Question

1. How to predict what will happen to allelic and genotypic frequencies with different selective pressures?

2. Given an example of independent assortment.

Answer 1

1. First of all, let us be clear about what selective pressures are: Any cause that reduces reproductive success in a portion of a population potentially exerts selective pressure, driving natural selection.The factors that we will be looking at here are Natural Selection, Genetic Drift and Gene Flow.

These are the mechanisms that cause changes in allele frequencies over time. When one or more of these forces are acting in a population, the population violates the Hardy-Weinberg assumptions, and evolution occurs.

Directional selection leads to increase over time in the frequency of a favored allele. Once an advantageous allele has reached a high frequency, deleterious alleles are necessarily rare and thus mostly present in heterozygotes, such that the final approach to fixation is more rapid for an advantageous recessive than for an advantageous dominant allele. As a consequence, natural selection is not as effective as one might naively expect it to be at eliminating deleterious recessive alleles from populations.

Balancing selection, in contrast to directional selection, maintains genetic polymorphism in populations.For example, if heterozygotes at a locus have higher fitness than homozygotes (a scenario known as heterozygote advantage or overdominance), natural selection will maintain multiple alleles at stable equilibrium frequencies. A stable polymorphism can also persist in a population if the fitness associated with a genotype decreases as that genotype increases in frequency (i.e., if there is negative frequency-dependent selection).

Genetic drift removes genetic variation within demes but leads to differentiation among demes, completely through random changes in allele frequencies.

Gene flow is the movement of genes into or out of a population. Such movement may be due to migration of individual organisms that reproduce in their new populations, or to the movement of gametes

In the absence of natural selection and genetic drift, gene flow leads to genetic homogeneity among demes within a metapopulation, such that, for a given locus, allele frequencies will reach equilibrium values equal to the average frequencies across the metapopulation. In contrast, restricted gene flow promotes population divergence via selection and drift, which, if persistent, can lead to speciation.

2. Mendel's Law of Independent assortment states that allele pairs separate independently during gamete formation. This law is only true for traits which are not linked and are heritable. e.g. seed color and seed shape in a pea plant.