Final answer:
Stabilizing selection is the type of selection that would most likely act on a well-adapted population in a constant environment by favoring average phenotypes and reducing genetic variance.
Step-by-step explanation:
The type of selection most likely to act on a well-adapted population in a relatively constant environment is stabilizing selection. This form of selection favors the average phenotype and reduces genetic variance within the population by selecting against the extremes. In other words, it maintains the status quo of adaptation that the population has achieved.
In a constant environment, extreme traits are typically disadvantageous, as they may lead to lessened survival or reproduction rates. For example, in a forest with a consistent shade of brown on the forest floor, mice that blend in due to their brown fur color will be most likely to survive and reproduce. Mice with fur color that deviates too much from this average will stand out and be more likely to be preyed upon. Over time, this selective pressure would lead to a decrease in the color variation of the mice population.
For example, in a population of mice living in the woods, natural selection would favor mice with fur color closely matched to the color of the forest floor, as they are less likely to be spotted by predators. Mice with lighter or darker fur would stand out and be more vulnerable to predation. As a result, the genetic variance within the population decreases.