Sexual selection (subset of natural selection): traits evolved solely because they were advantageous in the competition for mates. Two kinds of sexual struggle: intrasexual selection: struggle between individuals of same sex (typically males) driving away or killing rivals. Kudu horns: intersexual selection: struggle between individuals of same sex to excite or charm individuals of the opposite sex. The ess distribution of gamete size was a large, energy packed gamete coupled with a small, energy free gamete. With selection on the egg to be large to support zygote growth. And selection on the sperm to parasitize the egg small size can mean many sperm relative to egg number, which would increase the ability of sperm to find the egg (compared to larger sperm that are less numerous) Also there would be stronger selection on sperm to find the egg than on the egg to find sperm because the sperm are short-lived.