Speciation

AP Biology· difficulty 3/5

Two species meet in a hybrid zone. Hybrid fitness drops over generations due to mate-choice changes in the parental species.

Position across hybrid zone Hybrid fitness earlier (dashed) later (solid)

What process is most likely responsible for the further reduction of hybrid fitness through reduced interspecies mating?

  • A

    Stabilizing selection within each species

  • B

    Reinforcement: selection strengthens prezygotic isolation when hybrids have low fitness

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  • C

    Genetic drift in the hybrid zone

  • D

    Hybridization: hybrids are favored

Explanation

Reinforcement is the evolution of stronger prezygotic barriers (e.g., mate discrimination) when hybrids have lower fitness. Selection on parental species reduces costly interspecies mating, completing speciation.

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