Biology · Rung 3 of 4

Population change

Selection is not the only evolutionary mechanism. Drift changes frequencies by chance, with larger effects in small breeding populations. A bottleneck or a few founders can reduce diversity. Gene flow moves variants between populations when individuals or gametes contribute to reproduction.

Where this sits

Comes after
Natural selection

Exercises from this rung

Generated by the app, from easiest to hardest

  1. Twelve mice colonise an island, and their descendants lack the long-tail variant that was common on the mainland. Which mechanism is changing the population?

    • Genetic drift: chance among few individuals
    • Natural selection: those who leave more offspring
    • Gene flow: alleles arriving from another population
    • Mutation: a brand-new variant

    Why Genetic drift: chance among few individuals. When few are left, who gets to reproduce is largely a matter of luck, and an allele can be lost or spread without being any better or worse. A bottleneck and a founder effect are the same thing: a small sample that does not represent the population it came from.

    easier

  2. A neutral variant changes frequency by chance as a population reproduces. What is this process called?

    • Selection directed by need
    • Transcription
    • Genetic drift

    Why Genetic drift arises from random sampling of variants across generations. Frequencies can change without an allele improving survival or reproduction. Its relative effect is usually larger when the breeding population is small.

    in between

  3. In a population of fruit flies, 25% are black-bodied, which is recessive (aa). If the population is in Hardy-Weinberg equilibrium, what percentage of individuals are heterozygous (Aa)?

    Answer 50

    Why q² = 25%, so q = √0.25 = 0.5 and p = 1 − q = 0.5. Heterozygotes: 2pq = 50%; AA: p² = 25%; aa: q² = 25%. Hardy-Weinberg says what to expect if nothing is pushing: comparing these numbers with what is measured is how you find out that something, selection, drift, migration, is going on.

    harder