SYNTHETIC THEORY OF EVOLUTION: HISTORY, POSTULATES, EVIDENCES, STRENGTHS - BIOLOGY - 2024

The synthetic theory of evolution, also known as neo-Darwinian theory or modern synthesis of evolution, is a theory that proposes a link between Darwinian theories of natural selection and the theories of inheritance proposed by Gregor Mendel.

This theory provides explanations for the transformation of a species by natural selection and for the division of a species into isolated subgroups (speciation). He conceives of evolution as the sum of random events (mutations and recombination) and non-random events such as natural selection.

Darwin and Mendel (Source: Original images: undetermined.Arranged by SteinBike via Wikimedia Commons)

In the synthetic theory of evolution the fundamental evolutionary event is a change in the frequency of appearance of an allele in a population. Therefore, this theory is based on the analysis of all the factors that influence the changes in population allele frequencies, namely: mutation, selection and gene drift.

This theory reinforces the essential role of natural selection as the "motor" of evolution, but unlike the first evolutionary theories, it is based on different theoretical elements that facilitate its interpretation and analysis.

History

In order to tell the story of the synthetic theory of evolution, it is necessary to make a historical account of the antecedents that took place so that this theory had a place in the scientific world.

Darwin and Wallace

It could be said that it all began in 1858 with the English naturalists Charles Darwin and Alfred Wallace, who independently arrived at the deduction that natural selection is the mechanism responsible for the origin of phenotypic variations and, therefore, of speciation.

Alfred Wallace. User Tagishsimon on en.wikipedia

In some texts it is indicated that both authors presented a hypothesis known as "descendants with modification by natural selection", by means of which they affirmed 5 things:

1. All organisms produce more offspring than the environment in which they live can support
2. The intraspecific variability (within the same species) of most traits is extremely abundant
3. Competition for limited resources ends in a "struggle for survival"
4. In nature, there is the inheritance of modified traits, that is, some modifications can be inherited from the parents to their offspring
5. When the "modifications" are considerable, this can result in the evolution or appearance of a new species

Both naturalists supported their theories with detailed observations of fossil records and of living organisms in their natural environments.

Gregor Mendel

Gregor Mendel

In the same decade (1856), the Austrian monk Gregor Mendel carried out a series of experiments with pea plants, by means of which he determined that the characters are inherited as "physical entities" from the parents to the progeny.

Thanks to his discoveries, Mendel was able to formulate the "laws of inheritance of traits", which describe the principles of dominance, segregation and independent distribution of genes, which are now the fundamental bases of genetics.

There is evidence that Darwin read the works published by Mendel to the Brünn Natural History Society in the mid-1860s. However, he made no reference to them in his famous book The Origin of Species, probably because he did not understand with certainty. what the latter was referring to.

Neo-Darwinism

Mendel's works were "shelved" until the early 1900s and have become popular ever since. Its laws were applied to solve problems related to biological inheritance, but they did not seem to have any relationship with evolutionary biology or with the postulates of Darwin and Wallace.

This apparent “divorce” between the two approaches was due to the fact that the “supporters” of the two theories did not conceive a joint vision to analyze the continuous variation of the species.

It was the biologist and statistician Ronald Fisher in 19018 who used the statistical tools of the day to "reconcile" the inconsistencies between Darwin's ideas of natural selection and Mendel's experiments on the inheritance of characters.

Ronald Fisher. See page for author

The birth of Neo-Darwinism or the synthetic theory of evolution took place at the hands of Ronald Fisher himself and a large group of theoretical biologists, including Sewall Wright, John Haldane, and others.

Later, Theodosius Dobzhansky made important contributions by demonstrating, through experimental population studies, the effect of natural selection on the variability of natural populations using the integration of Mendelian genetics and chromosome theory.

Many other scientists, although some more than others, took place in the synthesis of evolutionary theory that prevails today, but only the most prominent were mentioned here.

Postulates of Synthetic Theory

Skeletons of large and medium primates. The original uploader was TimVickers at English Wikipedia.

The synthetic theory of evolution or "modern synthesis of evolution" explains this process in terms of the genetic changes that occur in populations and that lead to the processes of speciation. This theory defines evolution as "changes in the allele frequencies of a population."

According to it, the mechanisms that direct the evolutionary process are based on natural selection, which is supported by some of the postulates contemplated by Darwin and Wallace, especially those related to the overproduction of offspring, its variation and inheritance. of the features.

Thus, the factors involved in this theory are:

- Mutation rates

- Migration processes

- Chance or gene drift

- Recombination or variation

- Natural selection

Mutation

Interaction between mutations and natural selection. Wilfredor

Mutations are changes that occur in the sequences of genes and that generally produce different phenotypes. Some types of mutations can be deleterious or harmful, but others can be advantageous from many points of view (or simply neutral).

