Distinguish allopatric and sympatric speciation.

• A. Allopatric: geographic isolation causes speciation; sympatric: speciation occurs without geographic isolation due to reproductive barriers or disruptive selection
• B. Allopatric: speciation occurs without geographic isolation; sympatric: geographic isolation causes speciation
• C. Allopatric occurs only in animals; sympatric only in plants
• D. Allopatric: geographic isolation causes speciation; sympatric: speciation occurs without geographic isolation due to reproductive barriers or disruptive selection

Answer: (D)

Distinguishing allopatric from sympatric speciation hinges on whether geographic separation drives the divergence. In allopatric speciation, a physical barrier splits a population, cutting gene flow. Each group accumulates different mutations, experiences genetic drift, and adapts to its local environment; over time, these differences build reproductive barriers, so even if the barrier were removed, interbreeding would be unlikely. Sympatric speciation occurs without geographic separation, within the same area, because strong reproductive barriers or disruptive selection push individuals toward different niches or mating preferences. For example, polyploidy in plants can instantly create a new species in one location, while in animals, shifts in host use or assortative mating can reduce gene flow despite shared space. So the statement that allopatric involves geographic isolation and sympatric occurs without geographic isolation due to reproductive barriers or disruptive selection captures the key distinction.