Question28 Recently-diverged species that still have overlapping geographic ranges are likely to evolve __________________ isolation mechanisms because _____________________.Select one alternative: postzygotic; postzygotic isolation mechanisms expend time and energy on offspring that have higher fitness prezygotic; postzygotic isolation mechanisms expend time and energy on offspring that have lower fitness prezygotic; postzygotic isolation mechanisms expend time and energy on offspring that have higher fitness postzygotic; isolation mechanisms expend time and energy on offspring that have lower fitness ResetMaximum marks: 1 Flag question undefined

Question

BlackTom AI · Accepted Answer

Prezygotic isolation mechanisms are more likely to evolve in recently diverged species with overlapping ranges because selection favors preventing costly hybrid matings.

1) Relevant knowledge points and concepts
- Prezygotic isolation: barriers that prevent mating or fertilization (behavioral, temporal, mechanical, gametic).
- Postzygotic isolation: barriers that reduce fitness of hybrids after fertilization (hybrid inviability, sterility, or reduced fitness).
- Reinforcement: natural selection strengthening prezygotic barriers when hybrids have reduced fitness, to avoid producing unfit offspring.
- Sympatry versus allopatry: when populations overlap (sympatry), selection can act directly against mating that produces low-fitness hybrids.

2) Step-by-step reasoning
- Two recently diverged taxa that still overlap geographically may occasionally mate.
- If hybrids suffer reduced viability or fertility (postzygotic cost), then matings waste parental time, gametes, and resources and produce fewer surviving offspring.
- Natural selection therefore favors mutations or behaviors that prevent interbreeding before zygote formation (prezygotic changes), because preventing a costly mating yields a direct fitness benefit.
- This process is reinforcement: selection increases prezygotic isolation in sympatry to avoid the costs imposed by postzygotic incompatibilities.

3) Calculations
- No numerical calculations are required; the argument is qualitative evolutionary reasoning based on fitness costs.

4) Why the correct answer is correct
- The correct choice states “prezygotic; postzygotic isolation mechanisms expend time and energy on offspring that have lower fitness.” That captures that prezygotic barriers are favored when postzygotic hybrids are low-fitness, because preventing those matings conserves time and reproductive effort.

5) Why other options are incorrect
- Options claiming postzygotic isolation will evolve faster contradict reinforcement: postzygotic barriers act after costly mating and cannot prevent wasted reproductive effort.
- Options asserting hybrids have higher fitness are incorrect because reinforcement requires hybrids to have lower fitness; if hybrids were high fitness, selection would not favor prezygotic isolation.

6) Conclusion
- Therefore, prezygotic mechanisms evolve in recently diverged, overlapping species because postzygotic isolation produces low-fitness offspring, matching the provided answer.

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