Basically, the ladies are choosy, so any male with a unique trait or the ability to grow/make one gets to mate. In some cases when a trait gets to be too common (*cough* beards), they lose their appeal.
I say it's the females who are choosy, but this really depends on the level of commitment given to offspring. In species where the female cares for the offspring, she is the choosier partner and the males have ostentatious secondary sexual traits, including being larger in stature. On the other hand, in species in which males commit more to parenting, like seahorses for example, it's the females who have to compete for the best mates and so they are the larger and showier sex.
Another development of sexual selection, particularly of vivipary (live birth, as opposed to egg laying), is parent-offspring conflict. We sometimes jokingly refer to the developing fetus as a parasite, but this is actually far truer than people think. Parent-offspring conflict in utero is basically the conflict between mom, who wants to ensure she has enough resources to give to all offspring, and baby, who wants to hog all the resources for itself. This has led to a dynamic in which offspring evolve tactics to drain mom's resources and mom counter-evolves to ensure she can allocate resources to all offspring (note: this is a MAJOR simplification of this process). This is the process through which the placenta evolved. But the evolution of the placenta creates a shift in parental investment. Rather than putting all her resources into eggs that need to be fertilized (and are therefore very valuable), females became able to allocate resources after mating, lowering the risk of investing too much in offspring from a less-fit male. Presumably, this meant that females could mate with multiple males of varying genetic fitness, and that the offspring from the fitter males would hog more resources than the offspring from the less-fit males.
|A green swordtail, from |
the family Poeciliidae
|An example of a fish placenta. Source. I apologize|
to any squeamish readers.
These results mean that parent-offspring conflict, and the evolution/counter-evolution tactics used by mother and offspring, have influenced mate choice and male sexual characteristics. My Biology of Sex prof told us that the absence of gaudy sexual characteristics in humans was due to both parents contributing equally (arguably) to parenting. The results from this study seem to indicate that it might be a bit more complicated than that. Clearly, what's going on on the inside has a pretty large effect on what goes on on the outside.