Genetic drift is the process of changes of allele frequencies over generations in a random fashion. Genetic bottlenecks and founder effects are the most common effects of genetic drift. The rate of loss of alleles in a genetic drift is ½ N where N is the number of individuals in the initial population. The strength of genetic drift is proportional to ½ N.
In an idealized population, under the ideal rates of loss of alleles, it will take four generations on average to coalesce the alleles.

The founder effect and bottleneck effects made the ecologists to redefine the concept of population size. Effective population size is the estimate of the average number of individuals of a population contributing to the next generation and is denoted by Ne. It is a measurement of the strength of genetic drift. It represents the effect of influence of the genetic drift on the feature of interest in the population under actual and ideal conditions of drift across the same number of generations.

Isolated demes of a single population may become more diverse in their genetic features due to this drift.

Genetic bottleneck

The allele frequency is changed due to sudden decrease in populations. These bottlenecks are the result of habitat fragmentation and/or overexploitation of the species. The resultant drastic loss of genetic diversity affects the survival capacity of the species especially over the next generations.

Population bottlenecks are defined when at least one generation faces a sudden decrease in allele frequency. These genetic bottlenecks can have a drastic effect if the genetic drift occurs in smaller populations.

The effect of genetic drift is inversely proportional to the size of the population. In small populations, most of the individuals tend to be homozygous for deleterious genes. The situation is similar to the effects of inbreeding. Hence the endangered species which are few in number are at higher risks of genetic bottlenecks and conservationists employ this as a valuable indicator for selection of measures in conservation of species.

Examples of species which have encountered severe genetic bottlenecks in the past include northern elephant seals, cheetah etc. Our own species the Homo sapiens and the ancient humans have been found to be affected by genetic bottlenecks.

The genetic drift is rather rapid to cause sudden fluctuations in genetic diversity so as to make the population less fit. When the genetic variations are greatly reduced, the population is less able to adapt to the pressures of natural selection such as extreme climate changes, habitat loss, predation, competition etc. Sometimes the process results in the loss of features on which the selection acts upon making the population vulnerable to extinction. If the genetic drift happens to exclude such target features of natural selection, then the probability of reviving the population is higher.

Effective population size (Ne) after a the sudden changes in population size can be estimated by

Ne = t / ∑ (1/ Ni)

Ni = population size of the ith generation and t is the number of generations

These genetic bottlenecks can be due to natural causes or due to human intervention. In the case of elephant seals, the bottleneck was the result of excessive hunting by humans. Once there has been a widespread awareness and constant efforts to conserve species, the population rebounded to higher figures, but the loss of alleles is irreparable and the loss of genetic diversity is evident.

Founder effects

Founder effect is closely linked with the genetic drift. In this case, a few individuals of the original population start a new colony isolated from the other members of the species. This results in loss of genetic variation since the probability of all the alleles of the individuals of the colony being passed on to the next generation is very low. Chances are also that the sample colony is not a random one and does not represent the entire population.

The founder effect is easier to recognize with the presence of genetic diseases in the subsequent generations. For example, the Huntington's disease was found to be the effect of one such genetic drift. For the founder effect to be evident, the new colony thus formed must be reproductively isolated from the original one.

Founder effect is prominent in cases of migration and closed communities where the barriers of reproduction are stricter due to social or geographical isolation.

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