por 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 heredity theories 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.
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 changes in population allele frequencies, namely: mutation, selection, and genetic drift.
This theory strengthens 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.
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History
To tell the story of the synthetic theory of evolution, it is necessary to make a historical account of the background that took place for this theory to have 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.
In some texts it is indicated that both authors presented a hypothesis known as «descendants with modification by natural selection», through which they affirmed 5 things:
All organisms produce more offspring than the environment they live in can support.
Intraspecific (within the same species) variability of most traits is extremely abundant
Competition for limited resources ends in a “fight for survival”
In nature, the inheritance of modified traits occurs, that is, some modifications can be inherited from parents to their offspring.
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 the fossil record and of living organisms in their natural environments.
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 the inheritance of characters», 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 On the Origin of Species, probably because he did not fully understand them. what the latter was referring to.
neodarwinism
Mendel’s works were «archived» until the early 1900s and became popular ever since. His 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 of 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 time to «reconcile» the inconsistencies between Darwin’s ideas of natural selection and Mendel’s experiments on the inheritance of characters.
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, though some more than others, took part in the synthesis of evolutionary theory that prevails today, but only the most prominent are mentioned here.
Postulates of the synthetic theory
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 considered by Darwin and Wallace, especially those related to the overproduction of offspring, with their variation and with heredity. of the traits.
Thus, the factors involved in this theory are:
– Mutation rates
– Migration processes
– Chance or genetic drift
– Recombination or variation
– Natural selection
Mutation
Mutations are changes that occur in gene sequences and 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
Migration 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 its allelic frequency.
Chance or genetic drift
Chance or genetic drift is a genetic event that modifies the genotypic composition of a population by the random appearance of a rare modification, either by 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 between the chromosomes of the two individuals that reproduce to give rise to a new individual, which is characterized by having a different genetic combination from that of its parents.
Through this process deletions, inversions, duplications, translocations, polyploidies, etc. can occur.
Natural selection
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 Darwin proposed, which means that they are slow, gradual and continuous within each given lineage.
evidence
The anthropological intervention of ecosystems has provided «natural experiments» that serve to demonstrate the neo-Darwinian hypotheses.
The Biston betularia moth, for example, is an abundant arthropod of woodland areas of England, where two color morphs, one light and one dark, have been distinguished. A single gene is involved in the differences between both phenotypes and the dark color allele is known to be dominant.
The allele frequency of the dark form has increased considerably since 1850, especially in the more industrialized areas of Manchester and Birmingham, presumably as a «camouflage» 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
The main strengths of neo-Darwinian theory relate to three basic principles: causality, efficacy, and scope.
Causation 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 driver for speciation.
Fitness refers to the ability of organisms to generate «evolutionary novelty» and eliminate maladaptive individuals in populations, something like «survival of the fittest.»
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 considered as «exceptions to the rule».
Among the main omissions highlighted by this author are:
– The absence of a link between somatic and germ (sex) cells 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
Frias, L. (2010). Omissions in the synthetic theory of evolution. Biological Research, 43(3), 299-306.
Gardner, JE, Simmons, JE, & Snustad, DP (1991). Principal of Genetics. 8 ‘“Edition. John Wiley and Sons.
Gould, SJ (1982). Darwinism and the expansion of evolutionary theory. Science, 216(4544), 380-387.
Henderson, M. (2009). 50 genetics ideas you really need to know. Quercus Books.
Kutschera, U., & Niklas, KJ (2004). The modern theory of biological evolution: an expanded synthesis. Naturwissenschaften, 91(6), 255-276.
Matsuda, H., & Ishii, K. (2001). A synthetic theory of molecular evolution. Genes & genetic systems, 76(3), 149-158.
Salisbury, FB (1971). Doubts about the modern synthetic theory of evolution. The American Biology Teacher, 33(6), 335-354.
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