Final answer:
The factors that contribute to natural selection are genetic variation and environmental changes. These factors ensure that there are traits for selection to act upon and that there are changing conditions that can confer advantages to certain traits.
Step-by-step explanation:
Two primary factors contribute to natural selection: genetic variation and environmental changes. Genetic variation within a population is crucial as it provides the raw material for selection. Without variation, there would be no different traits for natural selection to act upon. Environmental changes create new challenges and opportunities, to which organisms must adapt. When the environment changes, certain traits may become more beneficial, leading to an increase in allele frequencies for those traits.
Predation pressure can also contribute to natural selection by favoring traits that allow organisms to avoid predators. Social preferences may play a role in sexual selection, which is a form of natural selection, by influencing mate choice and, consequently, the propagation of certain traits.
Mutations and gene flow are additional sources of genetic variation that contribute to natural selection, by adding new alleles to the population. Ultimately, natural selection results in changes in allele frequencies over time, leading to adaptations and potential speciation events.