Evolution in a broader aspect can be defined as encompassing changes over time. In the context of evolution, existence of genetic variation becomes immensely important as they form the substrate for evolution. The key forces that shape the pattern of genetic variation are Mutation, Recombination, Genetic Drift, Natural Selection, Assortative mating and Migration. This section presents a brief overview of each of the aforementioned evolutionary forces.
Mutation
Mutation is the random change in the actual genetic code, including changes in single DNA bases, insertion or deletion of DNA sequences, and other rearrangements of DNA sequences(John, 2012).Differing by the number of bases affected, mechanism of mutation, and regions of localization these variants exist as single nucleotide polymorphisms (SNPs), insertion and deletion (InDel), short tandem repeats (STRs), variable number tandem repeats (VNTRs), copy number variations (CNVs), inversion and translocations. Mutation is the primary source of variation and has a significant impact in the process of evolution. The mutants that are found in less than 1% of a population are called variations. A polymorphic locus is defined by the presence of the most common allele in equal to or less than 99% of the chromosomes (Nei, 1987).
Genetic Drift
Genetic drift is a stochastic process in which a subset of a population undergoes mating that result in allele frequency variation in the next generation. At the level of an entire population, this means that each generation may not have the exact same set of allele frequencies as the previous generation (Relethford, 2012). Drift increases with the reduction in the size of the population and causes changes in allele and genotype frequencies over time. Either of the alleles is eliminated or is fixed as equilibrium is reached, but it is difficult to predict the identity of this allele due to the stochastic nature of the process. Wright (1931) proposed a model to explain the sampling in populations that leads to drift. The model assumes a finite population with a constant size and non-overlapping generations where all individuals are equally fit. Effective population size (Ne) is the number of individuals of an idealized population breeding among themselves and will have the same amount of variation in allele frequencies as the latter under drift. Ne can be used for the comparison of genetic drift among different populations. It is also different for different parts of the genome. Two population events that occur due to genetic drift in populations that had a small size in the past are population bottlenecks (reduction in size and diversity of a single previously larger population) and founder effect (genetic separation and subsequent colonization of a subset of the total diversity of a source population).
Natural selection
Natural selection is Charles Darwin’s contribution to evolutionary theory, referring to differential survival and reproduction. It occurs when there is a difference in fitness (the probability of surviving and reproducing) for different genotypes, such that there is a change in allele frequency overtime. The change in color of peppered moths in England is an example of natural selection. Natural selection acts upon phenotypic variation to bring about the difference in reproductive ability of individuals with different genotypes. The rate of evolution by natural selection is proportional to the genetic variation in fitness (Fisher, 1930). The viability, mate selection, fertility and fecundity together constitute the fitness of a genotype and its relative fitness in comparison to other genotypes is given by a selection coefficient, s. When a mutation leads to a less fit genotype, it is subjected to negative or purifying selection while one which increases fitness undergoes positive or diversifying selection. When a mutant allele occurs in a diploid locus and increases the fitness of the heterozygote over homozygotes, the homozygotes are subjected to different degrees of selective pressures due to difference in individual fitness. Over-dominant selection is seen resulting in a balanced polymorphism. Frequency dependent selection also leads to a balanced polymorphism where low frequency allele has more fitness or high frequency allele has less fitness. Detection of positive selection signals has been done by studying candidate genes with a known function and genome wide scanning approaches (Kreitman, 2000). The signals of selection are differentiated from those of demographic events by their occurrence on selected regions of the genome in comparison to demographic events that affect the entire genome (Harris and Meyer, 2006).
Gene flow or migration
Migration is the inter-region movement of people and causes gene flow when migrants contribute to the next generation in the new location. Gene flow balances out genetic differentiation and lessens allele frequency differences between populations. Several population geneticists have proposed models to explain change in gene frequencies caused by gene flow mediated by migration. The “Island Model” proposed by Wright (1931) is the simplest model that shows a meta-population divides into smaller population islands of equal size which exchange genes at the same rate per generation. The “Stepping Stone Model” (Kimura and Weiss, 1964) added that genetic exchange is higher in geographically proximate populations. Another model that has gained prominence is the “Isolation by Distance” model which was based on the fact that mating choice is limited by distance and showed that increasing geographical distance decreases gene flow.
Recombination
Genetic recombination is the exchange of segments of homologous chromosomes during meiosis. It leads to an increase in genetic diversity due to formation of new allelic combinations. Alleles in closely spaced loci do not undergo random segregation during meiosis as a result of infrequent recombination. This is why evolutionary force acting on one locus might affect adjoining loci. Two genomic events have been observed in relation to this phenomenon (i) hitchhiking results when positive selection at a locus leads to rise in frequency of another allele and (ii) selective sweep occurs due to reduced genetic diversity at loci linked to a recently fixed allele. Specific regions in different chromosomes called recombination hotspots have been reported to have higher rates of recombination. This shows a nonuniform distribution of recombination rates over the entire genome.
Non-random mating or Assortative mating
Individuals in populations do not always follow random mating patterns. Assortative mating occurs because of more or less frequent mating among specific individuals than expected in a random mating population. The genetic contribution to variance in polygenic traits is increased due to Assortative mating (Wright, 1921). Preferred mating partners can have more similar (positive Assortative mating) or dissimilar (negative Assortative mating) phenotypes than expected in random mating. Phenotypically similar individuals might have similar genotypes which are observed in consanguineous mating resulting in inbreeding. Assortative mating is however not the same as inbreeding as the former is among phenotypically similar individuals, while inbreeding occurs between individuals with similar genotypes.