Effects of migration and other evolutionary processes on allele frequency and fitness
Life originated from a common ancestor and due to various mechanisms of evolution, the genotype of organisms has changed. Mutation, migration, genetic drift and selection are natural processes of evolution that affect genetic diversity. Mutations are spontaneous changes in genomic sequences (Robert, et al., 2006); it is one of the processes that influence allele frequency. A mutation can either have a positive, negative or a neutral effect on an organism’s fitness. When organisms of the same species exhibit different phenotypes, the organism is polymorphic for that particular trait. A beneficial mutation that gives rise to polymorphic traits can improve the chance of survival. For example, the grove snail, Cepaea nemoralis, is famous for the rich polymorphism of its shell. A mutation in the locus responsible for colour produces different shell colours, ranging from yellow, pink, white and brown (Ozgo, 2005). Snails with brown shells are found in beechwoods where the soil is dark. Snails with brown shells are able to camouflage with the soil, thus avoiding being detected by predators (Jones, et al, 1977). As a result of avoiding predation, the frequency of alleles that code for brown shells will increase. However, according to the hitchhiking model, fixation of a beneficial mutation will decrease the diversity at linked loci (Chevin, et al., 2008). If a new mutation increases the fitness of members of a particular species, a strong selective sweep on allele frequency will result to very few haplotypes existing in the population. The frequency of alleles that are positively selected and those that are closely linked will increase, but the other alleles will decrease. A mutation can be neutral, having neither a beneficial effect nor a negative effect. However, some mutations are lethal because they have a negative effect on fitness. The accumulation of deleterious mutations and the prevention of recombination reduce the fitness of individuals (Muller's ratchet). Experiment carried out on asexual and sexual yeast strains showed that sexually reproducing parts of the genome improved survival than asexually reproducing parts (Zeyl and Bell, 1997). Asexual strains decreased overtime because of Muller’s ratchet. On the contrary, sexual strains were able to stop the build-up of deleterious mutation due to recombination between chromosomes. Mutation in collagen-I gene is another example of lethal mutation reducing fitness. Collagen is a group of naturally occurring proteins found in animals, it is one of the major components of blood vessels. An experiment carried out on mouse embryonic stem cells showed that mutation in collagen-I gene impairs the function of collagen-I (Lohler, et al., 1984). During the experiment, 13 embryos died because a mutation in mouse collagen-I gene caused the major blood vessels to rupture. According to background selection model, because a deleterious mutation reduces the fitness of individuals, deleterious mutations are selected against (Innan and Stephan, 2003); this will decrease the allele frequency of a population. Genetic drift is a stochastic process that refers to the fluctuations of genotype frequencies (Maynard, 1998); alleles are either fixed or permanently lost from the population. Due to the randomness of the process, genetic drift can eliminate beneficial alleles that could have improved survival. Genetic drift can also eliminate lethal alleles from a population and therefore improve survival rate. Genetic drift has larger effect on small populations than a large population (Maynard, 1998); this is because the rate of allele fixation or elimination is faster in a small population compared to a large population. Moreover, population bottleneck is an evolutionary process that increases the effect of genetic drift; it involves random events that prevent species from reproducing (van-Heerwaarden, et al.,...
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