Speciation Practice Test

Two closely related frog populations occupy the same forest. Males call from similar locations, and females approach calling males. Playback experiments show that females from population 1 approach only the call frequency pattern typical of population 1, while females from population 2 approach only the pattern typical of population 2. When researchers place males and females together in an enclosure, mating occurs mostly within populations even though breeding seasons overlap and hybrids, when produced, develop into viable and fertile adults. Genetic analyses show increasing divergence in allele frequencies over time. Which prezygotic barrier most directly maintains reproductive isolation?

A plant species has two populations that overlap geographically. Cross-pollination experiments show that pollen from population A lands on stigmas of population B and germinates, but pollen tubes usually fail to reach ovules; the reciprocal cross shows the same pattern. Within-population crosses produce seeds at high rates. Field observations show pollinators visit both populations, and flowering times overlap. Allele frequencies differ between populations at multiple loci. Which reproductive barrier most directly limits gene flow between the populations?