Final answer:
Speciation is the process by which a new species forms, often due to reproductive isolation. Allopatric speciation arises from geographic barriers, whereas sympatric speciation occurs without those barriers. Variation and vestigial structures provide further insight into the process and history of speciation.
Step-by-step explanation:
Speciation is the evolutionary process where a new species arises. Allopatric speciation is a type of speciation that happens when biological populations of the same species become isolated from each other to an extent that prevents or interferes with genetic interchange. This can occur due to geographic barriers like mountains or rivers, leading to the independent evolution of the isolated groups. When isolation occurs without geographic barriers, often due to ecological factors or differences in behavior, it is referred to as sympatric speciation.
For speciation to take place, there must be reproductive isolation between the evolving populations. This may arise from physical barriers (allopatric) or behavioral changes (sympatric). Variation in the genetic makeup of a population contributes to speciation, and sometimes, vestigial structures visible in a species point to a common ancestry with other species from which they have diverged.