Final answer:
Marion Petrie focused on the study of the male peacock's feathers in relation to mate selection, contributing to the field of sexual selection within Biology.
Step-by-step explanation:
Within the subject of Biology, the research focused on the male peacock's feathers, particularly in relation to mate selection, was conducted by Marion Petrie. This area of study is part of sexual selection, a key component of Charles Darwin's theory of evolution by natural selection. The peacock's extravagant tail is an example of the handicap principle, which operates under the premise that only the most fit males can survive with such an encumbrance, thus the impressive tail serves as a signal of quality to females. Meanwhile, Charles Darwin, along with Alfred Wallace, are known for their independent discovery and development of the theory of natural selection.