Final answer:
Survival and reproduction in Darwin's finches are influenced by inherited traits, with those better suited to environmental conditions being selected for nonrandomly. Peter and Rosemary Grant's studies of finch beak size changes over time due to natural selection are a prime example. Adaptive radiation also accounts for the diversification of finch species in the Galápagos Islands.
Step-by-step explanation:
The fourth postulate of Darwin's theory of evolution by natural selection implies that survival and reproduction are not random processes, but rather are heavily influenced by the traits an organism possesses. With regard to Darwin's finches, studies by Peter and Rosemary Grant have shown that environmental factors such as food availability led to a nonrandom survival of birds with certain beak sizes. For example, during a period with higher rainfall and fewer large seeds, finches with smaller beaks had an advantage and were more likely to survive and reproduce. Hence, the population evolved to have smaller beak sizes over time, illustrating natural selection in action. This process exemplifies how specific traits that enhance survival and reproductive success in a given environment can become more common in the population.
Adaptive radiation is another concept that explains the diversity observed in Darwin's finches on the Galápagos Islands. Reflecting this, these birds spread to different niches and developed distinct beak morphologies suited to their environment's particular food resources. This phenomenon is a showcase of natural selection and descent with modification.