Final answer:
Polar bears are an example of animals that have adapted heavier bodies with thick fur and blubber to maintain heat in cold climates, illustrating natural selection. Rodent populations in similar environments will evolve to have an optimal body size to balance heat retention with the ability to hide from predators, a process known as stabilizing selection.
Step-by-step explanation:
An example of the development of rounder, heavier bodies as a result of natural selection in cold climates is seen in polar bears (Ursus maritimus). These mammals have adapted to their ice-covered habitats by evolving thick fur and a dense layer of fat, known as blubber, under their skin, which helps in maintaining body temperature by reducing heat loss. These adaptations allow the polar bears to survive the harsh Arctic conditions. Additionally, the white fur of polar bears is also an adaptation that provides camouflage against the snow and ice, aiding in hunting and protection from predators.
In the scenario where a population of rodents settles near the Arctic Circle, individuals with too large a body cannot hide well from predators like hawks, while too small bodies may lead to difficulties in maintaining a warm internal temperature. Therefore, this population will likely evolve to have an optimum body size that balances the ability to hide and the ability to retain heat. The selective pressures affecting this population will lead to a shift towards individuals with bodies that are larger than the smallest ones, which would freeze, but not so large that they cannot hide from predators. This is an example of stabilizing selection, where the intermediate trait is favored over both extremes.