the compatibility of his proposal with the theses of current synthetic theory: «The punctuated equilibrium does not attempt to redefine or criticize the conventional microevolutionary mechanisms at all (because it arises as the anticipated expression, after scale change, of the theories microevolutionary on speciation in the radically different domain of geological time) »[ Gould 2004: 812]. However, Gould defends that his proposal is original, and that originality lies in the change of perspective with which evolution is observed. For Gould, on the geological time scale, the subject of evolutionary selection would no longer be the individual of a population but would be the same species. As Gould puts it: ‘But the crux of the potential novelty of punctuated equilibrium for biological theory is that these classical microevolutionary mechanisms do not have the exclusive power of evolutionary explanation, and that their domain of action must be restricted (or at least shared) to level of the macroevolutionary pattern on a geological scale, because the punctuated equilibrium ratifies an effective macroevolutionary mechanics based on the recognition of species as Darwinian individuals. In other words,Gould 2004 : 812].

Therefore, according to this author, saltationism could be seen as an extension of the synthetic theory in which its principles are reaffirmed but in a different theoretical domain. These statements indicate, as we have also seen Ayala do, a certain level of independence between macro and microevolution, but always within the common framework of synthetic theory.

This discussion serves to allude to a debate that also has resonances in philosophical studies on evolution: the determination of which is the unit of selection. The gene, the individual and other population groups such as the species have been proposed as selective units. Geneticists, and more specifically neutralists, are more likely to consider the gene as a unit or target of selection. Mayr considers that “since no gene is directly exposed to selection, but only in the context of the entire genotype, and since a gene can have different selective values ​​in different genotypes, it does not seem appropriate to consider it the target of selection” [ Mayr 2005: 218]. This author thinks that the individual is the main target of selection, although he also admits the possibility of so-called group selection, such as species selection.

7.3. Notion of species

It has already been mentioned that a key notion, as is clear from everything seen so far, is the notion of species. In any work on the theory of evolution, like this one for example, it is one of the most used words. The notion has been the subject of considerable debate since the beginning of the theory of evolution. The debate on this notion also has special philosophical connotations and for this reason it is also important to treat it here even if it is very brief.

The problem discussed could be expressed in a simple way as an alternative: do species have a real existence or are they instead a product of our mind that simply facilitates the organization of our knowledge about nature? Darwinian gradualism blurring its contours is opposed to a notion of species conceived as something perfectly determined morphologically and temporally. If one species derives from another by gradual evolution, where to put the boundary between the two species? Or, what differences must there be between two individuals to be considered as belonging to different species? Darwin, for example, stated: ‘I consider the term species as arbitrarily given, for convenience’s sake, to a group of individuals very similar and not essentially different from the termvariety , which occurs in less precise and more fluctuating forms ”[ Darwin 2002 : 104]. For Haldane the concept of species was a concession to our linguistic habits and neurological mechanisms [ Sarkar 2007 : 70].

The difficulties in giving a definition of species that does not present some problem or limitation seem to support these opinions and reduce reality, or rather realism, to the notion of species. However, despite its limitations, much importance has been attached to the previously defined notion of biological species. In fact, this notion is useful within the conceptual scheme that serves to explain evolution itself and, as we have seen, in some authors like Gould, it even becomes a target of Darwinian selection, that is, a Darwinian subject. This notion of biological species is clear and clearly defines what a species is, but it does not completely avoid the general problem of lack of delimitation between species when gradualness in evolution is accepted.

Currently this debate is still open. The definition of a biological species is seen by some as insufficient. For example, the field of microbiology offers a greater diversity than that which we are used to ordinarily contemplate and which has been the object of the most common taxonomic proposals. In this area, the definition of a biology species is useless since the majority type of reproduction is not sexual. Despite the difficulties, they continue to search for taxonomic criteria that are useful for making classifications and reduce as much as possible the shortcomings of the existing ones. What seems clear to all biologists is the need to have a criterion to differentiate species, although they do not always agree to establish the most appropriate one. Above the disagreements, What does seem to be a coincidence is that the reality of the species is related to the existence of population units grouped in ecological niches in which natural selection avoids confusion between them. It is also accepted that there may not be a unique and optimal characterization for the species, but that one or the other should be used according to the level or branch in the nature tree that is being studied [Zimmer 2008 : 72-73].

