ch 23 Population Genetics and the Evolution of Populations

Population Genetics and the Evolution of Populations

Genetic Variation in Populations

Genetic variation is the foundation of evolution and refers to differences in DNA sequences among individuals in a population. This variation is essential for populations to adapt to changing environments.

• Formation of New Alleles (Mutation): Mutations are changes in the DNA sequence that can introduce new alleles into a population. These are the ultimate source of genetic diversity.

• Rapid Reproduction: Organisms that reproduce quickly (such as bacteria) can accumulate mutations faster, increasing genetic variation.

• Sexual Reproduction: Processes such as crossing over (exchange of genetic material during meiosis), independent assortment (random distribution of chromosomes), and fertilization (combining genetic material from two parents) all contribute to genetic diversity.

Microevolution refers to evolutionary change within populations, focusing on changes in allele frequencies over time.

Measuring Genetic Variation

• Gene Variability (Average Heterozygosity): Measures the percentage of loci in a population that are heterozygous. High heterozygosity indicates a large genetic "backup" and greater potential for adaptation.

• Nucleotide Variability: Assesses the percentage of differences in the actual DNA sequence among individuals.

• Heritable Variation: Genetic differences that can be passed from parents to offspring.

• Non-heritable Variation: Differences caused by environmental factors, not passed to offspring.

Phenotypic Variation can be:

• Discrete: Traits with distinct categories (e.g., brown eyes vs. blue eyes).

• Continuous: Traits that vary along a spectrum (e.g., height).

Hardy-Weinberg Equilibrium

The Hardy-Weinberg principle provides a mathematical model to study genetic variation in populations under ideal conditions. It predicts that allele and genotype frequencies will remain constant from generation to generation if certain conditions are met.

• No mutations

• Random mating

• Extremely large population size (prevents genetic drift)

• No gene flow (no movement of alleles into or out of the population)

Hardy-Weinberg Equations:

• Allele frequencies: $P + q = 1$

• Genotype frequencies: $P^2 + 2Pq + q^2 = 1$

Where P is the frequency of the dominant allele and q is the frequency of the recessive allele.

Causes of Changes in Allele Frequency

• Genetic Drift: Random changes in allele frequencies, especially significant in small populations. Can lead to loss of genetic variation and fixation of harmful alleles.

  • Bottleneck Effect: A sudden reduction in population size (e.g., due to catastrophe) causes loss of alleles.

  • Founder Effect: A small group establishes a new population, leading to reduced genetic diversity.

• Gene Flow: Movement of alleles between populations due to migration of fertile individuals or gametes (e.g., pollen transfer).

• Natural Selection: The only mechanism that consistently leads to adaptation. Favors alleles that increase fitness.

  • Disruptive Selection: Favors individuals at both extremes of a trait (e.g., both large and small beaks).

  • Directional Selection: Favors individuals at one extreme (e.g., only large beaks).

  • Stabilizing Selection: Favors intermediate variants (e.g., medium beaks).

  • Sexual Selection: Favors traits that increase mating success, leading to sexual dimorphism (differences in appearance between sexes).

    • Intrasexual Selection: Competition among individuals of one sex for mates.

    • Intersexual Selection: Mate choice, often by females, based on certain traits in males.

Preservation of Genetic Variation

• Diploidy: Maintains genetic variation by allowing recessive alleles to persist in heterozygotes, even if not currently advantageous.

• Balanced Polymorphism/Selection: Selection that maintains two or more phenotypes in a population.

  • Heterozygote Advantage: Heterozygous individuals have higher fitness than either homozygote, maintaining multiple alleles (e.g., sickle cell trait and malaria resistance).

  • Frequency-Dependent Selection: The fitness of a phenotype depends on its frequency in the population (e.g., scale-eating fish with left- or right-sided mouths).

Example Table: Types of Natural Selection

Type of Selection	Description	Example
Directional	Favors one extreme phenotype	Large beak size in finches during drought
Disruptive	Favors both extremes over intermediate	Black and white coloration in certain animals
Stabilizing	Favors intermediate phenotype	Medium beak size in stable environments

Additional info: The Hardy-Weinberg principle is a null model; deviations from its predictions indicate that evolution is occurring. Understanding the mechanisms that alter allele frequencies is crucial for studying how populations evolve over time.