Disruptive selection example and outcome.

Disruptive selection example and outcome.

• A. Stabilizing selection on human birth weight
• B. Allopatric speciation due to a geographic barrier
• C. Finch beak sizes in environments with two food types can favor small and large beaks while intermediate beak sizes fare worse
• D. Punctuated equilibrium with rapid change after long periods of stability

Answer: (C)

Disruptive selection happens when individuals at the extremes of a trait have higher fitness than intermediates, causing the population to shift toward two distinct phenotypes rather than a single average. In the finch scenario, two food types favor very small beaks (good for small seeds) and very large beaks (good for large seeds). Birds with intermediate beaks aren’t as efficient at either task, so their survival and reproductive success are lower. Over time, the beak-size distribution becomes bimodal, with more individuals at the two ends of the spectrum. If those with similar beak sizes prefer to mate with each other, this can further promote separation into distinct groups. Compared to other patterns: stabilizing selection would favor intermediates and narrow variation; geographic barriers causing separation lead to allopatric speciation, which is about isolation rather than the selection pattern itself; punctuated equilibrium describes rapid change after long periods of stability, not a mode of selection.

Disruptive selection happens when individuals at the extremes of a trait have higher fitness than intermediates, causing the population to shift toward two distinct phenotypes rather than a single average. In the finch scenario, two food types favor very small beaks (good for small seeds) and very large beaks (good for large seeds). Birds with intermediate beaks aren’t as efficient at either task, so their survival and reproductive success are lower. Over time, the beak-size distribution becomes bimodal, with more individuals at the two ends of the spectrum. If those with similar beak sizes prefer to mate with each other, this can further promote separation into distinct groups.

Compared to other patterns: stabilizing selection would favor intermediates and narrow variation; geographic barriers causing separation lead to allopatric speciation, which is about isolation rather than the selection pattern itself; punctuated equilibrium describes rapid change after long periods of stability, not a mode of selection.