Final answer:
The biological species concept uses reproductive isolation as its main criterion for defining species, with gene flow playing a role in maintaining species identity. Allopatric speciation and sympatric speciation involve reproductive isolation, leading to the diversification of populations into different species. Option 3 is correct answer.
Step-by-step explanation:
The biological species concept defines species based on reproductive isolation rather than solely on physical traits or morphology. According to this concept, a species is a group of actually or potentially interbreeding natural populations that are reproductively isolated from other such groups.
Reproductive barriers, whether they are prezygotic (before fertilization) or postzygotic (after fertilization), prevent different species from producing offspring or ensure that any offspring are infertile. Gene flow plays a crucial role in maintaining species identity, but reproductive isolation is the primary criterion for determining species boundaries.
Allopatric speciation is a key form of speciation resulting from geographic isolation. For instance, if a river changes course and divides a population, or seeds are carried across the ocean to an island, the separated groups may evolve independently through natural selection, genetic drift, mutations, and sexual reproduction. However, two primary sources of genetic variation are mutations and sexual reproduction. Genetic variation is important for speciation because it can lead to differences in traits that contribute to reproductive isolation.
Sympatric speciation can also occur, wherein species diverge while inhabiting the same geographical area, often through mechanisms like polyploidy, sexual selection, or ecological specializations.
The correct option, which demonstrates the primary criterion for determining species boundaries according to the biological species concept, is reproductive barriers, which corresponds to option 3) gene flow.