Evolutionary Processes: Mechanisms of Evolution (Ch. 23) – Study Notes

Evolutionary Processes

Overview of Evolutionary Change

Evolutionary change occurs in populations over time, driven by changes in allele frequencies. Understanding these mechanisms is fundamental to the study of evolution in biology.

• Population: A group of individuals of the same species living in the same area and interbreeding.

• Allele Frequency: The proportion of a specific allele among all alleles for a gene in a population.

• Evolution: Defined as a change in allele frequencies in a population over generations.

There are four main mechanisms that shift allele frequencies:

• Natural Selection: Differential survival and reproduction of individuals due to differences in phenotype.

• Genetic Drift: Random changes in allele frequencies, especially in small populations.

• Gene Flow: Movement of alleles between populations due to migration.

• Mutation: Introduction of new alleles into a population by changes in DNA sequence.

Analyzing Changes in Allele Frequencies: The Hardy-Weinberg Principle

Hardy-Weinberg Equilibrium

The Hardy-Weinberg principle provides a mathematical model to study genetic variation in populations. It predicts genotype frequencies under certain conditions, serving as a null hypothesis for evolution.

• Assumptions: No selection, no mutation, no migration, large population size, and random mating.

• Genotype Frequencies: If allele frequencies are p (for A) and q (for a), then genotype frequencies are:

$p^2$ = frequency of AA $2pq$ = frequency of Aa $q^2$ = frequency of aa

• Equation: $p^2 + 2pq + q^2 = 1$

• Allele Frequency: $p + q = 1$

Example: In a population with 10 individuals: 3 AA, 5 Aa, 2 aa. Calculate allele frequencies: Number of A alleles = (2 x 3) + 5 = 11 Number of a alleles = (2 x 2) + 5 = 9 Total alleles = 20 Frequency of A ($p$) = 11/20 = 0.55 Frequency of a ($q$) = 9/20 = 0.45

Types of Natural Selection

Patterns of Selection

Natural selection acts on phenotypic variation, leading to changes in allele frequencies. There are several types of selection:

• Directional Selection: Favors one extreme phenotype, shifting the average trait value in one direction.

• Stabilizing Selection: Favors intermediate phenotypes, reducing variation and maintaining the status quo.

• Disruptive Selection: Favors extreme phenotypes at both ends, increasing variation and potentially leading to speciation.

• Balancing Selection: Maintains genetic diversity; no single allele is favored. Can result from heterozygote advantage or variable environments.

Type of Selection	Effect on Variation	Example
Directional	Decreases	Antibiotic resistance in bacteria
Stabilizing	Decreases	Birth weight in humans
Disruptive	Increases	Beak size in African finches
Balancing	Maintains	Sickle cell allele in malaria regions

Genetic Drift

Random Changes in Allele Frequencies

Genetic drift refers to random fluctuations in allele frequencies, especially in small populations. It can lead to loss of genetic variation and fixation of alleles.

• Founder Effect: When a small group starts a new population, allele frequencies may differ from the original population.

• Bottleneck Effect: Sudden reduction in population size due to environmental events, leading to reduced genetic diversity.

Example: A population bottleneck in humans can be seen in the reduced genetic diversity of certain isolated populations.

Gene Flow (Migration)

Movement of Alleles Between Populations

Gene flow occurs when individuals migrate between populations, introducing new alleles and increasing genetic similarity between populations.

• Effect: Can increase genetic diversity within a population and reduce differences between populations.

• Example: Migration of pollen between plant populations.

Mutation

Source of Genetic Variation

Mutation is the ultimate source of new alleles. It occurs when DNA polymerase makes errors during replication.

• Types: Point mutations, insertions, deletions.

• Effect: Most mutations are neutral or deleterious; beneficial mutations are rare but important for evolution.

Example: Mutation leading to antibiotic resistance in bacteria.

Nonrandom Mating

Patterns of Mating Affecting Genotype Frequencies

Nonrandom mating occurs when individuals choose mates based on specific traits, leading to changes in genotype frequencies but not necessarily allele frequencies.

• Inbreeding: Mating between close relatives increases homozygosity and can expose deleterious alleles.

• Assortative Mating: Individuals mate with those similar to themselves, affecting genotype distribution.

Example: Self-fertilization in plants increases homozygosity.

Sexual Selection

Selection for Traits That Enhance Mating Success

Sexual selection favors traits that increase an individual's chances of attracting mates. It often leads to sexual dimorphism—differences in appearance between males and females.

• Intersexual Selection: Mate choice, often by females.

• Intrasexual Selection: Competition among males for access to females.

Example: Bright plumage in male birds or large manes in male lions.

*Additional info: The notes include diagrams and tables comparing types of selection, Hardy-Weinberg calculations, and examples of genetic drift and sexual selection. All equations are provided in LaTeX format for clarity.*