Natural selection primarily causes changes in populations over time. It acts on individuals within a population, favoring those with advantageous phenotypes (observable traits), which are often linked to their genotypes (genetic makeup). However, it is the population as a whole that evolves as certain traits become more or less common over generations.
In summary:
- Natural selection affects individuals, but it does not change them directly; rather, it influences their reproductive success and survival.
- It acts on phenotypes, as it is the observable traits that determine an individual's fitness.
- The underlying genetic variation (or genotypes) may change over time as certain alleles become more prevalent in the population.
So, the most accurate answer is that natural selection causes changes in populations.