Final answer:
The selection differential S measures the difference in mean trait values between the overall population and the individuals favored by selection. It serves as an indicator of how strong selection, whether natural or artificial, is influencing evolutionary change.
Step-by-step explanation:
The selection differential S is a measure of the difference between mean trait values of the total population compared to the mean trait value of individuals that are favored by selection. It quantifies the strength of selection in causing evolutionary change by comparing the phenotypic traits of the entire population against those traits favored by natural or artificial selection. This measure is significant because it helps biologists understand how selective pressures, driven by environmental factors, influence the distribution of phenotypes and thus contribute to evolutionary processes.
For example, directional selection might result in larger beak sizes in birds if larger beaks have become favorable due to a change in the available food sources, while stabilizing selection would favor an average phenotype, such as an optimal birth weight in humans. Diversifying selection might result in sexual dimorphism, where two different phenotypes are favored in males and females of the same species. All these forms of selection can lead to changes in the genetic and phenotypic makeup of a population over time.