Final answer:
Speciation occurs when populations become geographically isolated and diverge genetically, leading to the formation of different species. This can happen through prezygotic and postzygotic barriers that block reproduction. Even when brought back together, reproductively isolated populations will retain their species identity.
Step-by-step explanation:
Speciation occurs along two main pathways: geographic separation (allopatric speciation) and through mechanisms that occur within a shared habitat (sympatric speciation). In both cases, populations become reproductively isolated. When populations become geographical isolated, the free-flow of alleles is prevented. Over time and because of different selective pressures-the populations diverge and become genetically independent species. Prezygotic barriers block reproduction prior to formation of a zygote, whereas postzygotic barriers block reproduction after fertilization occurs. Obviously, if two populations are separated by a vast ocean, they will not come in contact with each other to reproduce. However, if speciation has occurred, even when brought back together, they will retain their species identity. There are many examples of this in nature, including Darwin's finches, northern and Mexican spotted owls, and Hawaiian honeycreeper. Adaptive radiation occurs when a single ancestral species gives rise to many new species. This may occur, for example, when new habitats become available. It can also be seen historically in the rise of mammals following the extinction of dinosaurs. Other examples of prezygotic isolating mechanisms include mating seasons and unique courtship behaviors. Sometimes mating occurs between two different species, resulting in a hybrid such as the mule, which is a cross between a horse and a donkey. However, most hybrids are inviable or sterile.