9/9 Lecture: Population Genetics and the Hardy Weinberg Principle

Name 3 mechanisms of evolution other than natural selection

Gene flow

Genetic drift

Non-random mating

Gene Flow

the movement of alleles between populations, often caused by migration of individuals between populations

The effects of gene flow are more significant in smaller populations. The change in allele frequencies also depend on how much migration occurs.

Genetic Drift

Random fluctuations in allele frequencies between generations

The effects of genetic drift are more significant in smaller populations.

Non-Random Mating

refers to any pattern where mate pairing is not due to chance

Can include sexual selection, which is specifically when certain traits increase the chances of attracting mates (through mate competition and choice) and thus increases reproductive chances

Can explain why some traits that are detrimental to survival are retained

Fixation

occurs when the allele for a given gene reaches 100% allele frequency

This can happen because of genetic drift, especially in small populations.

Population Bottlenecks

a significant decrease in population size (caused by natural events, disasters, and human intervention) accompanied by a reduction in genetic variation due to genetic drift

Can you determine genotype frequency from allele frequency?

Sometimes. Although you can determine allele frequency from genotype frequency, the reverse is not possible unless the population being studied is in Hardy-Weinberg equilibrium.

Hardy-Weinberg Model

a probability-based mathematical model used to predict genotype frequencies from allele frequencies under specific conditions:

• No mutations occur that introduce new alleles

• No selection occurs that favors the survival/reproductive success of certain genotypes

• No gene flow or migration occurs

• Population size is large enough that genetic drift is insignificant

• Random mating occurs in terms of the gene of interest (the gene being studied should not be impacted by non-random mating)

When these conditions are met over successive generations, a population is in Hardy-Weinberg equilibrium.

Hardy-Weinberg Equilibrium

When a population is in Hardy-Weinberg Equilibrium, the allele and genotype frequencies of that population remain constant over successive generations. Even a population that is not in Hardy-Weinberg equilibrium can be brought to HW equilibrium after just one generation of random mating if the conditions are met.

Genotype frequencies can be predicted based on those allele frequencies (if known).

Hardy-Weinberg equilibrium can be represented by the formula: p² + 2pq + q² = 1

p is the dominant allele frequency and q is the recessive allele frequency.