Final answer:
Natural selection acts on phenotypes because it is these observable traits that are subject to environmental pressures, which determine reproductive success and survival. While allele frequencies in a population change over time, it is the advantageous or deleterious phenotypes.
Step-by-step explanation:
Natural selection is a critical mechanism of evolution that predominantly acts on the phenotypes rather than the genotypes of individuals within a population. Phenotypes are the physical expressions or characteristics of an organism that are influenced by its genetic makeup, the genotype. Although allele frequencies may change as a result of selection, natural selection directly interacts with the traits that are manifested and subject to environmental pressures, which in turn influence reproductive success and survival.
Individuals with phenotypes that confer advantageous traits have a higher likelihood of surviving and reproducing. This increased reproductive success results in a higher frequency of the beneficial alleles associated with these advantageous phenotypes in successive generations. Over time, this can lead to these alleles becoming more common or even fixed within the population, contributing to the adaptive evolution of the species.
Crucially, because the environment can only 'select' organisms based on their exhibited traits, or phenotypes, natural selection cannot directly affect the genotypes. However, genotypes are indirectly affected because those that result in advantageous phenotypes will become more common as a byproduct of this selection process. It is the manifested phenotype that is subject to environmental factors, which ultimately determines an individual’s fitness and thus its contribution to future generations.
Examples of Natural Selection on Phenotypes:
The process of natural selection acting on phenotypes is illustrated vividly through the peppered moth example. Here, a phenotype change from lighter to darker wing coloration, in response to soot-covered tree trunks during the Industrial Revolution, led to increased survival of the darker moths. This change was not due to a preference for certain genotypes but rather because the darker phenotype offered better camouflage from predators in a sooty environment, demonstrating how a change in environmental conditions can select for different phenotypes.