Final answer:
Populations are always variable, as per Darwin's postulates; this variability is essential for natural selection and facilitated Darwin's finch observations of speciation on the Galápagos Islands.
Step-by-step explanation:
Concerning Darwin's postulates and the variability of populations, the correct answer is A) Yes, always. One of Darwin's fundamental insights was that populations do exhibit variation; no population is completely uniform. In his observations of finches on the Galápagos Islands, Darwin noted that the finches exhibited a variety of beak shapes and sizes, adapted to different food sources. This variability within a population is essential for natural selection to act upon, leading to the survival and reproduction of individuals best suited to their environment. Speciation may occur when two populations become reproductively isolated and evolve in response to different environmental pressures, ultimately leading to the emergence of new species.Microevolution, which refers to small changes within a species, and macroevolution, which involves the evolution of new species, have both been observed in Darwin's finches after thousands of years of isolation. Thus, populations are always variable, providing the raw material for evolution through natural selection.