Mutations or changes in the DNA sequence can be inherited from parents to their children and are the main source of variation in offspring.

Migration

Migratory processes between different populations of the same species can induce an increase in genetic variability due to the introduction of new alleles to the allelic set of a population, altering the allelic frequency of this.

Random or genetic drift

Randomness or genetic drift is a genetic event that modifies the genotypic composition of a population by the random appearance of a rare modification, either due to deletions, translocations, inversions, duplications, etc., which can end in the disappearance of the alleles less frequent.

Recombination or variation

This is the process that occurs during sexual reproduction and involves the combination of the chromosomes of the two individuals that reproduce to give rise to a new individual, which is characterized by having a genetic combination different from that of its parents.

Through this process, deletions, inversions, duplications, translocations, polyploidies, etc. can occur.

Natural selection

Darwin investigated the Galapagos finches as an example of natural selection (Source: Robert Taylor Pritchett via Wikimedia Commons)

Natural selection is a "force" that produces changes in the frequency of genes between one generation and the next, favoring the differential reproduction of the "best adapted" individuals.

According to the predictions of the “neo-Darwinian” models, evolutionary changes are gradual, as proposed by Darwin, which means that they are slow, gradual and continuous within each determined lineage.

Evidence

The anthropological intervention of ecosystems has provided “natural experiments” that serve to evidence neo-Darwinian hypotheses.

The Biston betularia moth, for example, is an abundant arthropod of forested areas of England, where two color forms have been distinguished, one light and one dark. A single gene is involved in the differences between the two phenotypes and the dark color allele is known to be dominant.

Photograph of a Biston betularia pair (Source: Follow via Wikimedia Commons)

The allelic frequency of the dark form has increased considerably since 1850, especially in the more industrialized areas of Manchester and Birmingham, supposedly as a "cloaking" mechanism to evade predators, that is, due to natural selection.

The frequency of the dark form with respect to the light one increased from 1 to 90% in less than 100 years, but in other less industrialized regions the dark form is still very "rare".

Strengths

Species of the genus Panthera. Omicroñ'R

The main strengths of Neo-Darwinian theory are related to three basic principles: causality, efficacy, and scope.

Causality establishes that the mechanism of natural selection is sufficient to drive the evolutionary process and the observed trends, that is, that natural selection is the main engine for speciation.

Efficacy refers to the ability of organisms to generate "evolutionary novelties" and eliminate poorly adapted individuals in populations, something like "survival of the fittest."

The scope has to do with the ability of the mechanism to explain microevolutionary and macroevolutionary processes.

Weaknesses

According to Frías (2010), the weaknesses of the synthetic theory of evolution have to do with some omissions that this theory makes to some processes or events that are often listed as “exceptions to the rule”.

Among the main omissions highlighted by this author are:

- The absence of a link between somatic and germ cells (sexual) in some invertebrate phyla, the inheritance of somaclonal variation and the conception of vertical transmission of genes

- Lateral or horizontal gene transfer to eukaryotes mediated by bacteria or viruses

- The lack of a "holistic" concept of the gene, determinism and genetic reductionism

- Non-coding DNA, epigenesis and genes that are not transcribed

- Homeotic mutations and the genesis of development

- Sympatric speciation.

References

1. Frías, L. (2010). Omissions in the synthetic theory of evolution. Biological research, 43 (3), 299-306.
2. Gardner, JE, Simmons, JE, & Snustad, DP (1991). Principal of Genetic. 8 '"Edition. John Wiley and Sons.
3. Gould, SJ (1982). Darwinism and the expansion of evolutionary theory. Science, 216 (4544), 380-387.
4. Henderson, M. (2009). 50 genetics ideas you really need to know. Quercus Books.
5. Kutschera, U., & Niklas, KJ (2004). The modern theory of biological evolution: an expanded synthesis. Naturwissenschaften, 91 (6), 255-276.
6. Matsuda, H., & Ishii, K. (2001). A synthetic theory of molecular evolution. Genes & genetic systems, 76 (3), 149-158.
7. Salisbury, FB (1971). Doubts about the modern synthetic theory of evolution. The American Biology Teacher, 33 (6), 335-354.
8. Solomon, EP, Berg, LR, & Martin, DW (2011). Biology (9th edn). Brooks / Cole, Cengage Learning: USA.
9. Suzuki, DT, & Griffiths, AJ (1976). An introduction to genetic analysis. WH Freeman and Company.
10. Watson, JD (2004). Molecular biology of the gene. Pearson Education India.