Population Genetics, Microevolution, and Natural Selection

Population Genetics and Microevolution

Introduction to Evolution in Populations

Evolution acts on populations, not individuals. While natural selection may kill or favor individuals, evolutionary change is measured as a shift in allele frequencies within a population over generations. This process is known as microevolution.

• Microevolution: A change in allele frequency over generations; the basis for phenotypic (expressed) evolution.

• Evolution is tracked by observing changes in the gene pool of a population.

Mechanisms of Allele Frequency Change

There are three primary mechanisms by which allele frequencies can change in a population:

1. Natural Selection

2. Genetic Drift

3. Gene Flow

Genetic Variation and Its Sources

Genetic Variation in Populations

Genetic variation is essential for evolution, as it provides the raw material for natural selection to act upon. Not all genetic variation leads to noticeable change, but it is necessary for populations to adapt to changing environments.

• Mutations: Changes in nucleotide sequences due to copying mistakes or outside influences.

• Duplication: Errors in DNA replication can result in gene duplication, providing new genetic material.

• Rapid Generation Time: Organisms like bacteria can evolve quickly due to short generation times.

• Sexual Reproduction: Increases genetic variation through:

  • Crossing over during meiosis

  • Independent assortment of chromosomes

  • Random fertilization

Quantifying Evolution in Populations

Defining a Population

A population is a group of individuals of the same species that live and interbreed together.

Gene Pool and Allele Frequencies

• Gene pool: All versions of an allele, for each gene, in a population.

• An allele is fixed if it no longer changes its frequency in the population (i.e., all individuals are homozygous for that allele).

Hardy-Weinberg Equilibrium

The Hardy-Weinberg equation allows us to determine if a population is evolving by comparing observed and expected allele frequencies:

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

• $p$ = frequency of allele 1

• $q$ = frequency of allele 2

• $p^2$ = expected frequency of homozygotes for allele 1

• $q^2$ = expected frequency of homozygotes for allele 2

• $2pq$ = expected frequency of heterozygotes

If observed and expected frequencies are the same, the population is in equilibrium and not evolving.

Mechanisms That Cause Allele Frequency Change

1. Natural Selection

• Environmental changes can favor certain alleles, increasing their frequency in the population.

2. Genetic Drift

Genetic drift is a random change in allele frequencies, especially significant in small populations. It can occur through:

• Founder Effect: A small group splits off from a larger population, carrying only a subset of the genetic diversity.

• Bottleneck Effect: A sudden environmental change drastically reduces population size, leaving a small, unrepresentative sample of alleles.

3. Gene Flow

• The movement of alleles between populations, often due to migration of individuals.

Natural Selection in Detail

Principles of Natural Selection

• Natural selection does not always mean "survival of the strongest"; sometimes, traits that improve mating success are favored.

• Sexual selection: Traits that increase mating success (e.g., bright plumage in birds) can be favored even if they do not improve survival.

Types of Natural Selection

• Directional Selection: Favors one extreme of a trait (e.g., larger beaks in birds when only large seeds are available).

• Disruptive Selection: Favors both extremes of a trait (e.g., birds with either very small or very large beaks, but not intermediate sizes).

• Stabilizing Selection: Favors intermediate variants (e.g., human babies are most likely to survive at intermediate birth weights).

Summary Table: Mechanisms of Evolutionary Change

Mechanism	Description	Effect on Genetic Variation	Example
Natural Selection	Alleles that confer a survival or reproductive advantage increase in frequency	Can increase or decrease variation	Antibiotic resistance in bacteria
Genetic Drift	Random changes in allele frequencies, especially in small populations	Decreases variation	Bottleneck effect after a natural disaster
Gene Flow	Movement of alleles between populations	Can increase variation within a population	Migration of individuals between populations

Key Terms

• Allele: Different forms of a gene

• Homozygote: Individual with two identical alleles for a gene

• Heterozygote: Individual with two different alleles for a gene

• Gene Pool: The total collection of genes in a population

• Hardy-Weinberg Equilibrium: The condition in which allele and genotype frequencies remain constant from generation to generation in the absence of evolutionary influences