In reality, the greatest difficulties at this point arise, above all, for those who defend proposals of a creationist or fixist type. In reality, this fixation belongs only to our way of thinking natural beings, that is, it belongs to the objectification that we make of them. It is not easy to think objectively the proper movement of life. A movement that, as in the case of evolution, involves periods of time that completely escape the magnitudes that capture our ordinary knowledge.

7.4. Importance of natural selection in evolution

The degree of intervention of natural selection in the evolutionary process has also been a constant object of debate since the formulation of Darwin’s theory. The controversy remains open in the purely scientific field. It must be borne in mind that this mechanism has always been given a central role within the orthodoxy of synthetic theory. Much of the originality of Darwin’s proposal rests on it.

However, at present reasons are adduced to diminish its importance in evolution, such as those presented by the aforementioned neutralists. Important reasons continue to be offered to emphasize its importance as well. An argument frequently used in its defense consists in the verification, especially in the field of macroevolution, of the existence of “convergent evolution”: there are living beings that are very far apart from the phyletic point of view, or that have evolved from a In isolation, but which have developed similar organisms and have reached remarkably similar functional solutions.

The problem of the importance of natural selection parallels the problem of the degree of contingency of evolution. The difficulty that arises is the following: if evolutionary history began again, would we have a panorama in nature similar to the one we find today? If it is admitted that mutations are what generates variety, and that these are blind, the answer to the question has to do with how strong or weak the role of natural selection is in evolution. The answer to the question has confronted various scientists. Gould, for example, has made contingency one of the central points of his thesis. Simon Conway Morris, on the other hand, has emphasized convergence in a special way. In general it seems that among biologists there is agreement that there is both one and the other, and they allow for contingency and convergence within modern synthetic theory. Some scientists have even formulated the action of natural selection in the form of a theorem, specifying which are the necessary premises that must be fulfilled for natural selection to act or not [Meléndez-Hevia 2001 : 18]. These formulations try to explain the why of the contrasts indicated within the framework of synthetic theory and offer a perspective as close as possible of the importance of the action of natural selection in evolution.

7.5. More we know, more debates

They are not the previous ones, far from it, the only debated questions. In any case, none of the important 20th century authors who have contributed to the establishment of synthetic theory think that this large number of controversies endangers the validity of modern synthetic theory for the time being. Mayr says in this regard: “For many evolutionary problems there are multiple possible solutions. Although all of them are compatible with the Darwinian paradigm. The lesson this pluralism teaches us is that, in evolutionary biology, generalizations are almost never correct. Even when something happens “usually”, this does not mean that it always has to happen ”[ Mayr 2005 : 223].

The reality is that there are many issues up for debate. The pluralism that Mayr refers to may seem excessive to some who consider the need to achieve greater unity and simplicity, perhaps with the formulation of a new synthesis. This new “postmodern” synthesis, as it is qualified with some apprehension in an article in Nature [ Whitfield 2008], it should be able to explain what happens in areas of biology that have not yet been successfully integrated with synthetic theory. One of these areas is, for example, developmental biology, on which genetics is currently providing a great deal of information. The emerging discipline called “evo-devo” (evolution and development) tries precisely to unite these two areas of biology, but it is still far from maturity. In this field of biology there are many mysteries to unravel, and the accumulating data leads to the question, for example, of the need to assume a richer relationship than that accepted by the synthetic theory between genotype and phenotype. This relationship should not be, for example, as unidirectional as the aforementioned central dogma of biology establishes. Or at least,

