Distinguish natural selection, genetic drift, gene flow, and population-genetic models.
Part of the free Knowledge Mountain biology library. Read an explanation, try a question, and use a memory cue to check what you can recall.
Natural selection occurs when heritable differences affect survival or reproduction. Over generations, alleles associated with greater reproductive success can become more common.
Individuals do not evolve because they need a trait. Selection acts on existing variation, while new mutations arise without being directed toward the organism’s needs.
After an antibiotic is used, resistant bacteria become common. Did the antibiotic deliberately teach individual bacteria to resist?
No. Resistant variants survived and reproduced more successfully under that condition.
Resistance can arise through mutation or acquisition of genes. The antibiotic selects among variants rather than purposefully creating the exact trait needed.
Genetic drift changes allele frequencies by chance, especially in small populations. Gene flow moves alleles between populations. Natural selection changes frequencies through differences in reproductive success.
A bottleneck sharply reduces population size. A founder effect occurs when a small group establishes a population. Both can make chance strongly influence the gene pool.
A storm randomly kills most beetles on an island, leaving an unusual allele frequency among survivors. Which mechanism best describes this change?
Genetic drift through a bottleneck.
The key clue is random survival with respect to the trait. If a trait specifically caused greater survival in the storm, natural selection could also contribute.
For two alleles, p + q = 1. Under Hardy–Weinberg conditions, expected genotype frequencies are p², 2pq, and q². The model provides a baseline for testing evolutionary change.
Its assumptions include a very large population, random mating, and no mutation, migration, or natural selection. Real populations may violate several assumptions.
Under Hardy–Weinberg assumptions and complete dominance, 9% of a population is aa. What are q and the expected heterozygote frequency?
q = 0.3; heterozygotes = 2pq = 0.42, or 42%.
The aa frequency is q² = 0.09. Take the square root to get q = 0.3. Then p = 0.7, so 2 × 0.7 × 0.3 = 0.42.