Evolution commonly assumes that organisms evolve randomly in a single direction. Some of the natural processes are random for example, the mutations. However natural selection is not random and tries to preserve the variations introduced in the organisms in a random fashion.
Populations have long been thought to be of infinite size. This is good for statistical analyses. However, in practical they are finite and limited in their size. When the fertility and fecundity rates are similar among the individuals of a population, the allele frequencies of the gametes equal that of the fertile adults in the population.
The population size is finite or limited. Hence only a small proportion of the gametes finds a role in fertilization and hence gets a representation in the next generation. The random sampling is affected though chance and therefore the allelic frequencies of the gametes and zygotes differ. The maintenance of allele frequencies in populations over generations requires infinite population size. Changes in allele frequency due to random sampling are exhibited in populations due to its finite nature. This is the cause of genetic drift.
Random genetic drift results in major allele frequency differences among the subpopulations and result in genetic differentiation. The allele frequencies change over the subpopulations; however the average value for the total population remains fairly constant. Genetic drift acts only on alleles with the same fitness. Fitness is a measure of the relative ability to reproduce for a combination of a phenotype and genotype.
After random genetic drift over numerous generations, the subpopulations may get fixed for one of the alleles. The probability of such fixation is equivalent to the allele frequency of the original population. The process by which alleles get lost from the population is called allelic extinction. Eventually only one allele becomes fixed in the entire population. Some genes get lost and some others get on to next generation.
Suppose there are two alleles at the same locus 'A' and 'a'. Let the allele frequencies of both be represented as p and q. the probability that 'a' will get extinct is p and the probability of allelic extinction is q. The divergence from the initial frequencies varies over time. For a population of size N, the average time to get one of the alleles fixed is equal to 4 N generations.
When the genetic drift is analyzed with a large number of populations, the total heterozygosity will decrease over time. Genetic drift affects the Hardy Weinberg equilibrium also. The effect is greater for small populations. The individuals who successfully transfer the genes to the next generation constitute the effective population. Genetic drift is faster with smaller effective population sizes.
Effective population size is denoted by Ne and depends on various factors such as number of fertile individuals in a population, variation in the number of progeny, sex ratio and natural selection. Ne gets reduced by population fluctuations and inbreeding mechanism.
Genetic drift can be reduced or retarded due to other external factors such as mutation, migration, heterozygote superiority etc which help to reintroduce the lost alleles. If there is no external factors affecting the genetic drift it will ultimately result in the extinction of most of the parental alleles. These factors contribute to polymorphism.
Sometimes these random fluctuations create a sudden temporary decrease in the effective population size which is termed as a 'population bottleneck'. The loss of genetic variations due to such bottleneck is called 'founder effect'. Genetic diversity is lost easily when either the number of founders or the population growth rate is low. Mutations help to restore the genetic variation in such populations.
Genetic drift on the average remains constant over the population since the size is finite. Mathematical models can be evolved to predict the basis and pattern of genetic drift among the subpopulations. The pattern follows a binomial distribution in most populations and hence is predictable. The variations can be sometimes advantageous though in effect they are neutral.
Evolution involves the progress of a species in the extent to which it gets genetically adapted to the physical environment. This means the increase in frequency of alleles which are favorable. Random genetic drift is one of the processes contributing to such variations in allele frequency and hence an important driving force of evolution.
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