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 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 a population.

Any change in allele frequency constitutes evolution, and each process has unique consequences for genetic variation and fitness.

The Hardy–Weinberg Principle

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:

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

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

• Genotype frequencies: The Hardy–Weinberg equation predicts the frequencies of three genotypes: p2 (homozygote), 2pq (heterozygote), and q2 (homozygote).

When allele frequencies remain unchanged across generations, the population is in Hardy–Weinberg equilibrium.

Assumptions of the Hardy–Weinberg Principle

• Random mating

• No natural selection

• No genetic drift (large population size)

• No gene flow

• No mutation

Case Study: Hardy–Weinberg Equilibrium in Butterfly Populations

To determine if a population is in Hardy–Weinberg equilibrium, follow these 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.

If observed and expected frequencies match, the population is in equilibrium. If not, evolution is occurring at the gene in question.

Nonrandom Mating and Inbreeding

Nonrandom mating, especially inbreeding (mating between relatives), alters genotype frequencies but not allele frequencies. Self-fertilization is the most extreme form of inbreeding, common in many flowering plants.

• Inbreeding increases homozygosity and decreases heterozygosity.

• It does not cause evolution directly, but can accelerate the removal of deleterious alleles via natural selection.

• Inbreeding depression is a decline in average fitness due to increased homozygosity.

Sexual Selection

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

Natural Selection

Natural selection occurs when heritable variation leads to differential survival and reproduction. Individuals with advantageous phenotypes produce more offspring, increasing the frequency of associated alleles.

• Selection can occur in multiple modes:

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

• Stabilizing selection: Reduces variation by favoring intermediate phenotypes.

• Disruptive selection: Favors extreme phenotypes, increasing variation.

• Balancing selection: Maintains genetic variation by favoring multiple alleles.

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	Beak depth in finches	Reduced
Stabilizing selection	Favors intermediate phenotypes	Human birth weight	Reduced
Disruptive selection	Favors extreme phenotypes	Gill raker number in whitefish	Increased
Balancing selection	No single phenotype favored	Guppy coloration	Maintained

Genetic Drift

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.

• Drift is random with respect to fitness.

• Founder effects and bottlenecks are specific cases of genetic drift.

Gene Flow

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

Mutation

Mutation is the ultimate source of genetic variation, creating new alleles. It occurs randomly with respect to fitness and can be beneficial, neutral, or deleterious. Mutation alone is usually slow, but combined with other mechanisms, it can have significant evolutionary effects.

• Point mutations: Change in a single base pair.

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

• Lateral gene transfer: Genes transferred between species.

Summary Table: Evolutionary Processes

Process	Description	Effect on Genetic Variation	Effect on Fitness
Natural selection	Certain alleles favored	Can maintain, increase, or reduce	Adaptations increase fitness
Genetic drift	Random changes in allele frequencies	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	Production of new alleles	Increases by producing new alleles	Most mutations lower fitness

Take-Home Messages

• Mutation is the ultimate source of genetic variation.

• Mutations are random with respect to fitness.

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

• Each evolutionary mechanism violates Hardy–Weinberg assumptions and contributes to biological diversity.