Population Genetics, Drift & Speciation

What you will investigate

This genetic drift virtual lab tracks allele and genotype frequencies in a mainland population and island comparisons. Investigate the Hardy–Weinberg reference, random change in finite populations, bottlenecks, founding events, and the effect of connecting island populations. The course distinguishes a genetic pattern from a claim that a new species has formed.

Begin by separating dominance from allele frequency. A dominant phenotype does not make its allele increase automatically from generation to generation. The Hardy–Weinberg comparison provides a null model under stated conditions. Use it to organize expectations and test a sample, rather than treating it as a claim that real populations can never evolve.

An ideal allele-frequency reference

The opening ideal comparison starts with allele frequency p = 0.2. The resulting reference genotype proportions are shown below. They describe the course's ideal Hardy–Weinberg setup, not an observed sample of birds from a field population.

Ideal genotype category	Expected proportion
AA	0.0400
Aa	0.3200
aa	0.6400

Calculate allele frequency from genotype counts rather than equating it with the frequency of the dominant phenotype. The heterozygote includes both allele types. This distinction matters when a population's visible categories hide the genotype information needed for the calculation.

The course's chi-square comparison asks whether observed genotype counts depart from the expected proportions under the selected test. Failing to reject the null hypothesis does not prove that no evolutionary process is operating. It describes what that sample and test support. Keep the sampling method and expected-count calculation attached to the interpretation.

Drift is random change in a finite population

Small populations can experience larger random changes in allele frequency through reproduction. An allele can become fixed or disappear without being the better-adapted option in the selected neutral comparison. Follow several island histories instead of treating one trajectory as a predictable schedule for all islands.

The bottleneck and founder comparisons alter the population's sampling history. A temporary reduction or a small founding group can change allele representation and later diversity. The new group is not automatically a miniature copy containing every allele in exactly the mainland proportion. The investigation connects that difference with random sampling rather than intentional changes made by the organisms.

Selection and drift can act in the same broader situation, but their explanations differ. A weak modeled selective advantage can be obscured by chance in a small group. Compare the controlled population-size and selection settings before attributing every allele-frequency change to adaptation.

Gene flow and the limit of a divergence measure

The paired-island comparison contrasts connected and isolated groups. Movement of genetic contributions can reduce differentiation in the model. The course's numerical divergence measure summarizes a selected genetic difference; it is not a species detector and does not establish whether or when speciation has occurred.

Reproductive barriers are discussed through separate qualitative scenarios. Distinguish a prezygotic barrier from a postzygotic barrier using the stated scenario, not by setting an arbitrary threshold on a neutral divergence graph. The migration representation also uses a defined mixing approximation rather than animating and modeling every individual organism's movement.

Standards and classroom use

The principal connection is AP Biology Unit 7, with selected environmental-science diversity and island ideas. Ask students to compute an allele frequency, interpret a sample against its null expectation, and compare several drift histories. A second discussion can distinguish neutral genetic differentiation from reproductive isolation. The model's counts and probabilities are educational references, and individual Real-mode trajectories should retain their conditions. Continue with natural selection in finches or biodiversity sampling.

Build an explanation from your records

Make a prediction before changing the conditions, then record a comparison that actually tests it. Keep the conditions attached to each row: a number without its setup can be misleading even when the arithmetic is correct. Explain what changed, what stayed fixed, and which observation supports your conclusion. If the result differs from your prediction, revise the explanation using the evidence rather than choosing a new answer without a reason.

The reference examples on this page describe the course model or its stated source data. They are not measurements collected from a physical laboratory by the reader. Compare like conditions, keep units visible, and distinguish rounded instrument displays from underlying model quantities. Use the investigation's stated limits when deciding whether a result can be generalized beyond the simulated setup. This guide supports preparation and interpretation; practical laboratory technique and safety training remain separate learning activities.