All that has been said so far may lead us to think that the conceptual framework of modern synthetic theory explains a lot, but that it is still insufficient to give true unity to all the phenomena that we witness in the biological world. In any case, what these debates do make clear is that life, in its apparent simplicity and simplicity, presents great complexity when analyzed from a scientific point of view. Biology is not physical and does not seem to be caught in the nets of a perfectly unified, defined and finished method. In reality physics, although it is more docile to mathematics, does not seem to allow it either. In any case, the vast amount of scientific knowledge that we have about biology, and in particular about evolution,

8. Philosophical reflection and theory of evolution

Philosophy is a discipline that seeks to achieve a global perspective in the face of reality. There is nothing that can escape the gaze of philosophy in its attempt to find the synthesis or connection with the globality of the real, that is, how each portion of the real fits into the broad landscape of reality [ Polo 1995 : 21 ].

For this reason, philosophy always transcends the scope of the plot on which it pauses. His vocation is to face the most radical questions. Philosophy is a discipline that seeks the principles or first causes of reality. This is the most demanding way to take a global perspective. To say that philosophy tries to reach the principles of the reality with which it deals is equivalent to saying that what is expected of philosophy is that it offer ultimate answers on the problems that arise, which is not the same as saying that it is expected her definitive answers.

The foregoing does not mean that philosophy is a kind of independent discipline or apart from what ordinary or scientific knowledge offers to our understanding. There is no pure and uncontaminated philosophy with issues that are considered minor or superficial. All authentic philosophy must be well rooted in what is known, whatever the method or the way in which that knowledge has been made present to us.

Therefore, the peculiarity and also difficulty of philosophical knowledge consists in its aspiration to achieve a global perspective. This aspiration means that philosophy cannot always be easily discerned from doctrines that we could call pseudo-philosophies. One could also speak of the existence of pseudosciences. These pseudo-doctrines have as a true point of support and are largely fed by ideologies which they serve as spokesmen. It is normal for pseudo-philosophy and pseudoscience to take advantage of the limitations of scientific knowledge to try to fill in their gaps with considerations that often contain an ideological component. Biology, with its complexity, its subjects and its current degree of development, is a fertile field for this type of pseudodoctrines.

On the other hand, it is not necessary to justify the need to make a philosophy of biology. It is enough to state that such a philosophy is inevitable, as the numerous publications and works carried out in this discipline prove.

The fundamental problem of the philosophy of biology, which is life, will not be dealt with here. Only some of the philosophical questions raised by the theory of evolution will be addressed very briefly. Some of them appear implicitly, or sometimes explicitly, supporting the debates referred to above.

8.1. Theory of evolution and evolutionism

It is important to distinguish between Theory of Evolution, which we have presented here as a strictly scientific theory, and Evolutionism.

All science is associated with a method that can be more or less explicit or defined. The method does not simply consist of a set of operating rules but includes elements of a very different kind and reaches great complexity in real science. In all cases, the use of a method always entails a reduction in the scope of the reality studied. This reduction is especially necessary if one wants to achieve one of the objectives pursued by empirical science and which consists of controlling, in some way, reality: empirical science is “that human activity in which a knowledge of nature is sought that allows obtain a controlled domain of it »[ Artigas 1999 : 15].

The methodical reduction that determines the way in which we contemplate reality with that science, what we observe and what we leave out of our consideration, is completely necessary to achieve the objectives of scientific activity. Problems arise when it is forgotten that using a method implies reduction or, simply, it is affirmed in a positive way that only what is made present through a particular method is real, however complex it may be. This statement, in reality, what it does is give a global character, which is typical of philosophy, to a particular science. The problem is that this way of proceeding leaves out of reality, in an arbitrary way, aspects that are real but cannot be captured by said method. As those aspects omitted or denied belong to reality, sooner or later they will claim their presence in our knowledge and, then, inadequate explanations will be offered to them because they do not conform to the method by which they are explained. A favorable situation will also be created for ideological responses to be offered to the problems that arise as a result of the aforementioned mismatch. The discipline that tries to cover the totality from its particular method slides down the slope of reductionism and, then, with property, the suffix “ism” can be added to the name of said discipline. A favorable situation will also be created to offer ideological responses to the problems that arise as a result of the aforementioned mismatch. The discipline that tries to cover the totality from its particular method slides down the slope of reductionism and, then, with property the suffix “ism” can be added to the name of said discipline. A favorable situation will also be created to offer ideological responses to the problems that arise as a result of the aforementioned mismatch. The discipline that tries to cover the totality from its particular method slides down the slope of reductionism and, then, with property the suffix “ism” can be added to the name of said discipline.

