Natural Selection and Ecology

Solution Preview

Natural Selection and Ecology (Case Mod 4)

1) Why is the Hardy-Weinberg law useful?

Hardy Weinberg theorem states that the frequencies of alleles in a population's gene pool remain constant over the generations during sexual recombination. In other words, sexual recombination of alleles by meiosis and random fertilization has no effect on the overall genetic makeup of a population. The Hardy Weinberg law can be formulated into an equation known as the Hardy Weinberg equation for calculating the frequencies of alleles and genotypes in population. It is ideal for the analysis of two alleles; one dominant , the other recessive. The letter p represent the frequency of one dominant allele and the letter q represent the frequency of the other recessive allele. Suppose that p = 0.8 and q = 0.2. The combine frequencies of all possible alleles must be 100% of the genes for that population; so the equation must be p+q = 1. If there are two alleles and we know the frequency of one, the frequency of the other can be calculated as 1 - p = q.
If gametes combine their alleles to form zygotes, the frequency of making a genotype AA is p2. If p = 0.8, and p2 = 0.64; the probability of an A sperm fertilizing an A ovum to produce a AA zygote is 64%. The probability of individualsfor the other recessive allele (aa) is q2 or 0.2 X0.2 = 0.04. Since there are two ways in which Aa genotype can arise, the frequency of heterozygous individual in population is 2pq ( 2 X 0.8 X 0.2) = 0.32. The frequencies of all possible genotypes should add up to 1:
P2 + 2pq + q2 = 1
Frequency of AA Frequency of Aa and aA Frequency of aa
So 0.64 + 0.32 + 0.04 = 1

The Hardy -Weinberg equation enable you to calculate the frequencies of alleles in a gene pool if you know the frequency of genotypes and vice versa. The Hardy Weinberg equilibrium tells you what to expect for a nonevolving population by comparing actual populations where the gene pools may be changing. Hardy Weinberg is maintained only if the population meets all five conditions:

1. Very large population size
2. Isolation from other populations
3. No net mutations
4. Random mating
5. No natural selection

So the Hardy Weinberg law is important because we can use the five laws to study microevolution because it is important to realize that Hardy Weinberg ideal population does not exist in nature.

2) Define genetic drift.

The Hardy Weinberg law states that there can be no change in a large population that would affect its overall gene pool, which is ideal for a very large population.
However, for a small population size, there is a greater chance that some genetic event by chance can change the gene pool. If the population is large, the existing gene pool is accurately represented in the next generation. If the population is small, the existing gene pool is not accurately represented in the next generation. There is random chance that can cause the frequencies of alleles in a small population to drift randomly from generation to generation. This is one of the microevolution caused by genetic drift, which are actually "random change in small gene pool due to ...