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The Microevolutionary Process - Vocabulary Flashcards

Microevolution is the change in allele frequencies over time, where allele frequencies are denoted by p and q such that p+q=1. Hardy–Weinberg (HW) Equilibrium describes a genetic state where allele frequencies are constant, meaning no evolution. This theoretical model assumes no mutations, isolated populations (no gene flow), no selection (alleles have no effect on fitness), and random mating. The genetic relationships are p+q=1 and p^2+2pq+q^2=1. However, HW equilibrium rarely occurs in nature due to the inevitability of mutations.

HWE serves as a crucial baseline to identify the forces that drive microevolutionary change. There are four primary mechanisms that move populations out of genetic equilibrium:

  • Mutation: The raw material for evolutionary change, which can be lethal, deleterious, neutral, or beneficial. Evolution occurs from the accumulation of these mutations across generations.

  • Genetic Drift: Particularly significant in small populations, leading to random changes in allele frequencies, loss of genetic diversity, and potential fixation (frequency goes to 1 or 0) of harmful alleles. Examples include bottleneck events and founder effects.

  • Migration/Gene Flow: The movement and transfer of alleles between populations, which promotes genetic diversity and has positive conservation implications, often facilitated by wildlife corridors.

  • Natural Selection: Operates in various modes:

    • Directional Selection: Shifts allele frequencies towards one extreme phenotype.

    • Stabilizing Selection: Favors intermediate phenotypes, while selecting against extremes.

    • Disruptive Selection: Favors extreme phenotypes, while selecting against intermediate forms.

Genetic drift poses significant negative implications for the long-term viability of small or endangered populations due to the loss of variation and the potential fixation of deleterious alleles. In essence, HW equilibrium is a null model, and any deviation from it reveals the evolutionary forces—mutation, genetic drift, gene flow, and selection—acting on populations.