Final answer:
The strength of selection, S, refers to the extent of selection for a particular trait within a population. It plays a significant role in population genetics and correlates with evolutionary changes driven by either natural or artificial selection. The correct option is 3.
Step-by-step explanation:
The strength of selection, S, is a measure of how much selection there is for a specific trait in a population. This concept is important in the study of population genetics, which examines how selective forces change allele frequencies over time and contribute to evolutionary change. Selection can be natural, where traits that improve survival and reproduction become more common in a population, or artificial, as seen in selective breeding practices conducted by humans. For example, in agriculture, farmers may deliberately choose cows that produce a large amount of milk for breeding to increase milk production in subsequent generations. This process is analogous to the natural selection described by Darwin, except those human preferences, rather than environmental pressures, guide the selection.