Population Genetics and Evolution: Hardy-Weinberg Equilibrium and Mechanisms of Evolution (Ch. 23)

Population Genetics and Evolution

Introduction

This chapter explores the genetic structure of populations, the Hardy-Weinberg equilibrium, and the mechanisms that drive evolutionary change. Understanding these concepts is fundamental to the study of how populations evolve over time.

Genetic Equilibrium and Hardy-Weinberg Principle

Genetic Equilibrium

Genetic equilibrium occurs when both allelic and genotypic frequencies in a population remain constant from generation to generation, provided that certain conditions are met. This concept applies to populations and gene pools, not individuals.

• Genotype: The genetic makeup of an organism.

• Phenotype: The observable traits of an organism.

Hardy-Weinberg Equilibrium

The Hardy-Weinberg equilibrium describes a population that is not evolving, meaning allele frequencies do not change over time. For a population to be in Hardy-Weinberg equilibrium, five conditions must be met:

• No mutations

• Large population size

• No gene flow (no migration)

• No natural selection

• Random mating

These conditions are rarely met in nature, but the model provides a useful null hypothesis for studying evolutionary processes.

Hardy-Weinberg Equations

• p = frequency of the dominant allele

• q = frequency of the recessive allele

The sum of allele frequencies:

$p + q = 1$

The sum of genotype frequencies:

$p^2 + 2pq + q^2 = 1$

• $p^2$ = frequency of homozygous dominant individuals

• $2pq$ = frequency of heterozygous individuals

• $q^2$ = frequency of homozygous recessive individuals

Practice Problem Example

• Population of 100 people, two alleles (A and a) at a gene locus.

• Genotypes: AA (curly hair), Aa (wavy hair, incomplete dominance), aa (straight hair).

• Given: 40 curly, 30 wavy, 30 straight.

• Calculate allele frequencies for A and a using the Hardy-Weinberg equations.

Mechanisms of Evolution

Overview

Evolution occurs when allele frequencies in a population change over time. The main mechanisms that cause evolution are:

• Natural selection

• Genetic drift

• Gene flow

• Mutation

Mutation and Evolution

• Mutations are rare (about 1 in every million copies).

• Must occur in germ cells to be heritable.

• Most mutations are harmful, but they are the ultimate source of genetic variation.

• Genetic recombination is a more common source of variation in populations.

Genetic Drift

Genetic drift is the random fluctuation of allele frequencies, especially in small populations. It tends to reduce genetic variation and can lead to significant changes in small populations due to chance events.

• Bottleneck effect: A sudden reduction in population size due to a disaster, leading to a loss of genetic diversity.

• Founder effect: When a small group of individuals establishes a new population, the allele frequencies may differ from the source population.

Example Table: Bottleneck Effect on Prairie Chickens

Location	Population Size (Pre-settlement)	Population Size (Peak Bottleneck)	Percentage Hatched
Illinois	100,000	50	5.1
Kansas	100,000	100,000	93.0
Nebraska	100,000	100,000	92.0

Additional info: Table inferred from slide showing loss of genetic variation in Illinois prairie chickens after a bottleneck event.

Gene Flow

Gene flow is the movement of alleles between populations due to migration. It increases genetic variation within populations and makes different populations more similar.

• High migration can cause large changes in allele frequencies.

• Low migration results in only small changes.

• Example: Lake Erie water snakes (Nerodia sipedon) show gene flow between island and mainland populations, affecting adaptation.

Natural Selection

Natural selection is the only mechanism that consistently leads to adaptive evolution. It acts on phenotypic variation, favoring traits that increase fitness.

• Random reproduction with respect to genotype is required for Hardy-Weinberg equilibrium.

• No differential reproductive success with regard to genotype in equilibrium.

Modes of Selection

• Directional selection: Favors one extreme phenotype.

• Disruptive selection: Favors both extreme phenotypes over intermediate forms.

• Stabilizing selection: Favors intermediate phenotypes, reducing variation.

Example: Beak Size in Medium Ground Finch

• During droughts, larger beak sizes are favored as birds feed on larger seeds.

• Evidence: Average beak depth increased after drought years.

Preservation of Genetic Variation

Diploidy

Diploidy maintains genetic variation by hiding recessive alleles from selection in heterozygotes.

Balancing Selection

Occurs when natural selection maintains stable frequencies of two or more phenotypic forms in a population.

• Heterozygote advantage: Heterozygotes have higher fitness than either homozygote (e.g., sickle-cell allele confers malaria resistance).

• Frequency-dependent selection: The fitness of a phenotype depends on its frequency relative to other phenotypes.

Example Table: Sickle-Cell Allele

Genotype	Phenotype	Fitness in Malaria Regions
AA	Normal hemoglobin	Susceptible to malaria
AS	Carrier (heterozygote)	Resistant to malaria
SS	Sickle-cell disease	Severe health problems

Adaptive Evolution and Its Limits

• Natural selection cannot produce perfect adaptations because environments and selection pressures change.

• Lack of genetic variation can limit evolution.

• Ecological trade-offs and evolutionary history also constrain adaptation.

Summary Table: Mechanisms Affecting Allele Frequencies

Mechanism	Effect on Genetic Variation	Effect on Adaptation
Mutation	Increases	Rarely adaptive
Genetic Drift	Decreases (especially in small populations)	Random, not adaptive
Gene Flow	Increases within, decreases between populations	Can introduce or remove adaptive alleles
Natural Selection	Can increase or decrease	Consistently adaptive