One of the biggest questions about evolution today is, "How did sex evolve?" The first place we should start is the definition of sex: exchanging genetic material with other members of a species. Bacteria have a way of exchanging genetic material; although it is more akin to the copy and paste functions of a computer than it is to sex. The next step would be for a population to evolve which went through cycles of giving and recieving genetic material. This population would be akin to the species of frogs that can spontaneously change sex from male to female. The final step would be for individuals to be born of only a single sex (Males that stay as males and females that stay as females). Richard Dawkins describes roughly the same scenario in "The Ancestor's Tale".
Why Sex?
The best supported reason for the evolution of sex is "The Red Queen Hypothesis". That is, sex evolved so a species could stay one step ahead of a parasite. There is even experimental evidence confirming this:
"Scientists from Rutgers University in New Jersey have tested this idea by observing different groups of small fish called topminnow in Mexico. Some populations of the topminnow reproduce sexually, while others reproduce asexually, so they provide the perfect opportunity to test these ideas. The topminnow is under constant attack by a parasite, a worm that causes something called black-spot disease.
The researchers found that identical populations ("clones") of the asexually reproducing topminnows harbored many more black-spot worms than did those producing sexually, a finding that fit the Red Queen hypothesis: The sexual topminnows could devise new defenses faster by recombination than the asexually producing clones." (Found at PBS.org)
More Interesting Stuff:
Virgin Whiptail Lizards that evolved from Sexuality to Asexuality
Scientific Book which presents a plausible scenario for the evolution of sex (called here "Amphimixis")
