Chapter 23: Evolutionary Processes – Structured Study Notes

Evolutionary Processes

Introduction to Evolutionary Processes

Evolution is defined as a change in allele frequencies within a population over time. Four primary mechanisms drive evolutionary change, each with distinct effects on genetic variation and fitness:

• Natural selection: Increases the frequency of alleles that enhance reproductive success in a specific environment.

• Genetic drift: Causes random changes in allele frequencies.

• Gene flow: Occurs when individuals migrate between populations and breed, altering allele frequencies.

• Mutation: Continuously introduces new alleles into the population.

Any change in allele frequency constitutes evolution, and populations can evolve even without natural selection. Each process has unique consequences for genetic variation and fitness.

The Hardy–Weinberg Principle

Null Hypothesis in Population Genetics

The Hardy–Weinberg principle provides a mathematical null hypothesis for studying evolutionary processes. It predicts genotype and allele frequencies in a population under specific assumptions, allowing researchers to detect deviations caused by evolutionary mechanisms.

• Gene pool concept: All alleles from gametes in a generation are pooled and combine randomly.

• Allele frequencies: For a gene with two alleles, frequencies are represented by p and q, where p + q = 1.

• Genotype frequencies: Three possible genotypes (AA, Aa, aa) with predicted frequencies:

$p^2$ for AA, $2pq$ for Aa, $q^2$ for aa

The sum of genotype frequencies equals 1:

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

Assumptions of the Hardy–Weinberg Principle

The Hardy–Weinberg model is based on five key assumptions:

• Random mating: No mate choice; gametes combine randomly.

• No natural selection: All individuals contribute equally to the gene pool.

• No genetic drift: Allele frequencies do not change by chance (large population size).

• No gene flow: No new alleles added or lost from the gene pool.

• No mutation: No new alleles introduced.

Case Study: Hardy–Weinberg Equilibrium in Butterfly Populations

Researchers analyzed two populations of endangered butterflies to determine if the Pgi gene (controlling flight metabolism) was under selection. The gene has two alleles: A and C. The analysis involves four steps:

1. Estimate genotype frequencies.

2. Calculate observed allele frequencies.

3. Use observed allele frequencies to calculate expected genotype frequencies.

4. Statistically compare observed and expected values.

Population 1 conformed to Hardy–Weinberg predictions, indicating equilibrium. Population 2 did not, suggesting evolution at the Pgi gene.

Nonrandom Mating

Inbreeding and Its Effects

Inbreeding, the most studied form of nonrandom mating, occurs between relatives and is especially pronounced in self-fertilizing plants. Inbreeding increases homozygosity and decreases heterozygosity in each generation, but does not change allele frequencies overall.

• Self-fertilization: Homozygous parents produce all homozygous offspring; heterozygous parents produce both homozygous and heterozygous offspring.

• Inbreeding depression: Decline in average fitness due to increased homozygosity and decreased heterozygosity.

Inbreeding can accelerate the elimination of deleterious alleles by natural selection, but is not an evolutionary process itself.

Sexual Selection

Sexual selection is a form of nonrandom mating where organisms choose mates based on physical or behavioral traits. Unlike inbreeding, sexual selection changes allele frequencies and increases fitness, making it a form of natural selection.

Natural Selection

Mechanism and Patterns

Natural selection occurs when heritable variation leads to differential survival and reproduction. Individuals with advantageous phenotypes produce more offspring, causing associated alleles to increase in frequency. Selection can occur in several modes:

• Directional selection: Changes the average value of a trait in one direction; reduces genetic diversity.

• Stabilizing selection: Reduces variation in a trait; maintains average value.

• Disruptive selection: Favors extreme phenotypes; increases variation.

• Balancing selection: Maintains variation; no single allele has a distinct advantage.

Summary Table: Modes of Selection

Mode of Selection	Effect on Phenotype	Example	Effect on Genetic Variation
Directional selection	Favors one extreme phenotype, changing average phenotype in one direction	Beak depth in finches during drought	Reduced
Stabilizing selection	Favors phenotypes near the middle, maintaining average phenotype	Human babies of average size	Reduced
Disruptive selection	Favors extreme phenotypes at both ends	Whitefish with low or high gill rakers	Increased
Balancing selection	No single phenotype is favored at all times	Guppies with rare color patterns	Maintained

Genetic Drift

Definition and Effects

Genetic drift is the random change in allele frequencies due to chance, especially pronounced in small populations. It can lead to the loss or fixation of alleles, reducing genetic variation and often decreasing average fitness.

• Founder effect: Occurs when a new population is established by a small group, leading to different allele frequencies.

• Bottleneck effect: Sudden reduction in population size causes a loss of alleles and changes in allele frequencies.

Gene Flow

Movement of Alleles Between Populations

Gene flow is the movement of alleles between populations through migration and breeding. It tends to equalize allele frequencies between populations, reducing genetic differences. Gene flow can increase or decrease fitness depending on the context.

Mutation

Source of Genetic Variation

Mutation is the ultimate source of genetic variation, creating new alleles and restoring diversity lost through other mechanisms. Mutations can be point mutations, chromosome-level changes, or lateral gene transfer. Most mutations are random with respect to fitness and often deleterious, but some can be beneficial or neutral.

• Point mutations: Change in a single base pair.

• Chromosome-level mutations: Changes in chromosome number or composition.

• Lateral gene transfer: Genes transferred between species.

Summary Table: Evolutionary Processes

Process	Definition/Description	Effect on Genetic Variation	Effect on Average Fitness
Natural selection	Certain alleles are favored and increase in frequency	Can maintain, increase, or reduce	Adaptations increase fitness
Genetic drift	Random changes in allele frequencies; most important in small populations	Reduces via loss or fixation	Usually reduces fitness
Gene flow	Movement of alleles between populations	May increase or decrease	May increase, decrease, or have no effect
Mutation	Random production of new alleles	Increases by producing new alleles	Most mutations lower fitness

Take-Home Messages

• Mutation is the ultimate source of genetic variation; only mutation creates new alleles.

• Mutations are random with respect to fitness.

• If mutation did not occur, evolution would eventually stop.

• Mutation alone is usually inconsequential in changing allele frequencies at a particular gene.

• All four evolutionary mechanisms violate Hardy–Weinberg assumptions and contribute to biological diversity.