Remember how I said evolution was random here? Well here’s an exception.
Sexual selection is a driver of evolution that is more streamlined than natural selection. While natural selection is “survival of the fittest”, sexual selection is “survival of the prettiest”. In a very basic sense.
Sexual selection is most often driven by the “choosier” sex. This is often the sex that has to carry or take care of the babies. In some fish, the males take care of the baby fish and therefore will be pickier with which females they will mate with, male sea horses carry the offspring to term in their bellies and therefore will be pickier, while many large female cats will take care of the offspring and therefore will pick the males they want to mate with.
So this “pickier” sex can drive evolution in a specific direction just due to their preferences. Sometimes to extremes. For example:
There’s this bird, the long-tailed widowbird. The males have this long tail that the females love. Well turns out the females choose to mate with males with the longest tales, then their descendants have long tails, which have females selecting for even longer tails! So much so, that the males can have tails way too long that they can’t even fly and are very susceptible to predators! So evolution finds a balance – natural selection your tail can’t be too long that you can’t fly – and sexual selection – but long enough that a female still thinks you’re sexy. You can find more about this specific phenomenon here: Andersson, M. (1982). Female choice selects for extreme tail length in a widowbird. Nature, 299(5886), 818-820.
The pickier sex isn’t necessarily looking for just the prettiest opposite sex to mate with. They could be looking for the best partner to raise the descendants with, or the partner who sings the prettiest, or the partner who brings the most extravagant gifts. The opposite sex is looking for characters and attributes that are the best in the population because it wants it’s descendants to have the best genes. This is so they can succeed in their endeavours of finding the best or the “fittest” – as we say in biology – partner. If you want to see what would happen if humans flirted like other animals check out this funny article.
Here’s another cute example of a male practicing his dancing skills before showing them off to a female.