BSCI Topic 5: Sources of Variation and Its Application to Evolution by Natural Selection: Population Genetics

Darwin’s problem

How variation arises and persists.

Sources of variation

Mutation, recombination, migration, genetic drift.

Hardy-Weinberg equation

p² + 2pq + q² = 1; predicts allele/genotype frequencies under no evolution.

Hardy Weinberg assumptions

No mutation, migration, selection, or drift; random mating

Genetic drift

Random changes in allele frequencies (stronger in small populations).

Bottleneck effect

Sudden population reduction lowers diversity.

Founder effect

New population from small number of individuals.

Gene flow

Movement of alleles between populations.

Nonrandom mating

Inbreeding or assortative mating alters genotype frequencies.

a. Hardy Weinberg equilibrium

= when all individuals are randomly interbreeding and population is in genetic equilibrium (gene frequencies not changing) random mating, no immigration or emigration, no mutation, no selection, no genetic drift = NO EVOLUTION

then p2 + 2pq + q2 = 1 p = A alleles, q = a alleles

b. What does Hardy Weinberg do for you?

i) Comparison of expected vs. observed genotypic frequencies potentially allows you to reject the null hypothesis that no evolution is occurring, i.e., if observed gene frequencies are significantly different (statistically) from expected gene frequencies (p2 + 2pq + q2 = 1 ) can conclude that either mutation, migration, selection, nonrandom mating or drift IS OCCURRING. This may be interesting and important biologically and ecologically.

ii) If you know the allele frequencies, it allows you to predict the genotypic frequencies in the population, assuming no mutation, no migration, no selection, no drift, random mating, i.e., assuming that the population is in Hardy Weinberg equilibrium (i.e., if you already know that the population is in equilibrium, then you can calculate the gene frequencies, which may be useful).

iii) SIGNIFICANCE: What happens in small populations? When there is a biased sex ratio? When there are highly specialized mating systems? Populations often are NOT in Hardy Weinberg equilibrium, and then can undergo rapid and extensive genetic change