Evolutionism would mean, in this context, a worldview in which the natural world is fully contemplated and explained through the method developed by the theory of evolution. This claim, which can be found in some current authors, is not at all legitimate [ Artigas-Giberson 2007]. The situation is parallel, although with its own characteristics, to that derived from the birth of mechanics. The physics of the seventeenth century was a true novelty in the way of understanding natural reality and brought with it a multitude of benefits for humanity. But along with the scientific discipline, a globalizing way of thinking also developed, and therefore of a philosophical nature, which received the name of mechanism or mechanical philosophy. The birth of a new science in which satisfactory results and answers to previously unsolved problems are offered, and in which perspectives are opened to reach new and important knowledge, is always an occasion to incur reductionism.

Mechanism exerted a great influence on thought for three long centuries. It entered into crisis as a consequence of the advancement of physical science itself. Evolutionism, like reductionism, also currently exerts great influence in many different areas and is present in the writings of some scientific popularizers who have achieved a large audience today.

It would incur an evolutionary reductionism, therefore, the one that wanted to explain all of reality from the methodical elements that the theory of evolution uses. To try to explain with the theory of evolution all the phenomena of our experience, including such human realities as love, for example, the reality of God, morality, etc., would be to constitute said theory into a kind of philosophy in which It would necessarily be necessary to introduce elements outside of it. The experience of mechanics is very illustrative of what the claim to encompass all of reality with a scientific method entails. In the case of mechanics it was not only seen that it was insufficient to assume a role that is typical of philosophy, but it did not even serve to explain the whole reality of its own subject: that of physical movement.

The confusion of the theory of evolution with evolutionism is frequent and has given rise to controversies like the one that Darwinism has faced with creationism or, more recently, with “Intelligent Design”. The struggles of this type never reach any port because, ordinarily, the discussion focuses on aspects of the philosophical sphere. This is precisely the realm that the contenders cannot reach by pretending to stay within science. The recourse to ideologies, at least implicit, makes the agreement impossible.

The above distinction is related to the accusation leveled by some against the theory of evolution that it is not strictly science but philosophy. This accusation is not equivalent to what authors like Artigas point out when they say that all science has a series of philosophical presuppositions. What they are actually saying is that the claims that fall within the subject of said science are philosophical in scope and are not supported by a properly scientific method. At the base of this accusation is the failure to take sufficient account of the distinction that we are commenting on and understand by theory of evolution any of the forms of evolutionism.

The problems that philosophy had to face, especially during the first half of the 20th century, in relation to the so-called “problem of demarcation”, that is, the problem of determining whether something is science or not , has led to the adoption of rather broad criteria full of nuances in the delimitation of what constitutes a discipline as science. If, for example, it were required that a theory, in order to be scientific, had to have predictive capacity, as in the case of Physics, then effectively the scientificity of the theory of evolution would have to be put in brackets or denied. Dobzhansky himself states: “Those who claim that predictability is essential to a scientific theory may scoff at the theory of evolution as unscientific” [Dobzhansky 1983 : 405-406]. Today the emphasis is rather placed on systematicity as a peculiarity of science [ Hoyningen-Huene 2008 ], and it is not intended to establish a demarcation of its limits so precise that scientific consideration is denied to disciplines that are, although their method do not respond to such a clear and well established paradigm as that of mathematics or physics, for example.

