Final answer:
The slope of the scatterplot between trait and relative fitness is known as the selection gradient. This selection gradient quantifies the strength of natural selection on that trait. Stabilizing selection decreases genetic variance, whereas directional selection increases it.
Step-by-step explanation:
The slope of the best-fit line of a scatterplot of trait vs. relative fitness for all individuals in the parental population is referred to as the selection gradient. The selection gradient is a measure of the strength of natural selection acting on a given trait. When the trait is plotted against relative fitness, any upward or downward trend in the line would indicate that individuals with certain trait values have higher or lower relative fitness than others, which in turn can suggest that those traits are being selected for or against by the environment.
Stabilizing selection tends to reduce genetic variance as it favors an average phenotype and selects against extreme variations. In contrast, directional selection results in greater genetic variance in a population because it favors phenotypes that diverge in a particular direction from the mean of the population.
Understanding these evolutionary forces is crucial in population genetics, as they are the mechanisms by which allele frequencies within a population are altered over time, thereby driving evolutionary change. This is exemplified by the process of directional selection, which can lead to favorable genetic traits, like better running speed, being more common in future generations, due to the individuals with such traits having higher relative fitness and therefore, greater reproductive success.