Population Genetics and Evolution: Natural Selection, Migration, Mutation, and Speciation

Population Genetics and Evolution

Part 5: Natural Selection

Population genetics studies how allele frequencies change within populations and the evolutionary forces that drive these changes. Natural selection is a key mechanism that can alter genetic variation and lead to adaptation.

Conditions for Stable Allele Frequencies

• Random mating: Individuals select mates at random.

• Infinite population size: Prevents genetic drift.

• No natural selection: No differential survival or reproduction.

• No migration: No gene flow into or out of the population.

• No mutation: No new alleles introduced.

If any of these conditions are violated, allele frequencies can change.

Definition of Natural Selection

• Natural selection refers to differences in survival or reproduction among individuals, caused by genotypic differences.

• Selection acts on phenotypes, but only heritable genetic variation leads to evolutionary change.

Example: Industrial Melanism in Moths

• Light and dark forms of Biston betularia (peppered moth) show differential survival depending on environmental background (pollution).

• Dark moths increased in frequency in polluted areas due to better camouflage from predators.

Forms of Selection

• Directional selection: Favors one extreme phenotype, shifting the mean and changing variance.

• Stabilizing selection: Favors intermediate phenotypes, reducing variance but keeping the mean constant.

• Disruptive selection: Favors both extremes, increasing variance but keeping the mean constant.

Quantifying Differential Survival and Reproduction

Selection can be measured using genotype frequencies and fitness values:

Genotype	Frequency	Fitness
B1B1	p2	1
B1B2	2pq	1-s
B2B2	q2	1-t

After selection, genotype frequencies change, and random mating restores Hardy-Weinberg Equilibrium (HWE).

Selection Strength and Allele Frequency Change

• Strong selection (large s or t) leads to rapid increases in favored allele frequencies.

• Weak selection causes slower changes.

Example: Selection for Alcohol Tolerance in Drosophila

• Populations exposed to high-ethanol environments show increased frequency of AdhF allele over generations.

• Control populations in zero-ethanol environments do not show this increase.

Heritability and Response to Selection

The response to selection depends on the heritability of the trait, which is the proportion of phenotypic variance explained by genetic variance.

• Genetic variance (VG) comes from additive (VA), dominance (VD), and interaction (VI) components.

• Phenotypic variance (VP) is the sum of genetic and environmental variance: $V_P = V_G + V_E$

Broad and Narrow Sense Heritability

• Broad sense heritability: $H^2 = \frac{V_G}{V_P}$

• Narrow sense heritability: $h^2 = \frac{V_A}{V_P}$

• Additive genetic variance (VA) is most responsive to selection.

The Breeder's Equation

• Response to selection: $R = S h^2$

• Selection differential (S): Difference between population mean and selected trait value.

Estimating Heritability from Selection Experiments

• Heritability can be estimated as $h^2 = \frac{R}{S}$

• Example: Selection for oil content in corn shows response over generations.

Natural Selection Can Maintain Genetic Variation

• Balancing selection, such as heterozygote advantage, maintains multiple alleles in a population.

• Example: Sickle cell allele is maintained in regions with high malaria frequency due to heterozygote advantage.

Genotype	Frequency	Fitness
AA	p2	1-s
AS	2pq	1
SS	q2	1-t

Both alleles are maintained at equilibrium due to selection favoring heterozygotes.

Part 6: Migration and Mutation

Migration and mutation are additional forces that can change allele frequencies in populations, contributing to genetic diversity and evolution.

Migration (Gene Flow)

• Gene flow is the movement of alleles between populations due to migration.

• Gene flow can:

  • Change allele frequencies by introducing new alleles.

  • Equalize allele frequencies between populations.

• One generation of random mating after migration restores Hardy-Weinberg Equilibrium.

Population	A1 Frequency	A2 Frequency	n
Mainland	1.0	0.0	200
Island	0.4	0.6	800
Admixed	0.6	0.4	1000

Mutation

• Mutation introduces new alleles and provides the raw material for evolution.

• Mutation alone changes allele frequencies slowly, but is essential for generating genetic variation.

Part 7: Population Divergence, Speciation, and Phylogenetics

Population divergence and speciation occur when populations become reproductively isolated and accumulate genetic differences. Phylogenetics is the study of evolutionary relationships among species.

Reproductive Isolation and Divergence

• Populations can diverge due to reproductive barriers or colonization of new territories.

• Genetic drift, selection, and mutation can lead to increased frequency of certain alleles in isolated populations.

Evolutionary Consequences of Divergence

• Divergence can result in speciation, the formation of new species.

• Genetic differences accumulate, leading to reproductive isolation.

Representing Evolutionary Relationships with Phylogenetic Trees

• Phylogenetic trees depict evolutionary relationships among species.

• The root represents the common ancestor.

• Branching patterns show relatedness; species sharing a recent common ancestor are more closely related.

• Monophyletic clade: A group containing an ancestor and all its descendants.

Example: Maternal Ancestry of Humans

• Phylogenetic analysis of mitochondrial DNA reveals clades and migration patterns.

• African lineages are interspersed across the tree, indicating deep genetic diversity.

Additional info: These notes expand on the original slides by providing definitions, equations, and examples for key concepts in population genetics and evolution, including Hardy-Weinberg Equilibrium, forms of selection, heritability, gene flow, mutation, speciation, and phylogenetic trees.