Final answer:
The selective pressure applied in breeding gentle lab rats is artificial selection, where humans select rats with desirable traits for breeding.
Step-by-step explanation:
The selective pressure applied when rats in research laboratories are not used for breeding if they are difficult to handle, resulting in most lab rats today being very gentle, is artificial selection.
Unlike natural selection, where environmental factors cause certain phenotypes to become more prevalent because they increase an organism's chances of survival and reproduction, artificial selection involves humans choosing which individuals get to reproduce based on desired traits.
In this case, gentleness in lab rats is being selectively bred for by only allowing those that are easy to handle to breed. Thus, over time, the overall population of lab rats will tend to be gentle due to the breeding practices imposed by humans.