What is the main difference between sympatric and allopatric speciation?

The main distinction between sympatric and allopatric speciation revolves around the presence of geographic barriers. In allopatric speciation, a population becomes geographically isolated, often due to physical barriers such as mountains, rivers, or distance, preventing individuals from interbreeding. This isolation leads to genetic divergence as the separate populations respond to different environmental pressures and accumulate variations over time.

In contrast, sympatric speciation occurs without geographic isolation. Populations may live in the same area but diverge due to other factors such as behavioral changes, reproductive barriers, or the exploitation of different niches. This means that geographic barriers are not a requirement for sympatric speciation to occur, while they are critical for allopatric speciation.

The other choices do not accurately capture this key concept. The type of organisms involved, the time frame of divergence, and nutritional requirements do not fundamentally differentiate between the two modes of speciation. Therefore, the presence of geographic barriers is indeed the main difference between these two processes.