Modern Synthetic Theory of Evolution

The Modern Synthetic Theory of Evolution (also called Modern Synthesis) merges the concept of Darwinian evolution with Mendelian genetics, resulting in a unified theory of evolution. This theory is also referred to as the Neo-Darwinian theory and was introduced by a number of evolutionary biologists such as T. Dobzhansky, J.B.S. Haldane, R.A. Fisher, Sewall Wright, G.L. Stebbins, Ernst Mayr.

It describes the evolution of life in terms of genetic changes occurring in the population that leads to the formation of new species. It also describes the genetic population or Mendelian population, gene pool and gene frequency. The major concepts coming under this theory include genetic variations, reproductive and geographical isolation and natural selection.

The Modern Synthetic Theory of Evolution showed a number of changes as to how the evolution and the process of evolution are conceived. The theory gave a new definition of evolution as “the changes occurring in the allele frequencies within the populations, ” which emphasizes the genetic basis of evolution.

Factors of Modern Synthetic Theory of Evolution

The factors that contribute to the change in allele frequency of a population are as follows:

Genetic Recombination

Recombination is a process where new combinations of alleles are formed. The genetic recombination occurs during sexual reproduc­tion at the time of gamete formation. There occurs an exchange of genetic material between non-sister chromatids during meiosis that is called crossing over. It leads to recombination and is one of the causes of genetic variability present within a population.

Mutation

Mutations are the sudden inheritable changes that occur in the gene and have a certain phenotypic effect. Chromosomal mutations may be due to change in the genes or chromosome structure or number, e.g. deletion, inversion, duplication, translocation, aneuploidy, polyploidy, etc. Mutation produces a variety of changes that may be harmful. Many of the mutant forms of genes are recessive and are expressed only in the homozygous condition. Advantageous mutations may be selected by natural selection and gradual small changes get accumulated over time. These mutations cause variation in a population.

Genetic Drift and Gene Flow

Any change in the gene/allele frequency of a population due to sudden, random changes, is referred to as genetic drift. It occurs due to chance events. Genetic drift is more prominent in a small population. Gene flow is due to the immigration or emigration of individuals from one population to another. If the migration occurs multiple times it leads to gene flow and changes the allele/gene frequency of the populations.

Natural selection

Organisms that are better adapted to the environment are selected by nature. Natural selection produces a change in the frequency of the genes from one generation to the other favouring the differential form of reproduction.

Isolation

It is one of the significant factors responsible for the synthetic theory of evolution. The isolation helps in preventing the interbreeding of related organisms which is a reproductive form of isolation.

In addition to these factors, other factors such as hybridization between two species increases the genetic variability of the popu­lation.