Two lizard populations are separated by a newly formed river. Over thousands of generations they accumulate different mutations and adaptations. When a land bridge later reconnects their ranges, they no longer interbreed. This is best described as:
ASympatric speciation via polyploidy
BAllopatric speciation
CPrezygotic isolation without divergence
DConvergent evolution
Allopatric speciation occurs when a geographic barrier separates a population, allowing divergence through natural selection and genetic drift without gene flow. When the barrier is removed, reproductive isolation has become sufficient to maintain species boundaries. The river is the classic allopatric barrier.
Question 2 True / False
Two bird populations that look strikingly different from each other — different colors, different songs, different sizes — should therefore be separate species.
TTrue
FFalse
Answer: False
Under the biological species concept, the defining criterion is reproductive isolation, not morphological difference. Populations can look very different and still interbreed successfully (e.g., dog breeds), while populations that look nearly identical may be reproductively isolated (cryptic species). Appearance alone is not sufficient evidence of speciation.
Question 3 Short Answer
What is the difference between prezygotic and postzygotic reproductive isolation? Give one example of each.
Think about your answer, then reveal below.
Model answer: Prezygotic isolation prevents mating or fertilization from occurring in the first place — examples include different mating seasons (temporal isolation), different habitat use (ecological isolation), or incompatible courtship behaviors (behavioral isolation). Postzygotic isolation occurs after mating: hybrid offspring are produced but have reduced viability or fertility — examples include hybrid inviability (embryos fail to develop) or hybrid sterility (like the mule, offspring of a horse and donkey).
The prezygotic/postzygotic distinction matters because it describes where along the reproductive process isolation acts. Postzygotic isolation is costly — resources are wasted on failed reproduction — so natural selection often reinforces prezygotic barriers once postzygotic isolation exists (a process called reinforcement).