Final answer:
The environment is the key determining factor in Darwin's theory of natural selection, as it shapes which individuals are best adapted and thus more likely to survive and reproduce. These selective pressures lead to changes in genetic frequencies, driving evolution and speciation.
Step-by-step explanation:
The key point in Darwin's proposal is that the environment imposes the conditions that determine the direction of selection. Darwin's theory of natural selection highlights that it is the environment that influences which individuals have a higher evolutionary fitness, meaning their ability to survive, reproduce, and pass on their genes to the next generation.
Natural selection acts on populations through environmental pressures which may include factors like food availability, predators, or mating preferences. These pressures can determine which individuals are best adapted to their surroundings, thereby increasing their representation in the gene pool.
Over time, such selective pressures lead to a shift in genetic frequencies within populations, ultimately driving evolution and influencing speciation rates. Darwinian fitness therefore depends on how well an organism's phenotype, resulting from its genotype and its interaction with the environment, allows it to cope with environmental challenges.