As we saw in the entry on Natural Selection the environment of an organism can include other species. It also includes other individuals in the organism’s own species, this is where the concepts of sexual and kin selection can fit in.
Put simply Sexual selection is about ladies choice, in many species the males compete for mates but it is the ladies who have the pick of the winners. In nature there are examples of extreme behavior, ostentatious displays an over developed physical attributes, as there are no free lunches in the natural world what pays for these? The answer has been posited that they increase reproductive fitness not directly by allowing faster running, better hunting or other obvious means but but making the individual more attractive to mates. In fact these characteristics might even become harmful by making the individual an easier target for predators.
Ideally the characteristic signals genetic fitness which is expressed by immunity to disease and parasites and ability to acquire food and avoid predators. Bright plumage would be metabolically costly and make predator avoidance more difficult so only a healthy individual can afford the up keep, one that is susceptible to illness would have difficulty and so show that it was inferior. It may be that some sexual selection is a chance preference in females that spirals out of control. If a female happens to prefer longer than average tails in her mates then those males who have long tails will mate and pass on those genes but, significantly, the female will also pass on the preference for long tails. In this way a trait can rapidly change in a population. It may then become a cost to the male that only healthy males can afford and so becomes a de facto indicator of reproductive fitness. In other words the female did not choose the characteristic because it indicated a healthy mate.
In birds this process can be seen to be taken to ridiculous extremes as the aptly named Long-Tailed Widow bird clearly shows. But there is an interesting hypothesis that the human brain itself is a sexually selected characteristic. Instead of developing “higher” intelligence in response to a changing environment or as a survival trait our capacious craniums may have simply made us sexier. It was our ability to shoot off a witty one liner that was the real target of selection and our increased problem solving just came along for the ride. While this may seem far fetched experiments with fruit flies suggests that under normal circumstances greater intelligence is more trouble than it’s worth this combined with the observation that human like intelligence is not replicated in other species, even among our close relatives, means an explanation like this might be closer to the truth than we’d care to admit.