Final answer:
The evolutionary response to selection in a population is mainly determined by genetic variation and selection pressure. These factors provide the potential for change and the directional force acting on it, respectively, driving evolutionary changes through mechanisms that are studied in population genetics. Option b is the correct answer.
Step-by-step explanation:
The evolutionary response to selection in a population depends on several factors. Of the provided options, the correct answer for the two main factors that determine how much the population changes would be b) Genetic variation and selection pressure. The role of genetic variation is central to evolutionary processes, as it provides the raw material upon which selection pressures can act.
A population with high genetic diversity has a greater potential for evolutionary change. On the other hand, selection pressure results from environmental factors favoring or disfavoring specific phenotypes, thereby influencing the survival and reproduction of individuals with those traits.
Mutations introduce new genetic variation which can be acted upon by selection, while gene flow can alter genetic frequencies by introducing alleles from other populations. Genetic drift is a stochastic process that can also shift allele frequencies independent of selection pressures, particularly in small populations.
In addition to these, nonrandom mating and environmental variances can influence the genetic structure of a population, which could also result in evolutionary change. Understanding these forces is critical for the study of population genetics and evolutionary biology.
Overall, to predict and understand evolutionary changes in populations, a comprehensive view that includes all these evolutionary forces, their interactions, and their contributions to changing allele frequencies is necessary.