Final answer:
D. Direct fitness refers to the genetic contribution individuals make to the next generation through their own reproduction, and is a measure of their evolutionary (Darwinian) fitness.
Step-by-step explanation:
The concept in question refers to D. direct fitness, which is the component of an individual's genetic contribution to the next generation resulting from personal reproduction.
Direct fitness contrasts with indirect fitness, which involves genetic contributions to the next generation through supporting the reproduction of close relatives.
Fitness in an evolutionary context, often referred to as evolutionary (Darwinian) fitness, is measured by the individual's ability to pass on genes to the succeeding generation.
Selection acts at the individual level to ensure those with greater contributions to the gene pool are more likely to pass their genes on, thus enhancing their evolutionary (Darwinian) fitness.
This is foundational to the principle of natural selection, which dictates why more fit individuals are more likely to have more offspring, and why alleles that confer beneficial traits or behaviors are selected for.