The debate over naturalism also arises in this context. Naturalism, in its most common and strong sense, defends that all reality is resolved and explained by natural laws: ontological naturalism. Some critics of the theory of evolution have accused it of being a naturalist. Also in this case it seems that it is more just to accuse evolutionism of naturalism, in this strong sense. On the other hand, it seems justified to argue that natural laws can only be used when one wants to explain phenomena that do not go beyond the scope of material nature. To defend the latter would be to defend what could be called methodological naturalism. Science is legitimately naturalistic in this last sense, that is, when it does not set itself up as global knowledge, which is specific to philosophy.

8.2. Evolution and purpose

The theory of evolution has been a powerful incentive for philosophical reflection from its earliest formulations. Today numerous studies are published that bear the label of philosophical and that focus on the field of biology. Much of them, directly or indirectly, address issues related to evolution. On the one hand, there are the epistemological problems related to the theory, which have already been mentioned in the previous section and which have to do with the consideration of its scientific status. Also in the epistemological field appears the problem of the reductibility of the theory of evolution, and of biology in general, to other disciplines such as physics. This topic also has ontological implications. Adaptation, the role of natural selection and its legitimacy as a non-tautological notion, chance, the notion of function, what are the units of selection, the emergence of properties, the concept of progress in biology and the evolutionary continuity of man with respect to the rest of the animals, are some of the many other questions related to evolution that are the object of philosophical reflection today. It is not possible for us to address them in this encyclopedic writing.

It is appropriate to consider here, even briefly, the fact that the background for most of the questions raised revolves around reflection on the causes of evolution. These are some of the many other evolutionary issues that are the subject of philosophical reflection today. It is not possible for us to address them in this encyclopedic writing. It is appropriate to consider here, even briefly, the fact that the background for most of the questions raised revolves around reflection on the causes of evolution. These are some of the many other evolutionary issues that are the subject of philosophical reflection today. It is not possible for us to address them in this encyclopedic writing. It is appropriate to consider here, even briefly, the fact that the background for most of the questions raised revolves around reflection on the causes of evolution.

Reflection on causes, especially when it focuses on the most radical or first causes of any reality, is genuinely philosophical and forces us to adopt an approach that is global, that of philosophy. A danger that, directly or indirectly, is present in the consideration of the causes of evolution is to try to offer solutions that must be given from a global perspective, that is, philosophical, with elements of the scientific method and that, therefore, do not have that scope. For example, the affirmation of chance as the motor principle of evolution, in the way that Monod proposes [ Monod 1987], for example, incurs such a reduction. In reality, chance is part of a mechanism that, by itself, is not capable of explaining evolution from a global perspective.

This type of reductionism is frequently answered from science itself. Dobzhansky himself states: ‘I do not think that the modern biological theory of evolution is based on’ chance ‘to the extent that Auden fears it or Monod affirms it. The known and unknown of this question deserve detailed consideration. ”[ Dobzhansky 1983: 394-395]. In the same document he states the following: «Adaptability through culture and symbolic language transmitted extragenically has developed in a single species – man. Calling this “random” is a nonsensical solution. Ascribing it to predestination is incompatible with all that is known about the causes of evolution. The analogy with artistic creativity is, at least descriptively, more appropriate, since there are no obvious differences to the contrary “[ Dobzhansky 1983: 422-423].

The search for the deepest causes leads the reflection that leads Dobzhansky to see the analogy with artistic creativity as the best way to express his ideas about the causes of evolution. But the very notion of creativity that he employs has great limitations and presents problems when attributed to natural selection. The sincerity that moves his reflection is evident in the following words: «We neither appeared by chance nor were we predestined to appear. In evolution, chance and destiny are not alternatives. We have here an occasion in scientific theory where we must invoke some kind of Hegelian or Marxist dialectic. We need a synthesis of the “thesis” of chance and the “antithesis” of predestination. My philosophical competence is insufficient for this task.Dobzhansky 1983 : 419].

