Biology Today — January 2018

Table : Differences between Hugo de Vries’ mutation and Darwinian variation

Hugo de Vries’ mutation Darwinian variation

(i) Mutations appear all of a sudden. Darwinian variations are gradual.

(ii) Mutations are the raw material of evolution. Continuous variations are the basis of evolution.

(iii) Mutations are due to change in genetic makeup. Genes were not known to Darwin.

MODERN CONCEPT OF EVOLUTION

• The modern concept of evolution is a modified form of Darwin’s theory of natural selection and Hugo de Vries’ theories.
  It is also called synthetic theory of evolution.


• It is the most accepted theory of evolution in modern times. Many scientists like T. Dobzhansky, R.A. Fisher, J.B.S. Haldane,
  Swell Wright, Ernst Mayr have contributed to the modern theory of evolution but the final shape of ‘Modern Synthetic theory
  of Evolution’ was given by Stebbins.


• The modern synthetic theory of evolution includes the following factors:

Genetic Variations in Population

• Changes in genes occur in following ways:

(i) Changes in chromosome number (increases in number of chromosome set) and structure (change in the morphology of

chromosome), due to duplication, inversion, deletion or translocation.

(ii) Change in structure and expression of gene by mutations and mutated genes add new alleles to the gene pool.

(iii) Gene recombination which occur due to independent assortment of chromosomes, crossing over, random fusion of gametes, etc.

(iv) Gene migration (gene flow) is the movement of individuals from one place to another, which add new alleles to the local gene pool.

(v) Genetic Drift or Sewall Wright Effect is the drastic change in allele frequency when population size becomes very small and

it alters gene frequency of remaining population. Examples of genetic drift are:

(a) Founder effect : Small group of persons leave the population and find new settlement. Their genotypic frequency becomes

different from parent population.

(b) Bottleneck effect : Cyclic phenomenon of decrease and increase of a size of population.

• Non-random mating : Repeating mating between individuals for certain selected traits changes the gene frequency. For
  example, the selection of more brightly coloured male by a female bird will increase the gene frequency of bright colour in
  the next generation.


• Hybridisation : It is the crossing of genetically different organisms, usually in one or more traits. It helps in intermingling
  of genes of different groups of same variety, species, etc.


• All of these factors result in genetic variations in a population by sexual reproduction.

Fig.: Different Types of Natural Selection

Phenotypes

favoured

by natural

selection

Number of Individuals

with phenotype

Medium sized

individuals are

favoured

Stabilising selection (Balancing

selection) favours average sized individuals

while eliminates small sized individuals. It

maintains the mean value from generation

to generation.

Mean

Conserved by

selection

Eliminated

Directional selection (Progressive

selection) favours small or large-

sized individuals and the population

changes towards one particular

direction.

Mean

Eliminated

Disruptive selection (Diversifying selection) favours

both small-sized and large-sized individuals. It eliminates

most of the members with mean expression, so produces

two peaks in the distribution of the trait that may lead to

the development of two different populations.

Two peaks form

Eliminated

Conserved