Final answer:
The major criterion for determining if two populations are part of the same species according to the phylogenetic species concept is reproductive isolation. This helps maintain species boundaries by preventing genetic mixing between distinct populations.
Step-by-step explanation:
According to the phylogenetic species concept, the major criterion for determining if two populations are part of the same species is reproductive isolation. The phylogenetic species concept focuses on the unique traits and genetic makeup that distinguish species from one another. Reproductive isolation ensures that genetic differences are not diluted through interbreeding and thus helps maintain species boundaries over time.
Speciation may occur when two populations become reproductively isolated from each other due to various mechanisms. For instance, ecological speciation involves the adaptation of populations to different ecological niches, leading to reproductive isolation. Another mechanism is peripatric speciation, where a small group becomes isolated at the edge of a larger population and evolves into a new species due to different selective pressures and genetic drift.
Scientific definitions of subspecies often come down to identifying unique genetic and morphological characteristics that are consistent within a group, but distinct from those of other populations or species, as indicated by reproductive isolation or phylogenetic history.