Different flowering times reduce gene flow, allowing allele frequencies to diverge over time

Different flowering times increase gene flow, keeping allele frequencies similar across habitats

Different flowering times only change plant size, so reproductive isolation cannot develop over time

Different flowering times completely stop mutations, preventing island and mainland populations from diverging

Postzygotic isolation evolved as hybrid offspring had reduced fitness due to incompatible traits

Prezygotic isolation evolved as hybrids had reduced fitness due to incompatible traits after birth

Speciation cannot occur because lab-produced hybrids prove the two populations are the same species

Speciation occurred only because the river changed course, not because selection acted over generations

Habitat-based mating preferences reduce gene flow, so divergence can accumulate without separation

Habitat-based mating preferences increase gene flow, so divergence cannot accumulate without separation

Habitat-based mating preferences only change diet choice, so reproductive isolation cannot emerge in insects

Habitat-based mating preferences completely prevent mutation, so allele frequencies cannot differ over time

Behavioral isolation limits mating, reducing gene flow even if the populations come into contact

Behavioral isolation promotes mating, increasing gene flow when the populations come into contact

Behavioral isolation only affects development, so mate choice cannot contribute to speciation in frogs

Behavioral isolation completely eliminates natural selection, so allele frequencies cannot diverge over time

Divergent selection favors different alleles, and low hybrid fitness reinforces reproductive isolation

Stabilizing selection favors the same alleles, and high hybrid fitness reinforces reproductive isolation

Divergent selection favors different alleles, and high hybrid fitness reinforces reproductive isolation

Genetic drift favors different alleles, and poor hybrid growth proves selection played no role at all

Individuals from the two populations fail to recognize each other's mating signals and do not interbreed when reunited

The two populations show minor differences in body coloration patterns but successfully interbreed when placed together

Both populations maintain identical mating behaviors but have adapted to consume different prey items in their habitats

Genetic analysis reveals the populations share ninety-eight percent sequence similarity across their entire nuclear genomes

Temporal isolation prevents pollen transfer between species because their reproductive periods do not overlap during the year

Mechanical isolation prevents successful pollination because the flower structures are incompatible between the two species

Hybrid sterility alone maintains species boundaries by eliminating all gene flow through nonviable hybrid offspring

Habitat isolation keeps the species separate because they occupy completely different ecological niches within the meadows

Divergent sexual selection in different microhabitats can drive sympatric speciation by creating reproductive isolation within a shared area

Allopatric speciation occurred first when lake levels dropped and then secondary contact reunited the now-distinct populations

Genetic drift alone produced the observed differences because small population sizes in each microhabitat caused random allele fixation

Stabilizing selection maintained ancestral traits in turbid water while only clear-water populations experienced any evolutionary change

Pollinator specificity creates prezygotic isolation by ensuring pollen transfer occurs only between individuals of the same species

Postzygotic barriers eliminate all hybrid embryos before seed development can occur in natural pollination events between species

Competitive exclusion prevents the two species from accessing the same pollinators due to limited nectar resources in the habitat

Genetic incompatibility prevents pollen tube growth in natural conditions but laboratory techniques bypass this molecular barrier

Natural selection accelerated genetic divergence beyond neutral expectations as populations adapted to different Pacific and Caribbean conditions

Founder effects during initial colonization of each ocean basin explain all observed genetic differences between the separated populations

Gene flow continues across the isthmus through larval dispersal, preventing complete reproductive isolation between Pacific and Caribbean forms

Sympatric speciation within each ocean basin produced the observed species pairs independently of the geographic barrier formation