Biology 1 - Lesson 23: Population Genetics and Microevolution

Population genetics explores how allele frequencies in a population’s gene pool shift over time. Microevolution refers to small-scale evolutionary changes—often observable over relatively short timescales—driven by forces like natural selection, genetic drift, gene flow, and mutation. This lesson details the Hardy-Weinberg equilibrium principle, factors disrupting equilibrium, and how these collectively guide population-level genetic dynamics.

Hardy-Weinberg Equilibrium

The Hardy-Weinberg (HW) principle states that, in a large, randomly mating population without mutation, migration, or selection, allele and genotype frequencies remain constant from generation to generation. If a population meets these conditions, it is said to be in HW equilibrium. Under HW assumptions, allele frequencies (p + q = 1) lead to genotype frequencies p² (homozygous dominant), 2pq (heterozygous), and q² (homozygous recessive).

In practice, few real populations perfectly satisfy these assumptions, so HW is often a useful model to detect deviations (e.g., evidence of selection, non-random mating, or migration).

Forces of Microevolution

Allele frequencies can shift through multiple mechanisms:

• Natural Selection: Differential reproductive success among phenotypes.
• Genetic Drift: Random fluctuations in allele frequencies, especially potent in small populations (e.g., bottleneck, founder effect).
• Gene Flow (Migration): Movement of individuals/alleles between populations, potentially altering allele frequencies.
• Mutation: Introduction of new genetic variants; ultimate source of all new alleles.
• Non-Random Mating: Inbreeding or assortative mating can shift genotype frequencies away from HW predictions.

Mermaid Diagram – Microevolutionary Forces

Below is a conceptual flow of how different processes act on a population’s gene pool:

flowchart LR A[(Gene Pool)] --> B[Mutation introduces new alleles] B --> A A --> C[Gene Flow: alleles enter/leave] C --> A A --> D[Genetic Drift: random changes] D --> A A --> E[Natural Selection] E --> A A --> F[Non-Random Mating] F --> A

Mermaid flowchart showing how mutation, gene flow, drift, selection, and non-random mating can each affect a population’s gene pool.

D3-Based Pie Chart: Allele Frequency Changes Over Time

The pie chart below simulates changing allele frequencies (for a single gene with two alleles, A and a) across a few generations, illustrating microevolutionary shifts:

D3-based mini pie charts showing hypothetical allele frequencies (A vs. a) across four generations. Slight shifts illustrate microevolution over time.

Hardy-Weinberg Calculations

Suppose a population has an allele frequency for the dominant allele A of p = 0.6 and for the recessive allele a of q = 0.4. If the population is in HW equilibrium:

• AA genotype frequency: p² = 0.36
• Aa genotype frequency: 2pq = 0.48
• aa genotype frequency: q² = 0.16

Real populations rarely meet all HW assumptions perfectly, but significant deviations can hint at selection, drift, non-random mating, or other evolutionary forces at play.

Conclusion

Population genetics provides the quantitative framework for exploring how alleles fluctuate under various forces, including natural selection’s adaptive direction, drift’s randomness, and the mixing or isolation of populations via gene flow. By applying Hardy-Weinberg principles, biologists can identify where real populations diverge from equilibrium—and thus infer which microevolutionary factors are shaping their genetic structure and driving the evolutionary process.