The theory of genetic drift was developed by a geneticist SE WALL WRIGHT in 1930. It is also known as Sewall Wright effect or ‘scattering of variability’. It denotes that the random fluctuations in the gene frequencies In a small population from generation to generation.
In demes of limited sizes, random genetic drifts arise by chance. These cannot arise in large population. For example if we compare two populations of two extreme sizes - Population ‘A’ consisting of 5,000 breeding individuals and population ‘B’ of only 50, the gene pool of each contains equal number of Land I. If their gene frequencies are represented by p & q
In the next generation, the gene frequency is expected to deviate from the original 0. 5 by an amount of equal to the ‘standard error’
Effects of Genetic Drift on Gene frequency
i) Homozygocity: In small populations, due to genetic drift gene frequencies continue to fluctuate until one of the allele lost and other fixed. This leads homozygosity in small populations. It means the genetic drift reduces genetic variability by eliminating one of the two alleles ‘either new or old one.
iii) Genetic divergence: The demes become progressively genetically different. In each sub population, the genes fixed and lost will be different. Thus, In due course of time, (each deme gradually diversifies from the other sister demes) lead to the establishment of new species.
Founder Effect: Whenever a few organisms from large population encroaches a new or isolated geographical region, these form the “founders or ‘founder members’. The founders carry only a limited portion of the parental gene pool. The descendants of the founder i.e. the founder population or marginal isolates in a new area will tend to have ratios similar to the founders. The resemblance of the descendants of the founders is called founders effect’ or ‘founder principle’ (Maw).
The diffusion of genes into populations through migrations and interbreeding is known as Gene flow. The gene flow links all the demes of a population. It tends to counteract the loss of variability due to genetic drift in small population.
- Completely revised new edition of an established textbook.
- Features new chapters and examples from exciting new research in genomics, including the human genome project.
- Excellent new co-author in Richard Twyman, also co-author of the new edition of hugely popular Principles of Gene Manipulation.
- Accompanying web-page to help students deal with this difficult topic at www.blackwellpublishing.com/primrose
Author: Heather Buschman, PhD