Final answer:
Scientists distinguish species by examining morphological differences, genetic makeup, geographic location, and reproductive behavior. Genetic variation from mutations and sexual reproduction is critical for speciation. Such variation can cause reproductive isolation, spurring the formation of distinct species.
Step-by-step explanation:
Scientists distinguish between species by focusing on a combination of factors, including physical appearance (morphological differences), genetic makeup, geographic location, and reproductive behavior. Specifically, they examine ecological niches and morphological differences as well as reproductive barriers and genetic changes that contribute to the separation of species. The relationship between genetic variation and speciation is pivotal because genetic variation can lead to reproductive isolation and eventually speciation, as organisms with different genetic makeup can no longer interbreed successfully. Two primary sources of genetic variation are mutations and sexual reproduction. Genetic variation is crucial as it provides the material for natural selection to act upon and can lead to the emergence of new species.