Answer:
A plausible hypothesis is that deleterious germline mutations in sperm cells are removed by natural selection since sperms have to compete to fertilize the egg
Explanation:
Sperm cells need to travel through the cervix in order to reach the uterus and fertilize the egg. During this process (which is called 'fertilization'), approximately 300 million sperm cells have to compete in a race to reach an egg. During fertilization, the majority of sperm cells die off quickly because male germinal cells carry mutations that lead to abnormalities (e.g., many sperms have double heads, others no tail, etc). The sperm cells are considered to be physiologically weak because their normal functions can be easily disrupted by mutations and, simultaneously, they are under strong selective pressure to be selected during the sperm race. In consequence, deleterious germline male mutations are expected to be eliminated by natural selection, thereby sperms are selected before fertilization so only the strongest ones can compete in the race to fuse with the egg cell and thus develop one zygote.
