A characteristic which is influenced by genes and passed from parents to offspring is called heritable. Over many generations heritable adaptive traits become more common in a population. This process is called evolution by natural selection. Evolution by natural selection leads to adaptation within a population. The term evolution by natural selection does not refer to individuals changing, only to changes in the frequency of adaptive characteristics in the population as a whole. For example, for the mice that lived on tan sand, none of the mice had a change in the color of their fur; rather, due to natural selection, tan fur was more common for the pups than for the mother mice. In summary, a heritable characteristic that helps an animal or plant to have more offspring which survive to reproduce and will tend to become more common in a population as a result of evolution by natural selection.

a. Explain why a heritable characteristic which helps an animal to live longer will generally tend to become more common in subsequent generations as a result of evolution by natural selection.
b. Suppose an unusual heritable characteristic helped animals to live longer but made them sterile so they could not have any offspring. Explain why this heritable characteristic would not become more common in subsequent generations as a result of evolution by natural selection.