Natural selection acts on individuals, but only populations evolve. Explain why this statement is true.

Natural selection is a process in which individual organisms with certain heritable traits are more likely to survive and reproduce than those without those traits. This differential reproduction and survival lead to gradual changes in the characteristics of a population over time, as advantageous traits become more common.

Evolution is the change in the allele frequencies of a population over time. These allele frequency changes happen due to different mechanisms such as natural selection, genetic drift, mutation, and gene flow. As these changes accumulate, they can result in the emergence of new species.

An individual organism is a single living entity, such as a plant, animal, or bacterium. A population is a group of individuals from the same species living in a particular area at a specific time. While the traits of individual organisms can vary significantly, a population represents the combined genetic information present in all its members.

Natural selection acts on individual organisms because it is their traits that determine how well they survive and reproduce in a given environment. For example, an individual cheetah with a faster running speed may be better equipped to catch prey and survive, resulting in more offspring carrying this trait. This does not mean that the individual cheetah evolves, but rather that its advantageous trait increases its reproductive success compared to other cheetahs in the same population with slower running speeds.

Although natural selection acts on the traits of individual organisms, the overall effect is observed at the population level, as the allele frequencies change over time. As the advantageous traits become more widespread in a population, the overall genetic makeup of the population evolves. In our cheetah example, the genes associated with faster running speeds would become more common in the population over time, leading to an overall increase in the average running speed within that cheetah population. In summary, the statement "Natural selection acts on individuals, but only populations evolve" is true because natural selection targets specific traits in individual organisms, resulting in them having different chances of survival and reproduction. These differences in reproductive success lead to changes in allele frequencies, which, in turn, cause the genetic composition of the population to evolve.