The debate about causes in nature is as old as philosophy. The reflections on movement center the reflections of the first Greek philosophers. The most mature fruits of this reflection are found in the Aristotelian doctrine of the four causes: material, formal, efficient and final. The evolution of thought after Aristotle affects in one way or another in the way in which these causes are understood. Experimental science, from its birth, has had a major impact on this understanding. In the field of biology, the way in which the final cause has been understood is of particular importance. The purpose, its peculiar way of causing or its non-existence as a cause is a constant in philosophical reflection. The birth of mechanics, for example, it substantially modified the way of understanding the four causes and, in a particular way, the final cause. The effect of this modification is important to take into account in order to understand the orientation of many of the philosophical debates around the theory of evolution.

The most important change introduced by mechanics with respect to the final cause is that it began to be seen as a cause external to nature. For Aristotle the finality is in the nature of things, which was especially evident for him in living beings. This perspective is maintained in the great medieval masters who see in finality a way to access the existence of God: the argument of finality. The change in perspective introduced by mechanics also led to a barely perceptible reformulation of the argument from finality. The fifth way argument of St. Thomas, that of finality, is no longer the teleological argument used by Paley (1743-1805) to demonstrate the existence of God. Both understand nature and its causes differently. Paley’s argument leads to affirm the existence of a God who is the explanation of the complexity of living beings, but who causes from outside.

The example that he uses of complexity is that of a watch: the order of its pieces cannot be explained by “natural causes”. The causes of Paley’s argument are no longer Aristotelian causes. In particular, the final cause is different. The purpose of the clock is external or extrinsic to the clock itself: a different conception from that understood by Aristotle and the Thomist tradition for the complexity that any living being presents. In it, the notion of nature is very important, in which there is a unity, which we could call intrinsic, between the formal and the final cause. The example that he uses of complexity is that of a watch: the order of its pieces cannot be explained by “natural causes”.

The causes of Paley’s argument are no longer Aristotelian causes. In particular, the final cause is different. The purpose of the clock is external or extrinsic to the clock itself: a different conception from that understood by Aristotle and the Thomist tradition for the complexity that any living being presents. In it, the notion of nature is very important, in which there is a unity, which we could call intrinsic, between the formal and the final cause. The example that he uses of complexity is that of a watch: the order of its pieces cannot be explained by “natural causes”. The causes of Paley’s argument are no longer Aristotelian causes. In particular, the final cause is different.

The purpose of the clock is external or extrinsic to the clock itself: a different conception from that understood by Aristotle and the Thomist tradition for the complexity that any living being presents. In it, the notion of nature is very important, in which there is a unity, which we could call intrinsic, between the formal and the final cause. In particular, the final cause is different. The purpose of the clock is external or extrinsic to the clock itself: a different conception from that understood by Aristotle and the Thomist tradition for the complexity that any living being presents. In it, the notion of nature is very important, in which there is a unity, which we could call intrinsic, between the formal and the final cause. In particular, the final cause is different. The purpose of the clock is external or extrinsic to the clock itself: a different conception from that understood by Aristotle and the Thomist tradition for the complexity that any living being presents. In it, the notion of nature is very important, in which there is a unity, which we could call intrinsic, between the formal and the final cause.

Before Darwin, Paley’s argument seemed to be convincing as an argument for accessing God. Mechanical philosophy thus fulfilled an apologetic role. Problems arise with Darwin because his proposal seems to render the finality argument without foundation. What should be highlighted is that the argument that is directly affected by Darwin’s proposal is the one sustained from mechanical philosophy. The theory of evolution seems to offer a way of explaining complexity without resorting to external agents who have to design or order the various organisms. This is immediately interpreted by many as an elimination of finality as the cause of nature. Mechanics seemed to erase the finality of the inanimate world and Darwin, for many, succeeded in doing the same in the living world. But to eliminate the purpose is to leave without foundation one of the most important arguments of access to God. Science, it is affirmed from these positions, has been snatching the causal role of supernatural agents in favor of science.

