A seed-eating bird population lives on an island that has only small and large seeds — very few medium-sized seeds. Over generations, the population's beak-size distribution becomes bimodal, with the middle beak-size class nearly absent. This pattern is most consistent with:
AStabilizing selection, which eliminates extremes and preserves medium phenotypes
BDirectional selection, which shifts the entire population toward larger beaks
CDisruptive selection, where medium-beaked birds have lower fitness than both extremes
DGenetic drift, which randomly eliminates intermediate phenotypes from the population
Disruptive selection favors both extremes at the expense of intermediates. In this environment, small beaks efficiently handle small seeds and large beaks handle large seeds, but medium beaks are poor at both. The bimodal distribution is the signature of disruptive selection — it is the opposite of stabilizing selection (which produces a unimodal distribution by eliminating extremes) and directional selection (which shifts the whole distribution toward one end).
Question 2 Multiple Choice
Why is disruptive selection alone often insufficient to produce complete sympatric speciation?
AIt reduces total genetic diversity, making speciation genetically impossible
BIt only occurs in allopatric populations, so cannot drive sympatric speciation by definition
CRandom mating continually produces intermediate offspring that are selected against, creating genetic load but preventing the two peaks from fully separating
DDisruptive selection only operates on morphological traits, not on traits that control reproductive isolation
If large-peaked individuals mate randomly with small-peaked individuals, recombination in offspring continually regenerates intermediates that are selected against. This genetic load drags against divergence but does not eliminate intermediate genotypes entirely. For speciation to proceed, disruptive selection must be coupled with assortative mating (large with large, small with small), which reduces recombination between the morphs and allows the two peaks to diverge into reproductively isolated lineages.
Question 3 True / False
Disruptive selection creates a bimodal phenotypic distribution by favoring both extremes while selecting against intermediate phenotypes.
TTrue
FFalse
Answer: True
This is the defining feature of disruptive (diversifying) selection. Where stabilizing selection produces a sharp unimodal distribution by culling extremes, and directional selection shifts the distribution toward one end, disruptive selection pulls in opposite directions simultaneously — both high and low trait values have above-average fitness, while values near the population mean have below-average fitness. The African seed-cracker finch (*Pyrenestes ostrinus*) provides a real-world example.
Question 4 True / False
Disruptive selection and directional selection are functionally similar because both ultimately favor one end of the phenotypic distribution over the other.
TTrue
FFalse
Answer: False
This conflates two qualitatively different selection modes. Directional selection favors one extreme — the entire distribution shifts toward that end, and the mean changes in one direction. Disruptive selection favors *both* extremes simultaneously while eliminating the middle — the result is divergence, not directional shift. The ecological requirements also differ: directional selection needs one peak on the fitness landscape; disruptive selection requires two fitness peaks with a valley in between, as when resources come in discrete types with a gap in the middle.
Question 5 Short Answer
Why does assortative mating amplify the evolutionary consequences of disruptive selection, and what outcome can this coupling potentially produce?
Think about your answer, then reveal below.
Model answer: Assortative mating — large-phenotype individuals preferring large-phenotype mates — reduces gene flow between the two morphs. Without it, recombination from random mating continually regenerates disadvantaged intermediates. With it, the two phenotypic classes become genetically more isolated from each other over time, allowing their gene pools to diverge. If reproductive isolation becomes strong enough, the result is sympatric speciation: two distinct species arising from a single ancestral population without geographic separation.
The coupling of disruptive selection with assortative mating is one of the leading theoretical mechanisms for sympatric speciation and is theoretically important because it requires no geographic barrier — the ecological and genetic divergence happen within the same habitat. The rarity of disruptive selection in nature is largely a rarity of the ecological conditions that favor intermediates consistently worse than both extremes.