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. 2008). Population bottleneck decreases allele frequency and it reduces a population’s ability to adapt to new environmental pressures. For example, the current cheetah populations have low genetic diversity caused by a demographic bottleneck that occurred 10,000 years ago (Charruau, et al. , 2011). The surviving cheetah populations are not representative of the original cheetah population because they have less variation (founder effect). Due to low genetic diversity and less adaptation skills, the modern cheetah population is close to extinction. Natural selection is another evolutionary process that changes allele frequency.
Organisms with advantageous alleles survive and reproduce, increasing the frequency of the advantageous alleles. Individuals with disadvantageous alleles do not survive or reproduce and therefore the frequency of the disadvantageous alleles is reduced or eliminated from the population (William and Michael, 2003). Biston betularia (peppered moths) is a common example used to demonstrated natural selection (Saccheri, et al. , 2008). Before the industrial revolution, non-melanic peppered moths avoided predators by camouflaging with lichen-covered trees.
Their ability to camouflage improved the rate of survival which increased the frequency of non-melanic alleles. Melanic peppered moths were not able to camouflage with the lichen trees, as a result, melanic moths were detected and predated by the song thrushes. This decreased the frequency of alleles that gave rise to melanic peppered moths. However, during the industrial revolution period, symbiotic lichens living on trees were killed because smog and soot were released when coal and other materials were burnt.
As a consequence of the tree trunks becoming more visible, non-melanic peppered moths were more susceptible to predation because they were unable to camouflage with the trees. The ability to camouflage helped melanic moths to survive and reproduce, changing the population allele frequency from mostly non-melanic alleles to mostly melanic alleles (Saccheri, et al. , 2008). Migration of species from one place to another can increase the rate of gene flow. Gene flow is the transfer of gene from one population to another (William and Michael, 2003); it changes the allele frequency of a population.
The effect of migration on the gene pool of a population depends on the rate of migration. Various studies have shown that migration rate is not the same for all species (Tajima, 1990). Species with low migration rate will have less DNA polymorphism and species with high migration rate will have more polymorphic alleles (Tajima, 1990). The benefit of plant migration, which increases the chance of hybridization between plant species, can be demonstrated by examining the adaptation skills of Iris species. Iris nelsonii is a species of hybrid origin, with traces of I. fulva, I. hexagona and I. revicaulis. I. nelsonii picked up characteristics that are not present in the parent population. For example, I. nelsnii can grow in sunny wet conditions whereas the parents can either grow in sunny dry conditions or wet and shady conditions (Taylor, et al, 2011). Given that I. nelsonii can survive in challenging environments, the allele frequency of the advantageous traits will increase. Furthermore, another benefit of gene flow through means of hybridization can be demonstrated by analyzing the genetic variation of Tragopogan species. Hybridization between T. dubious and T. pratensis produces T. iscellus, an allotetraploid that has multiple enzymes needed for various biochemical pathways (Tate, et al. , 2006). Hybridisation enabled T. miscellus and T. pratensis to survive because they were able to exploit the gene pool of both parents. However, migration can also have negative effects on survival. Given that I. nelsonii will exist in niches that parents cannot live in, gene flow between the hybrid and its progenitors will be reduced. If I. nelsonii does not have alleles that can resists infection caused by parasites, an outbreak of a pathogenic disease can wipe out the entire I. nelsonii species.
Although some evolutionary processes eliminate alleles from a population, multiple alleles can be maintained through frequency-dependent balancing selection (Matessi and Schneider, 2009). In negative frequency-dependent selection, the fitness of a phenotype increases as it becomes less common. An example of negative frequency-dependent selection is in the case of Cepaea nemoralis. C. nemoralis are regularly predated by song thrush birds called Turdus philomelos. These birds have a search pattern whereby it persists in targeting the most abundant morph, even if other morphs are available (Bond, 2007).
If snails with yellow shells are common, then these snails will be eaten by song thrushes. As a result, the frequency of alleles that code for yellow shells will decrease. The fitness of other morphs such as pink, white and brown shells will increase because song thrushes would not search for rare coloured morphs. In conclusion, the four fundamental processes of evolution, mutation, genetic drift, natural selection and migration (gene flow), alters allele frequencies in populations. The consequences on survival fluctuate. Occasionally, altering allele frequency gives rise to traits that increases fitness.
However, changing allele frequencies can also give rise to phenotypes that reduce fitness. Word count: 1390 Grade: A- My essay is easy to read and follow. I have given evidences and interpreted them where possible. I also gave examples from animals and plants to show that I have done outside reading. All of the points that were made are relevant as they ultimately answer4 the question e. g. whether the evolutionary processes increase of decrease allele frequency and fitness References Bond, AB, 2007. The evolution of color polymorphism: crypticity searching images, and apostatic selection.
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