Final answer:
A population of mice developed larger body sizes over generations due to directional selection, which occurred as an adaptation to colder temperatures, leading to an evolutionary change in average body size.
Step-by-step explanation:
Over many generations, a population of mice has developed a larger body size to cope with a drop in average temperature to which it is exposed. This type of evolutionary change is likely the result of directional selection.
In this scenario, the selection pressure comes from the colder environment, and thus the larger body size phenotype becomes more advantageous because it retains heat better than smaller body sizes. As a consequence, mice with larger bodies have higher survivability and reproductive success, passing their genes on to the next generations. This selective pressure shifts the average body size of the population in one direction — to larger body sizes over time.
Directional selection is evident when a histogram of body sizes over time shows an increased number of larger individuals and a decrease in the number of smaller ones. If a histogram were drawn, the new population line would skew to the right, towards larger body sizes, compared to the former population.