There have been a number of major books written on the evolution of sexuality that anyone truly interested in understanding the problem should read. You might begin with GC Williams "Sex and Evolution". Another, more advanced and formal treatment is Graham Bell's "The Masterpiece of Nature". Essentially, sexuality must provide a fitness advantage to individuals over asexuality to evolve and become fixed within a population. We can ignore the mechanics for a moment, some primitive forms of which have already been discussed below. It is much more important to understand the 'why' part of the question. Many hypothetical advantages have been proposed for evolution of sexuality and they are not mutually exclusive. We might touch on them one at a time, but there are far too many to deal with in a single post.
Let's review some facts first so there is stable ground for a discussion.
Asexuality is the primitive condition.
However, the vast majority of higher organisms are obligately sexual.
Sexual reproduction leads to more genetic variation in progeny than does asexual reproduction.
Sexual reproduction exacts a significant fitness cost for an individual compared to asexual reproduction, known as the 'cost of meiosis' (Maynard Smith). This arises because females only obtain 50% genetic representation in each offspring instead of the 100 % they would obtain if they produced asexually (all daughters the same as themselves). This means we must show how a sexual female can gain more fitness (reproductive success) than an asexual female.
Thus, the advantages of genetic diversity in one's progeny must somehow more than compensate for this cost in order for sexuallity to evolve.
I think there is great insight to be gleaned from organisms such as aphids (that I happen to study for a living, among others) that employ BOTH modes of reproduction. We can see that when environment is favorable (and food supply abundant and predictable) all reproduction is asexual. Males and sexual females are only produced at the end of the season when unpredictable conditions are faced (overwintering etc.) and a diverse range of progeny increases ones chances that some will survive. Williams calls this the 'lottery strategy'. Would you want to hold a thousand lottery tickets all with the same number, or a thousand with different numbers? However, this works only for organisms producing large numbers of offspring and investing very little in each.
There are many other scenarios where the cost of meiosis can be overcome. Perhaps I will have time to cover some others later.
Is anyone familiar with the 'Red Queen' theory? Named by Bell for the red queen in Alice in Wonderland who stated: "sometimes it takes all the running you can do to stay in the same place". It hinges on frequency dependent selection operating on populations inhabiting unpredictable or capricious environments.
Sex seems to pay off whenever uncertainty prevails for the next generation.
This message has been edited by EZscience, 09-22-2005 12:55 PM