Population Genetics and Mechanisms of Evolution: Hardy-Weinberg Equilibrium and Evolutionary Forces

Other Evolutionary Mechanisms

Learning Objectives

• Explain how changes in allele frequencies of a population's gene pool are related to the process of evolution or descent with modification.

• Identify when evolution has occurred in a population and explain which of the mechanisms of evolution is likely to be supported based on available evidence.

• Explain how the allele frequencies would potentially change for each mechanism of evolution.

• Compare and contrast random and selective forces of evolution.

• Explain the process of sexual selection and describe the role of mate choice and competition for mates.

Causes of Evolution (Evolutionary Mechanisms)

Hardy-Weinberg Equilibrium and Its Assumptions

The Hardy-Weinberg equilibrium is a foundational concept in population genetics, describing a state in which allele frequencies in a population remain constant from generation to generation in the absence of evolutionary forces. This equilibrium is maintained only if five key assumptions are met.

Assumption of H-W Equilibrium	Possible Outcomes if Broken (Cause of Evolution)
Population size	Genetic drift is especially relevant in small populations, where random chance events can lead to loss of alleles.
Random mating	Non-random mating due to inbreeding (not an evolutionary process itself) or due to sexual selection via mate choice.
Mutations	Mutations are the only evolutionary mechanisms that generate new alleles in a population by chance.
Gene flow	Gene flow tends to equalize allele frequencies in populations, but also brings in new alleles via migration of individuals.
Natural selection	Natural selection favors certain alleles, removing others from a population and can lead to adaptations.

Population Genetic View: Detecting Evolution

Calculating Allele Frequencies

Allele frequencies in a population can be calculated using genotype counts. For a gene with two alleles, A and a, the frequencies are denoted as p (frequency of A) and q (frequency of a).

• Formula for allele frequency:

$p = \frac{2N_{AA} + N_{Aa}}{2N_{total}}$ $q = \frac{2N_{aa} + N_{Aa}}{2N_{total}}$

• Example: In a population of 100 plants: - 20 AA (homozygous dominant) - 50 Aa (heterozygous) - 30 aa (homozygous recessive) Total individuals = 100, so total alleles = 200. $p = \frac{2(20) + 50}{200} = 0.45$ $q = \frac{2(30) + 50}{200} = 0.55$

Testing for Evolution Over Generations

By comparing allele frequencies across generations, we can determine if evolution has occurred. If allele frequencies remain unchanged, the population is in Hardy-Weinberg equilibrium and no evolution has occurred, even if genotype frequencies change.

• Example: Generation 1 and Generation 10 both have $p = 0.45$ and $q = 0.55$. No evolution has occurred.

Summary Table: Hardy-Weinberg Equilibrium and Evolutionary Mechanisms

Assumption	Evolutionary Mechanism	Effect
Large population size	Genetic drift	Random loss or fixation of alleles, especially in small populations
Random mating	Non-random mating (inbreeding, sexual selection)	Changes genotype frequencies; sexual selection can change allele frequencies
No mutations	Mutation	Introduction of new alleles
No migration	Gene flow	Movement of alleles between populations, increases similarity
No selection	Natural selection	Increase in frequency of advantageous alleles, adaptation

Additional info:

• Genotype frequencies can change due to non-random mating (e.g., inbreeding), but if allele frequencies remain constant, no evolution has occurred.

• Evolution is defined as a change in allele frequencies in a population over time.

• Hardy-Weinberg equilibrium serves as a null model to test for evolutionary change.