Which of the following variations could be subject to natural selection?
a) A dog with short legs due to malnutrition is able to crawl into holes better than his litter mates
b) A tree is not infested by a ground-dwelling beetle when the homeowner cuts the lower branches
c) A hyena is born with a spotted fur pattern that allows it to hide in the grass better than his litter mates
d) A pigeon learns than when its keeper comes near it will be fed
e) All of these variations may be acted on by natural selection