Speciation occurs when:

Speciation is the evolutionary process through which new biological species arise. This process is primarily driven by the reproductive isolation of populations, which means that two populations are unable to interbreed and produce fertile offspring. When populations are reproductively isolated, they can diverge genetically over time, leading to the development of distinct species. This isolation can occur due to various factors, including geographical barriers, behavioral differences, or temporal variations in mating behaviors.

In contrast, while extinction and adaptation to new environments are significant ecological processes, they do not directly lead to the formation of new species on their own. Similarly, while convergent evolution—where different species independently evolve similar traits in response to similar environmental challenges—does occur, it does not result in the creation of new species. Therefore, the essential mechanism behind the emergence of new species is indeed the reproductive isolation of two populations.