Modern Synthetic Theory of Evolution: Definition and Darwin’s Theory

Modern Synthetic Theory of Evolution: What animals did humans evolve from? Do you know how life evolved from a simple single cell to a complex multicellular organism? Many scientists came forward and gave their thoughts and formulated a theory for evolution, but the most accepted theory is the Modern synthetic theory of evolution. This theory has been formulated by considering and discarding different concepts of different scientists regarding evolution. This theory mainly accounts for the changes occurring when the allele frequency of the population changes.

What is Modern Synthetic Theory of Evolution?

This theory is a modified form of Darwin’s theory of Natural Selection, so it is also called Neo-Darwinism. This modern theory states that the variations which happen at the gene level are only inherited (mutation) and accountable for evolution and not all the variation as held by Darwin. The modern concept of evolution is the synthesis of Darwin’s and Hugo de Vries’ Theories, so it is called Synthetic theory.

The synthetic theory is the result of the work of many scientists, i.e., T.Dobzhansky, R.A Fisher, J.B.S Haldane, Swell Wright, Ernst Mayr and G.L. Stebbins.

Fig: Evolution of Man

Modern Synthetic Theory of Evolution – Description

Modern theory or Synthetic theory is also called Neo-Darwinism theory. Neo-Darwinism theory is the synthesis of Darwin’s theory and modern genetics. This theory has the following postulates:-

1. Gene variability
2. Natural Selection
3. Reproductive Isolation

In addition, some other accessory factors affect the working of these three basic factors; Migration of organisms from one population to another as well as hybridization between races or closely related species; both of these factors increase the amount of genetic variability available to a population.

1. Gene Variability

This postulate says that in a population, there is gene variability. That means in any given population, there are variations that exist because of certain genetic changes which are present in that population.

The causes of gene variability are:

i. Mutation

These are sudden heritable changes which occur due to permanent change in their genotypes. Mutations can be chromosomal mutations or gene mutations.
a. Chromosomal mutation– It refers to the changes in chromosome number or its structure. So chromosomal mutation can categorized as-
I. Morphological aberrations– When changes occur in the morphology of chromosomes, then it is called chromosomal aberrations. It is caused due to addition, deletion, translocation and inversion.
II. Numeric mutation– These mutations are caused by the change in chromosome number. These are of two types: Aneuploidy and Euploidy.

Aneuploidy: It is the condition when there are fewer or more chromosomes than normal. E.g.,
1. Monosomy– It refers to the absence of one chromosome from a pair of chromosomes \(\left( {2\,{\rm{N}} – 1} \right).\) E.g. Turner’s Syndrome \(\left( {44 + {\rm{X}}} \right).\)
2. Nullisomy– It refers to the absence of a pair of homologous chromosomes \(\left( {2\,{\rm{N}} – 2} \right).\)
3. Trisomy– It refers to the condition when an extra chromosome is present. E.g., Down’s Syndrome \(\left( {2\,{\rm{N}} + 1} \right).\)
4. Tetrasomy– It refers to the condition when there is the addition of two chromosomes \(\left( {2\,{\rm{N}} + 2} \right).\)

Euploidy: Chromosome number changes by a set. If one set of chromosomes is changed, then it is called haploid; if it is changed by two sets of chromosomes, then called diploid, likewise triploid, polyploid, etc.

b. Gene Mutation- It refers to the changeor mutation in a particular location or a particular point or a nucleotide is called a point mutation. It can be due to the addition or deletion of a nitrogenous base, or it could be a substitution of the nitrogenous base—addition or deletion of nitrogenous base results in a frameshift mutation.

ii. Recombination

This takes place during crossing over during meiotic cell division. The two homologous chromosomes undergo vertical splitting forming sister chromatids. The non-sister chromatids cross-over and exchange their genetic material is called recombination. This is also responsible for gene variability.

iii. Hybridization–

Two species are crossed, and the offspring which is obtained is called hybrid, and this hybrid shows variability.

iv. Genetic drift

Elimination of genes or alleles of certain traits from a population due to by chance events like disease, some natural calamities, etc. It dramatically alters the gene frequency of the remaining population. This is called genetic drift. It is also known as allelic drift or Sewall Wright Effect. It is always seen in small populations.

Fig: Genetic Drift due to a Chance Event

v. Heredity

Characters are transferred from parents to their offspring, also contributing to evolution.

2. Natural Selection

1. The organism which is selected by nature is stronger or better adapted to its environment is selected by nature, and the rest are eliminated from nature is called natural selection. This was emphasized by Darwin.
2. But actually, Natural Selection means differential reproduction. Some members of a population have genes that enable them to grow and reproduce at a faster rate.  When such members reproduce, they give more offspring. This will contribute a greater percentage of genes to the next generation.
3. If differential reproduction continues for many generations, genes of the individuals which produce more offspring will become predominant in the gene pool of the population. This leads to a change in the gene frequency of a population.
4. Some phenotypes within a population are better adapted to some environmental changes and hence are more likely to survive and reproduce than other phenotypes. A common example is an Industrial melanism, a phenomenon depicted by peppered moths.

Fig: Industrial Melanism

3. Reproductive Isolation

Reproductive isolation is the prevention of interbreeding between the populations of two different species. It maintains the character of a species but can lead to the origin of a new species. This can be achieved by-

1. Geographical isolation – It refers to the separation of groups of related organisms by physical barriers, such as sea, mountain, desert, river, etc.
2. Temporal Isolation – It is the failure of two species to interbreed because they breed at different times of the year.
3. Behavioural Isolation – It prevents interbreeding of two groups due to differences in courtship and mating behaviour.
4. Mechanical Isolation – It is the difference in size or structure of genitalia between species that makes interbreeding difficult or impossible.

Fig: Factors of Evolution According to Modern Synthetic Theory

Summary

From the above discussion, we came to know about the modern concept of evolution, which is the synthesis of Darwin’s and Hugo de Vries’ Theories, and so it is called Synthetic theory. Synthetic theory of evolution is the most accepted theory of evolution. According to it, variations at the gene level are only inherited to the next generation and accountable for evolution. Causes of gene variability are mutations, recombination, hybridization and genetic drift. Some members of the population are able to survive due to their genotypes and phenotypes and give more offspring.

Those offsprings will contribute a greater percentage of genes to the next generation and can change the gene frequency of a population. This evolution can also be brought about by reproductive isolation. So, we can conclude that this theory says about evolution which is due to gene variability, Natural Selection and Reproductive Isolation.

FAQs

Q.1. What are the three main concepts of modern synthetic theory?
Ans: a. Gene variability
b. Natural Selection
c. Reproductive Isolation

Q.2. Who developed the modern theory of evolution?
Ans: T.Dobzhansky, R.A Fisher, J.B.S Haldane, Swell Wright, Ernst Mayr and G.L. Stebbins are the scientists who gave the Synthetic theory of evolution.

Q.3. What is the modern concept of evolution?
Ans: Modern theory states that the variations which happen at the gene level are only inherited (mutation) and accountable for evolution and not all the variation as held by Darwin. The modern concept of evolution is the synthesis of Darwin’s and Hugo de Vries’ Theories, so it is called Synthetic theory.

Q.4. What is neo-Darwinism theory?
Ans: Modern Synthetic Theory of Evolution is also called neo-Darwinism as this concept of evolution is a modified form of Darwin’s theory of natural selection and often called neo-Darwinism.

Q.5. What are the main causes of evolution?
Ans: a. Mutations
b. Recombination
c. Hybridization
d. Genetic drift
e. Natural Selection
f. Reproductive Isolation

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