Natural Selection — Flashcards & Quiz

Q1: Compare directional, stabilising and disruptive selection.

Directional selection: favours one extreme phenotype, shifts the mean (e.g., increasing beak size during drought). Stabilising selection: favours intermediate phenotypes, reduces variation (e.g., average birth weight in humans). Disruptive selection: favours both extreme phenotypes over intermediate, can increase variation and may lead to speciation (e.g., beak sizes in seed-crackers: very large or very small, not medium).

Q2: What conditions are necessary for natural selection to occur?

Four conditions (VISA): 1) Variation — individuals differ in their traits. 2) Inheritance — traits must be heritable (genetic basis). 3) Selection pressure — environmental factor that differentially affects survival/reproduction. 4) Advantage — individuals with certain traits have higher fitness (more offspring). When these conditions are met, advantageous alleles increase in frequency over generations. Natural selection acts on phenotypes but changes genotype frequencies.

Q3: What is sexual selection and how does it differ from natural selection?

Sexual selection is a form of natural selection where traits are favoured because they increase mating success rather than survival. Two types: 1) Intersexual selection (mate choice) — one sex chooses mates with attractive traits (e.g., peahen choosing elaborate peacock tail). 2) Intrasexual selection (competition) — same-sex competition for mates (e.g., male elephant seals fighting for dominance). Sexual selection can produce traits that reduce survival but increase reproductive success.

Q4: What is the heterozygote advantage and how does it maintain genetic variation?

Heterozygote advantage (overdominance) occurs when the heterozygous genotype (Aa) has higher fitness than either homozygote (AA or aa). This maintains both alleles in the population because selection favours heterozygotes, preventing either allele from being eliminated. This is a form of balancing selection that preserves genetic diversity.

Q5: What is coevolution?

Coevolution is the reciprocal evolutionary change between two interacting species, where each species acts as a selective pressure on the other. Types: 1) Mutualistic — both species benefit (e.g., flowers and pollinators evolving matching structures). 2) Antagonistic — evolutionary arms race (e.g., predator develops speed, prey develops faster escape; parasite evolves immune evasion, host evolves stronger immunity). Changes in one species drive changes in the other, creating a feedback loop.

Q1: Stabilising selection favours individuals with extreme phenotypes over those with intermediate phenotypes.

Answer: FALSE

STABILISING selection favours INTERMEDIATE phenotypes and reduces variation. DISRUPTIVE selection favours extreme phenotypes. Stabilising selection narrows the distribution around the mean.

Q2: Natural selection acts directly on an organism's genotype.

Answer: FALSE

Natural selection acts on PHENOTYPES (observable traits). It is the phenotype that determines survival and reproduction. However, because phenotypes are influenced by genotypes, selection indirectly changes allele frequencies.

Q3: Heterozygote advantage maintains both alleles in a population because the heterozygous genotype has the highest fitness.

Answer: TRUE

When heterozygotes (Aa) have higher fitness than either homozygote (AA or aa), natural selection maintains both alleles in the population. Neither allele can be eliminated because the fittest genotype requires both.