Microevolution and Genetic Variation in Populations

Evolution of Populations

Microevolution and Its Mechanisms

Microevolution refers to the small-scale changes in allele frequencies within a population over time. These changes are the foundation of evolutionary processes and occur due to several mechanisms.

• Natural selection: Differential survival and reproduction of individuals with certain alleles leads to changes in allele frequencies.

• Mutation: Introduces new genetic variation by altering DNA sequences.

• Genetic drift: Random fluctuations in allele frequencies, especially significant in small populations.

• Gene flow: Movement of alleles between populations through migration of individuals or gametes.

Key Point: Natural selection acts on individuals, but only populations evolve.

Population Genetics Terminology

Genes, Alleles, and Loci

Understanding genetic variation requires familiarity with basic genetic terminology:

• Gene (locus): A segment of DNA that codes for a specific trait.

• Allele: Different forms of a gene, corresponding to variations in DNA sequence.

• Diploid individuals: Possess two alleles for each gene, one inherited from each parent.

• Heterozygous: Having two different alleles at a locus.

• Homozygous: Having two identical alleles at a locus.

Note: Alleles remain separate and do not blend; apparent blending is due to incomplete dominance.

Population and Allele Frequencies

Gene Pool and Genetic Structure

A population is a localized group of individuals of the same species that interbreed and produce fertile offspring. The gene pool consists of all alleles present in the population.

• Each individual has a unique combination of alleles drawn from the gene pool.

• Different populations may have distinct gene pools due to geographic separation or other factors.

Example: Yukon caribou populations may have different allele frequencies due to geographic isolation.

Genetic Variation and Its Importance

Sources and Significance of Variation

Genetic variation is essential for evolution, as it provides the raw material for natural selection. Heritable traits are a prerequisite for evolutionary change.

• Genotype: The genetic makeup of an individual.

• Phenotype: The observable traits, influenced by genotype and environment.

• Natural selection acts only on genetically-based (heritable) variation.

Example: Moth caterpillars (Nemorhaedus) can resemble twigs or flowers based on diet, but only genetically-based differences are heritable.

Mutation: The Source of Genetic Variation

Types and Effects of Mutations

Mutation is the original source of all genetic variation. It is any change in an organism's DNA sequence and occurs randomly, not in response to selection pressures. Only mutations in germ-line cells (eggs or sperm) are heritable and contribute to population-level variation.

• Point mutations and small indels: Single-base substitutions or small insertions/deletions; usually neutral or mildly deleterious.

• Chromosomal mutations: Large deletions, duplications, inversions, or translocations; often highly disruptive.

• Gene duplications: Duplication of small DNA segments, potentially leading to new gene functions.

• Genome duplications: Increase genome complexity and can drive major evolutionary transitions.

Significance: Mutations generate genetic diversity, providing material for natural selection, genetic drift, and gene flow.

Example: The Cornish Rex cat breed arose from a single gene mutation in 1950.

Summary Table: Mechanisms of Microevolution

Mechanism	Description	Effect on Genetic Variation
Natural Selection	Differential survival and reproduction of individuals with certain alleles	Can increase, decrease, or maintain variation depending on selection type
Mutation	Random changes in DNA sequence	Introduces new variation
Genetic Drift	Random fluctuations in allele frequencies	Reduces variation, especially in small populations
Gene Flow	Movement of alleles between populations	Can increase or decrease variation within populations

Key Equations

• Allele frequency: The proportion of a specific allele among all alleles for a gene in a population. $p = \frac{\text{Number of copies of allele A}}{\text{Total number of alleles at the locus}}$

• Genotype frequency: The proportion of a specific genotype among all individuals in a population. $f(AA) = \frac{\text{Number of individuals with genotype AA}}{\text{Total number of individuals}}$

Conclusion

Microevolution describes the processes that change allele frequencies in populations over time. These processes—natural selection, mutation, genetic drift, and gene flow—are fundamental to understanding how populations evolve and adapt to their environments. Genetic variation, generated primarily by mutation and maintained by sexual reproduction, is essential for the evolutionary potential of populations.