Final answer:
Natural selection is the process where individuals with traits that provide a survival advantage are more likely to reproduce, thus passing on those traits. It acts on the level of individuals, indirectly affecting genes and traits. Evolutionary fitness describes an individual's reproductive success, which combines genetic makeup and trait expression.
Step-by-step explanation:
Understanding Natural Selection and Evolutionary Fitness
Natural selection is a fundamental mechanism of evolution. It refers to the differential survival and reproduction of individuals due to differences in phenotype, which affect an organism’s ability to survive and reproduce in its environment. Essentially, individuals with heritable traits better suited for their environment are more likely to survive and reproduce, passing those traits on to the next generation.
Natural selection acts on the level of individuals, not directly on genes or separate traits. While it is the genetic makeup that defines potential traits, it is the manifested characteristics, or the phenotype, that undergoes the selective pressures of the environment. An individual's evolutionary fitness, referring to its reproductive success, is therefore a combination of its genetic makeup and how those genes are expressed as traits that interact with the environment.
In summary, while natural selection acts on individuals, it indirectly selects for genes associated with advantageous traits, leading to changes in allele frequencies in a population over time. This shift in allele frequencies within a population, brought about by the survival and reproduction of individuals, is the basis of evolutionary change.