The concepts presented are from discussions in two anthropology courses: Bio-Cultural Evolution taught by Dr. J. Kurland in Spring 2009 and The Evolution of Human Mating taught by Dr. D. Puts in Spring 2010, both at the Pennsylvania State University.
Parental investment in animals is the degree of care that a parent puts into it's offspring. For some species this may be carrying young to term, sitting on eggs, rearing young to adulthood, etc. Generally, the females of a species has a higher parental investment than males. In many species, males only contribute their sperm, which is low in parental investment. Sperm is practically an unlimited resource, whereas in some species eggs are limited, and energy costly to produce because they are such large cells.
Species with low parental investment generally create more progeny than those with high parental investment. When parental investment is different between the sexes, the sex that invests more is known as the slow sex (because they must go longer between matings due to their investment). Again, because females tend to invest more they are usually the slow sex. Due to the energy and time costs of their parental investment, the slower sex is the choosier sex; if the degree of parental investment is going to be high, the choosy sex will want to find the best genetic mate. Therefore, members of the fast sex must compete to mate with the slow sex.
This is where sexual selection comes in. The slow sex selects for good or attractive traits and mates with those in the fast sex that carry such traits. Sexual selection is the source for many sexually dimorphic traits between males and females. One popular example is the peacock's tail. A peacock has an iconic, but detrimental tail. The theory is that peahens favored bright and long tail feathers and eventually through runaway sexual selection the peacock's tail as we know it today was evolved. While the tail is certainly beautiful, it can land a peacock in a deal of trouble with predators making them easy to spot and catch. However, the catch is that while the individual is in danger, he is more likely to mate with a female because of the tail. It increases his Darwinian fitness while decreasing his individual survival.
The peacock's tail is also fitting for the "sexy-sons" hypothesis. In this hypothesis, it is thought that female selection results in male offspring with the attractive trait, and female offspring attracted to the trait. Thus the trait and it's attraction is preserved in successive generation guaranteeing the male offspring to have a good chance at mating.
Sexually dimorphic traits can be very costly to males, be it investing energy into developing the traits or the dangers that accompany them (ie. predators). However, despite the costs to the individual, it increases fitness which is what life is all about (in some opinions). This the core idea behind Richard Dawkins' book The Selfish Gene. Admittedly I have not finished the book but from my understanding it's about genes sometimes sacrificing the individual (through the means of altruism) in order to propagate themselves in the gene pool. Obviously I don't think Dawkins is suggesting that genes literally have hopes and dreams but rather that it is evolutionary adaptive for individuals of a species to cooperate (especially closely related individuals) to ensure survival of species (and genotypes). In a University of Portland commencement speech, Paul Hawken remarked, "We are here because the dream of every cell is to become two cells."