Final answer:
The opportunity for selection is greater when variation in individual fitness is significant and when there is a biased sex ratio, promoting competition and differential reproductive success.
Step-by-step explanation:
The opportunity for selection on a trait is greater when individual variation in fitness is large and the operational sex ratio is biased. This suggests that selection pressures are more intense and can lead to more rapid evolutionary changes when some individuals have higher reproductive success over others, and when there is a skew in the ratios of mating individuals, generally leading to competition. Natural selection thus favors individuals with traits that enhance their survival and reproductive success. Consequently, alleles that confer beneficial traits are more likely to be passed on to subsequent generations.