Question

1. Compare and contrast the mechanisms of genetic drift. Propose how genetic diversity can be increased in populations and why that is beneficial. Compare and contrast adaptive and nonadaptive mechanisms of evolution.

2. List and describe the conditions under which a theoretical population will be in Hardy-Weinberg equilibrium. Next, explain the relationship between evolution and Hardy-Weinberg equilibrium. Discuss whether the Hardy-Weinberg conditions are likely to occur in natural populations, and explain what that means in terms of evolution. Finally, given the frequency of one allele, calculate the allele and genotype frequencies of a trait controlled by two alleles in a nonevolving population using the Hardy-Weinberg equation.

Answer 1

1. (a)Genetic drift is the change in allelic or genotype frequency of a population because of reduced population size

Different mechanisms of genetic drift are founder effect and genetic bottleneck. Both types lead to a decrease in genetic variation.

Founder effect is seen when a small group of individuals becomes isolated/ separated from their larger populations. This small group is known as founder population. The population which it establishes has a different gene pool from the population it was originated. This variation from the parent population arises because of sampling difference. If a larger proportion of individuals were separated, then it was more likely to resemble the parent population. Example : The frequency of huntington disease gene is very high in afrikaner population in south africa. This is because the founder population of afrikaner happened to carry that gene in unusually high frequency.

On the other hand, Genetic bottleneck is seen, when due to certain evolutionary events a populations gets reduced in size.Some natural disasters can lead to killing of large number of individuals of a population In the resultant population. some genes may have more occurance then others and some other may be totally deleted off the population. This leads to change in genotype frequency. Genetic bottleneck is a random/ chance event.

1 (b) Genetic diversity is the total genetic characterstics of a particular species population. It can be increase by events like:

1) Mutations

2) out breeding i.e. random mating

3) random fertilization of gametes

4) recombination between homologous chromosomes during meiosis (gamete formation)

Genetic diversity is beneficial as it allows species to survive sudden changes in environmental conditions or certain natural events. Example If a species encounters a deadly disease which can wipe off its entire population, then a highly genetic diverse species has more chances of containing genes to resist the disease and carry forward the population.

Also genetic diversity allows the suppression of genetic disease. In poorly diverse small population, there are high chances of inbreeding. Inbreeding can lead to unmasking of genetic defects. Example Huntington disease in afrikaner population. Due to inbreeding, huntington disease is higly prevalent in the population.

1 (c) Adaptive evolution is driven by natural selection. It occurs when an organisms adapts to certain environmental condition. This adaptation provides survival advantage to that organism and it in turn becomes natural selected because the advantage. It can lead to speciation. Example: Giraffes whose neck were long could eat food that short neck individuals could not. This trait got naturally selected and long necked giraffe became the predominant population.

Non adaptive evolution is driven by genetic drift, gene flow and mutation. Genetic drift can lead to speciation and also reduce ability of a population to survive selection pressure. This type of evolution is more affected by population migration. When some individuals separate from their parent population and migrate to inhabit a new place.

2) A population will be in hardy weinberg equilibrium when it follows the following assumptions:

a Random mating : Individuals are mating randomly without any specific preferences.

b Same allelic frequencies in male and female

c absence of natural selection ; individuals of all different genotypes are equally viable and fertile.

d absence of mutation: mutation will lead to change in alleles and contribute to an increase proportion of mutant alleles.

e absence of migration : frequency of alleles may changes if a population migrates.

f large size of population: This ensures that change in allele frequency does not change by chance