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Genetic Drift (Sewall Wright Effect)
Random change of gene/allelic frequencies in a population merely by chance is called genetic drift.
It operates rapidly with a small population.
It is due to habitat fragmentation, isolation, natural calamities or any epidemics.
Founder effect and bottleneck effect are two forms of genetic drift.
1.0Founder Effect
Founder effect
When a section of population gets isolated or migrated or drifted from original population, then this section becomes genetically different from the original population due to change in allelic frequency because gene pool of this section may contain some alleles in a very low frequency or may lack a few alleles.
Sometimes the change in allelic frequency is so different in the new sample of population that they become a different species. The original drifted population becomes founders and the effect is called founder effect.
2.0Bottleneck Effect
Bottlenecks are the natural calamities like Earthquakes, volcanic eruptions, floods, storms etc. A sudden change in the environment may drastically reduce the size of a population and now this population may be genetically different from the original population. Certain alleles may have more frequency among the survivors, others may be less, and some may be absent altogether.
If a population that has passed through a bottleneck ultimately recovers in size, it may have low levels of genetic variation for a long period of time and this may produce a new species.
3.0Reproductive Isolation
It is the prevention of inter breeding between the populations of two different or closely related species.
It maintains the characters of the species but can lead to the origin of new species.
4.0Two Main Subtypes of Reproductive Isolation
1. Prezygotic isolation - Prevention of mating and the formation of hybrid zygote.
(i) Ecological isolation: Isolation due to different habitats of two species. For example, one may be living in fresh water and other in the sea.
(ii) Temporal isolation: Due to difference in breeding seasons or flowering times of two species.
(iii) Behavioural isolation: Due to difference in sexual behaviour of two species.
(iv) Mechanical isolation: Due to incompatible external genital organs.
(v) Gametic isolation: The sperms and ova of different species can't fuse due to difference in their surface chemicals.
2. Postzygotic isolation - A hybrid zygote is formed but it may not develop into a viable fertile adult.
(i) Hybrid inviability: Hybrid zygote fails to develop. In plants, embryos arising from interspecific cross are not viable.
(ii) Hybrid sterility: Hybrid adults are sterile and do not produce gametes.
e. g. Mules and hinny
(iii) Hybrid breakdown: Sometimes inter specific mating produces a hybrid, which give rise to next hybrid by back cross but they have reduced vigour or fertility or both.
∙ Exception: Tigon (African lioness x Asian tiger) and Liger (Male lion x Female tiger) hybrids are fertile but these species do not interbreed naturally.