The modern synthetic theory of evolution defines the evolution in terms of genetic variations in a population which leads to the formation of new species. Darwin’s book, The Origin of Species by Means of Natural Selection, has found a great impact on evolutionary theories that favours the idea that all the living organisms are descended from a common ancestor. Darwin proposed the theory of natural selection as a mechanism of evolution. After this neo-Darwinian theory of evolution came into light which described the importance of mutations and variations in a population, as a driving force of evolution. This perspective was obeyed for many years. The modern synthesis was prominent in the early 20th-century, synthesis conforming Charles Darwin’s theory of evolution and Gregor Mendel’s ideas on heredity was a linked mathematical substructure. Julian Huxley was the first to coin the term in the year 1942 in his book, Evolution: The Modern Synthesis.
Modern Synthetic Theory
The Modern Synthesis resulted in several changes to how evolution and other evolutionary processes were devised. It gave a new definition of evolution as “changes in allele frequencies within various populations”, thus emphasizing more on the genetic basis of evolution. (Alleles are mainly alternate forms of the similar kinds of gene, that is characterized via differences in DNA sequence that leads to the construction of proteins which differ in amino acid contents.) Four types of forces of evolution were determined as contributing to the changes in allele frequencies. They are as follows random genetic drift, gene flow, mutation pressure, and natural selection . Out of these natural selection is the one via which the best-adapted organisms possess the highest survival rate, and is also the only evolutionary force that makes organisms better adapted to their environments. Genetic drift helps in determining random changes in allele frequencies in a given population. It is generally powerful in small populations. Gene flow defines allele frequency as changes due to the immigration and emigration of various individuals from a population. Though mutation is a weak evolutionary force but is important because all genetic variation usually arises from mutation, variation in the DNA sequences that results due to specific errors that occur during replication or some other factors. The Modern Synthesis recognises that the majority of mutations are deleterious (and possess harmful effects), and mutations that are advantageous mainly have a small phenotypic effect.
Advantageous mutations are incorporated into the population via the process of natural selection. Changes in species therefore occur gradually via accumulation of small changes. The large differences that can be seen between species include gradual changes in an extensive time period. Speciation (i.e. formation of new species) have resulted from the evolution of reproductive isolation, mainly during a period of allopatry , where two populations are isolated from each other.
Factors of Modern Synthetic Theory of Evolution
The five major factors of modern synthetic of evolution are discussed below:
Gene Mutation
This refers to a change or mutation in a specific location or a specific point or a nucleotide is known as a point mutation. It can be mainly due to the addition or deletion of a nitrogenous base, or it can be a substitution of the nitrogenous base, or may be an addition or deletion of the nitrogenous base results in a frameshift mutation. Mutation creates several numbers of changes that may be harmful. Different mutant forms of genes are falling under the normal genes in a homozygous condition. These changes are the main reason for variations in an offspring. Mutations possess a variety of effects. They may or may not be harmful. Although it’s very difficult and complicated, the changes in DNA arrangement may even turn out to be useful to the organism.
Genetic Variation or Recombination
Genetic Recombination can be seen when newly formed genotypes are derived from the existing genes. The gene combinations possessing similar individuals having two types of alleles, and mixing of the chromosomes during sexual reproduction of two parents results in new individuals. Crossing over is the result of exchange of chromosomal pairs of alleles during meiosis. Crossing over results in the formation of new forms of gene combinations. Chromosomal mutations such as inversion, deletion, duplication, translocation or polyploidy all are the result of recombination of genes. Natural selection is usually the most important mechanism that causes evolution to happen. However, natural selection only chooses among the already existing variations in a population. It never yields new genetic variations but is also responsible for making new genetic combinations that were not found in earlier generations.
Natural Selection
This includes forces that have both physical and biotic factors and also explains how and in what direction an organism has to change. Natural selection possesses no favouritism. But it is obvious that the organisms that are better suited for environmental conditions survive over the power in the force of competition. Due to this better survivors remain in nature. This method of natural selection forms an adaptive relation among the environment and the population over several combinations of genes. In the year 1859, Charles Darwin gave his philosophy of evolution via natural selection as an explanation of adaptation and speciation. He clearly explained natural selection as the “principle by which even the smallest variation of trait, if useful, is conserved”. This idea though was simple but proved powerful: persons that are better adapted to their surroundings are more likely to survive and reproduce. Till there are some differences between them and that, these differences are heritable, there will be a particular selection of individuals with the most useful variations. If the differences are found to be genetic, then a degree of different reproductive achievement leads to a progressive evolution of certain specific populations of a species, and populations that can grow to be sufficiently different finally become different species.
Genetic Drift
Within a small interbreeding population, heterozygous gene pairs tend to become homozygous. Because of this, many non-functional characters are expressed and such organisms are excluded. These genetic drifts are mainly not theoretical. They are operated in a small population of Islands. Genetic drift thus provides a better way to determine the line of evolution. Genetic drift occurs mainly due to chance events that are sudden and random.
Isolation
Isolation is mainly one of the most crucial factors that results in the synthetic theory of evolution. It helps in preventing interbreeding among related organisms that represents a reproductive form of isolation. Isolating mechanism refers to a barrier that prevents genetic flow or exchange of genes among the isolating populations. These isolating mechanisms are mainly categorised into two types: geographical isolation and reproductive isolation.
Conclusion
From the topic which we discussed in this article, we came to know more about the modern concept of evolution, that is the synthesis of Darwin’s and Hugo de Vries’ Theories, and so it is known as Synthetic theory. Synthetic theory of evolution is the most widely accepted theory of evolution. According to this theory, variations present at the gene level can only be inherited to the next generation and only these genes are responsible for evolution. The main causes of gene variability are mutations, recombination, hybridization and genetic drift. Some of the members of the population are able to survive because of their genotypes and phenotypes and produce more offspring.
Those offspring contribute a greater percentage of genes to the next generation and could change the gene frequency of a population. Reproductive Isolation is responsible for bringing about this type of evolution. Thus, we can conclude that this theory is more about evolution which is solely responsible for bringing genetic variability, natural selection and reproductive isolation.