Evolution Ecology>Evolutionary Mechanisms Practice Test

A population of ground beetles inhabits a dark basalt flow near a coastal bluff. Historically, beetles occur in two color morphs: light brown and nearly black. Over 12 years following the establishment of a nesting colony of visually hunting shorebirds, researchers recorded a steady increase in the frequency of the black morph from 38% to 81%. In quarterly mark-recapture surveys, black beetles had higher survival between sampling periods than light beetles when foraging on the basalt surface, whereas survival did not differ inside shaded crevices where color contrast was minimal. Hatch rates and clutch sizes were similar between morphs, and the census population size remained above 40,000 individuals with no evidence of immigration or emigration based on stable capture locations and genotypes. Predation trials using clay decoys painted light or dark showed more attack marks on light decoys placed on basalt. Across the study, the direction of change was consistent and gradual rather than oscillating. The investigators concluded that the color shift reflects a response to a persistent environmental pressure.

On a small island, a population of ground-nesting finches with diverse bill patterns numbered about 6,200. In a single night, a Category 5 cyclone flooded low-lying nesting grounds, leaving only 54 adults that happened to be roosting on a rocky ridge. The surviving group showed a narrow range of microsatellite alleles and lacked several bill-pattern variants previously documented. Over the next 10 breeding seasons, the population rebounded to more than 5,000, but genetic diversity remained depressed and the missing variants did not reappear, despite similar food availability and no new arrivals detected by banding and genotyping. Bill pattern frequencies shifted abruptly after the storm and then stabilized, with no consistent association between pattern and diet or reproductive success in subsequent years. Researchers noted that the mortality induced by the storm appeared indiscriminate with respect to phenotype, as carcass surveys recovered both common and rare variants from flooded areas. The main change in trait distribution therefore coincided with a drastic, short-term drop in population size.