Final answer:
Changes in a population's gene pool lead to evolution. Evolution happens due to shifts in allele frequencies as a result of various selective pressures, which may include environmental changes, nonrandom mating, and genetic drift, ultimately affecting the population's genetic structure and adaptive traits.
Step-by-step explanation:
The correct answer to fill in the blank in the question is d) gene pool. Changes in a population's overall gene pool lead to the evolution of that population. Evolution in population genetics is characterized by a change in the allele frequencies within a population's gene pool over time. When individuals with different phenotypes reproduce at different rates, this causes changes in genotypic frequencies, which then affects the gene pool.
Different selective pressures such as environmental changes, nonrandom mating, and genetic drift can significantly influence the genetic structure of a population, leading to evolutionary change. Phenotypic variations that provide a survival and reproductive advantage — a higher relative fitness — tend to be passed on more frequently to future generations, thereby increasing those alleles' frequency in the gene pool. When beneficial traits are favored by natural selection, they become more common in the population, a process known as adaptive evolution.
An example of this is seen in the study of allele frequencies relating to pesticide resistance in mosquitoes. If a resistance allele allows mosquitoes to survive in an environment with pesticides, it will become more prevalent in the gene pool over time as these mosquitoes have more offspring who inherit the resistance. Over generations, this leads to a population that is more resistant to pesticides, highlighting the dynamics of evolution and population genetics.