Final answer:
Darwin's first postulate indicates that variability within a population, such as the beak size of Galápagos finches, can lead to inherited changes affecting a population's genetic makeup, as supported by research on finch populations by Peter and Rosemary Grant.
Step-by-step explanation:
Charles Darwin's first postulate indicates that individual members of a population exhibit variability, which can be inherited. In the case of Galápagos finches, this variability is evident in the differing beak shapes among the finches. Darwin proposed that these beak shapes adapted over time to allow finches to utilize different food sources more effectively. Among the finches, the process of natural selection has been demonstrated by the work of Peter and Rosemary Grant. They showed that beak size could change in response to environmental conditions, such as the types of seeds available due to varying rainfall. These changes in beak size, which are hereditary, can alter the genetic makeup of a finch population, leading to an evolved population with different phenotypic distributions. This is an example of evolution in action, as predicted by Darwin's theories and observable within natural populations.