Origin and Evolution of life - part 08 - Mechanism of organic evolution

Mechanism of organic evolution :

One has to give the importance to the population while considering the mechanism of evolution.
It is the population that evolves and not its individual members.
Individual's role is to pass its genetic variation to its offspring.
The following are the basic processes which bring about evolution viz. -

Mutations -

Gene recombination -

These are variation produce due to coming together of alleles during sexual reproduction.
Gene recombinations occur due to -

Gene flow -

It is the transfer of gene during interbreeding of populations that are genetically different.
As explained earlier in this chapter gene flow is due to emigration and imigration. Its brings about changes in the allele frequency.

Genetic drift -

Any alternation in allete frequency in the natural population by chance, is called genetic drift.
Concept of genetic drift was first given Sewall wright, hence, called as Sewall wright effect.
For example, elimination of a particular allele from a population due to events like accidental death prior to mating of an organism.
Genetic drifts are random or directionless.
The effect of genetic drift is more significant in small population than in large population.
Due to genetic drift, some alleles of a population are lost or reduced by chance and some others may be increased.
Some time, a few individuals become isolated from the large population and they produce new population in new geographical area.
The allele frequency of new population become different.
The original drifted population (i.e. colonizing ancestor/ pioneer) becomes `founders' and the effect is called founder effect.
A bottle neck effect is seen when much of a population is killed due to a natural disasterand only a few remaining individuals are left to begin a new population.

Natural selection -

It is a process by which better adapted individuals with useful variations are selected by nature and leave greater or more number of progenies (Differential reproduction).
Type of Natural selection :

1. Stabilizing selection : (Balancing selection)

Here more individuals of a population acquired a mean character value.
It tends to favour the intermediate forms and eliminate both the phenotypic extreme.
For e.g. More number of infants with intermediate weight survive better as compare to those who are over-weight or under-weight.
It reduces variations.
It does not lead to evolutionary change but tend to maintain phenotypic stability within population, therefore, it is described as stabilizing selection.
Genetically stabilizing selection represents a situation where a population is adapted to its environment.

2. Directional selection :

In this type, more individuals acquired value other than the mean character value.
Natural selection acts to eliminate one of the extremes of the phenotypic range and favour the other. e.g. systematic elimination of homozygous recessives.
Directional selection operates for many generations, it results in an evolutionary trend within a population and shifting a peak in one direction.
e.g. Industrial melanism, DDT resitant mosquito etc.

3. Disruptive Natural selection :

Here more number of individuals acquire peripheral character value at both ends of the distribution curve.
Nature select extreme phenotypes and eliminate intermediate. Hence two peaks are formed in distribution of traits.
This kind of selection is rare.
It ensures the effect on the entire genepool of a population, considering all mating types or systems.
Example - It was observed in the different beak size of African seed cracker finches. The birds have different size of beak and they feed on seeds.
The avilable seeds were of two kinds small and large sized seeds.
Large beak sized birds feeds on large seeds while small beak sized birds feed on small seeds and their number was increased.
Intermediate beak sized birds are unable to feed on either type of seeds so their population was decreased gradually and then eliminated by natural selection.

Isolation -

It is separation of a single interbreeding population into subunits.
Isolation restricts gene flow between descrete (non continuous population due to different barriers like geographical barriers.

Speciation -

The sub units of single interbreeding population due to the geographical barriers like river, mountains, desert, sea etc. become isolated in such a way that their interbreeding is prevented.
This will finally lead to origin of new species(i.e. speciation).

Yogy's biology