Which event leads to the formation of two or more separate species?

The formation of two or more separate species is specifically characterized by a speciation event. Speciation occurs when populations of a single species become genetically isolated from each other, often due to factors like geographical barriers, ecological differences, or reproductive isolation. Over time, these isolated populations evolve independently, leading to the accumulation of genetic differences that ultimately result in the emergence of distinct species.

In contrast, the other choices refer to different biological concepts. Homology refers to similarities between species due to shared ancestry, which does not directly lead to speciation. A phylogenetic event pertains to the evolutionary relationships and histories among species, providing a framework for understanding speciation but not being the actual process of forming new species. Gene flow involves the transfer of genetic material between populations, which tends to maintain genetic cohesion and prevent speciation rather than promote it.

Thus, a speciation event is the most accurate choice because it encapsulates the essence of how new species arise through evolutionary processes.