Ayala, for example, states: «The scientific advances of the 16th and 17th centuries had brought the phenomena of inanimate matter — the movements of planets in the sky and of physical objects on Earth — to the realm of science: explanation by means of natural laws. Similarly, natural selection provided a scientific explanation for the design and diversity of organisms, something that had been omitted by the Copernican revolution. With Darwin, all natural phenomena, inanimate or living, became the subject of scientific investigation »[ It is affirmed from these positions, it has been snatching the causal role of supernatural agents in favor of science. Ayala, for example, states: «The scientific advances of the 16th and 17th centuries had brought the phenomena of inanimate matter — the movements of planets in the sky and of physical objects on Earth — to the realm of science: explanation by means of natural laws. Similarly, natural selection provided a scientific explanation for the design and diversity of organisms, something that had been omitted by the Copernican revolution. With Darwin, all natural phenomena, inanimate or living, became the subject of scientific investigation »[ It is affirmed from these positions, it has been snatching the causal role of supernatural agents in favor of science. Ayala, for example, states: «The scientific advances of the 16th and 17th centuries had brought the phenomena of inanimate matter — the movements of planets in the sky and of physical objects on Earth — to the realm of science: explanation by means of natural laws. Similarly, natural selection provided a scientific explanation for the design and diversity of organisms, something that had been omitted by the Copernican revolution.

With Darwin, all natural phenomena, inanimate or living, became the subject of scientific investigation »[ «The scientific advances of the 16th and 17th centuries had brought the phenomena of inanimate matter — the movements of planets in the sky and of physical objects on Earth — to the realm of science: explanation through natural laws. Similarly, natural selection provided a scientific explanation for the design and diversity of organisms, something that had been omitted by the Copernican revolution. With Darwin, all natural phenomena, inanimate or living, became the subject of scientific investigation »[ «The scientific advances of the 16th and 17th centuries had brought the phenomena of inanimate matter — the movements of planets in the sky and of physical objects on Earth — to the realm of science: explanation through natural laws. Similarly, natural selection provided a scientific explanation for the design and diversity of organisms, something that had been omitted by the Copernican revolution. With Darwin, all natural phenomena, inanimate or living, became the subject of scientific investigation »[ Similarly, natural selection provided a scientific explanation for the design and diversity of organisms, something that had been omitted by the Copernican revolution. With Darwin, all natural phenomena, inanimate or living, became the subject of scientific investigation »[ Similarly, natural selection provided a scientific explanation for the design and diversity of organisms, something that had been omitted by the Copernican revolution. With Darwin, all natural phenomena, inanimate or living, became the subject of scientific investigation »[Ayala 2007 : 24-25].

Ayala’s words do not explicitly presuppose the expulsion of God from rationality, but they can give rise to the idea that God is confined to the world of the subjective and that, therefore, in the best of cases, there is no incompatibility between God and science because they belong to areas that have no points in common: the thesis of the double teaching that Ayala and also Gould defend, for example.

These dangers derive from a finalist vision passed through the filter of mechanism. It is enough to read a text by Thomas Aquinas to verify that in his proposal the purpose does not explain complexity in an external way but from nature itself and, therefore, through natural laws: «Nature is, precisely, the plan of a certain art (specifically, divine art), imprinted on things, by which things themselves move towards a certain end: as if the craftsman who made a ship could grant the logs to move by themselves to form the structure of the ship »[S. Thomas Aquinas, Commentary on Aristotle’s Physics , book II, lectio 14, n.8]. Thomas Aquinas is not opposed to a methodological naturalism.

The causal pluralism of the realist tradition is richer than that derived from mechanical philosophy and on which many of the debates that have to do with purpose and causes in general still rest. Causal pluralism faces monisms of different kinds that have been proposed as a causal explanation of evolution, the most important of which are of a materialistic nature. The proposal of the realistic tradition does not confront a methodological naturalism such as that which is evident in the words of Ayala cited above. The philosophy of the realist tradition assumes all that science can say in its field, but frames the purpose, as cause, in a broader context than that of the scientific method.

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