Final answer:
Extrinsic postzygotic isolation most likely occurs when parental populations are adapted to divergent environments, leading to offspring that may have lower fitness due to mixed adaptations. Allopatric speciation involves geographic barriers that enhance genetic divergence and reinforce reproductive isolation over time. so, option 2 is the correct answer.
Step-by-step explanation:
Extrinsic postzygotic isolation is most likely when parental populations are adapted to divergent environments. This type of isolation takes place after fertilization and typically results when individuals from two populations manage to mate, but the offspring are less likely to survive or reproduce due to the different adaptations they have inherited from each parent. For example, if one parental population lives in a dry, arid environment and the other in a moist, temperate one, hybrids might not be well-suited to either and have lower fitness.
In cases of allopatric speciation, this isolation occurs due to geographic barriers that restrict gene flow between populations, leading to genetic divergence. Over time, selection pressures in each environment act independently on the populations, enhancing differences and reinforcing isolation. Thus, when brought back into contact, the populations are more likely to display postzygotic barriers due to their adaptations to the specific environments they've evolved in.