Evolutionary Processes and Hardy-Weinberg Equilibrium: Study Guide

Evolutionary Processes and Hardy-Weinberg Equilibrium

Introduction

This study guide covers the major evolutionary processes that influence allele frequencies in populations, focusing on the Hardy-Weinberg equilibrium and its assumptions, as well as mechanisms such as non-random mating, natural selection, and genetic drift. These concepts are foundational for understanding how populations evolve over time.

Hardy-Weinberg Equilibrium

Assumptions of Hardy-Weinberg Equilibrium

• Random Mating: Individuals pair by chance, not according to their genotypes or phenotypes.

• No Natural Selection: All genotypes have equal chances of survival and reproduction.

• No Genetic Drift: Allele frequencies do not change due to random sampling; population size is large.

• No Gene Flow: No migration of individuals into or out of the population.

• No Mutation: No new alleles are introduced into the gene pool.

If all these conditions are met, allele and genotype frequencies remain constant from generation to generation, and no evolution occurs.

Application Example: Snapdragons

• Population with two alleles (B and W) showing incomplete dominance.

• BB: Blue flowers, BW: Lavender flowers, WW: White flowers.

• Observed: 500 lavender, 90 white out of 1000 plants.

• Question: Is this population in Hardy-Weinberg equilibrium?

Case Study: HLA Alleles in Humans

HLA Genes and Hardy-Weinberg Equilibrium

• HLA genes code for proteins in the immune system (major histocompatibility complex, MHC).

• Multiple alleles exist at both HLA-A and HLA-B loci.

• Observed genotype frequencies did not match expected frequencies: fewer homozygotes than predicted.

• Null hypothesis (Hardy-Weinberg equilibrium) rejected: at least one assumption is violated.

Table: Observed vs. Expected Genotype Counts for HLA Genes

Gene	Data Type	Homozygotes	Heterozygotes
HLA-A	Expected	41	81
HLA-A	Observed	28	94
HLA-B	Expected	31	107
HLA-B	Observed	20	118

Hypotheses for HLA Equilibrium Violation

• Mating is not random with respect to HLA genotype (e.g., odor cues influence mate choice).

• Heterozygote advantage: Heterozygous individuals may have higher fitness due to a wider variety of HLA proteins, improving immune response.

Non-Random Mating

Inbreeding and Its Effects

• Inbreeding increases the frequency of homozygous individuals and decreases heterozygotes.

• Does not cause evolution directly, as allele frequencies remain unchanged.

• Increases the rate at which recessive deleterious alleles are eliminated.

Inbreeding Depression

• Decline in average fitness when homozygosity increases and heterozygosity decreases.

• Example: Florida panthers show reduced offspring survival due to inbreeding.

Heterozygote Advantage

• Genes under selection for heterozygote advantage (e.g., disease resistance).

• Fitness declines in homozygous individuals.

Sexual Selection

• Form of nonrandom mating where females choose certain males or males compete for mates.

• Leads to changes in allele frequencies and increases fitness.

• Special form of natural selection that attracts mates and increases reproductive success.

Natural Selection

Mechanism and Effects

• Differential reproduction among genotypes increases frequency of alleles that improve reproductive success.

• Only mechanism that results in adaptation.

• Genetic variability produces phenotypic variability, allowing natural selection to act.

Types of Selection

Directional Selection

• Individuals of one extreme phenotype favored.

• Reduces genetic diversity and changes mean value of a trait.

• Example: Average beak depth increases in birds during drought.

Stabilizing Selection

• Individuals with intermediate phenotype favored.

• Reduces genetic diversity but does not change mean value of a trait.

• Example: Human babies of average size are most likely to survive.

Disruptive Selection

• Both extreme phenotypes favored.

• Increases genetic diversity but does not change mean value of a trait.

• Can cause speciation (formation of new species).

• Example: Whitetails with low or high numbers of tail spots are most likely to survive.

Balancing Selection

• No single phenotype has a distinct advantage.

• Maintains genetic variation.

• Examples: Heterozygote advantage (e.g., sickle cell trait), Frequency-dependent selection (e.g., guppy color patterns favored when rare).

Table: Modes of Selection

Mode of Selection	Effect on Phenotype	Example	Effect on Genetic Variation
Directional selection	Favors one extreme phenotype, causing the average phenotype in the population to change in one direction.	Average beak depth increases in ground finches during drought.	Genetic variation is reduced.
Stabilizing selection	Favors intermediate phenotypes near the middle of the range of phenotypic variation, maintaining average phenotype.	Human babies of average size are most likely to survive.	Genetic variation is reduced.
Disruptive selection	Favors extreme phenotypes at both ends of the range of phenotypic variation.	Whitetails with low or high numbers of tail spots are most likely to survive.	Genetic variation is increased.
Balancing selection	No single phenotype is favored in all populations at all times.	Guppies with rare color patterns are favored.	Genetic variation is maintained.

Genetic Drift

Definition and Effects

• Random changes in allele frequencies in very small populations (<100) due to chance events.

• Sampling error can cause a gene pool to differ from the parent population.

• Reduces genetic diversity through chance; fixation of alleles likely ($p=1$, $q=0$).

Causes of Small Population Sizes

Bottleneck Effect

• Natural disaster kills individuals non-selectively, causing drastic reduction in population size.

• Example: Cheetah populations reduced by bottleneck events.

Founder Effect

• Small number of individuals colonize a new habitat and start a new population.

• Smaller founding population is less likely to represent the genetic makeup of the original parent population.

• Genetic drift may further reduce diversity until population size increases.

Key Equations

Hardy-Weinberg Equation

• Genotype frequencies: $p^2 + 2pq + q^2 = 1$

• Allele frequencies: $p + q = 1$

• Where $p$ = frequency of one allele, $q$ = frequency of the other allele.

Summary

• Evolutionary processes such as non-random mating, natural selection, and genetic drift alter allele frequencies and genetic diversity in populations.

• Hardy-Weinberg equilibrium provides a baseline for detecting evolutionary change.

• Understanding these mechanisms is essential for studying population genetics and evolutionary biology.