Discovery News & Views has a good post on the problem sex presents for Darwinism. No, not the act of sexual intercourse, but the origin of sexual organisms (male and female). Asexual organisms have the ability to produce offspring at twice the rate as sexual organisms, and they never fail to reproduce on the grounds that they can’t find a mate. Given these clear biological advantages of asexual reproduction, how did sexuality ever evolve? How did it come to dominate in the struggle for survival? Sexuality should have been outcompeted very early on. Even more intriguing is the question of how asexual organisms could gradually evolve into sexual organisms without dying out in the process. A partially formed reproductive system does not result in progeny. Even if we find a way to hurdle the problem of a gradual transition, what is the likelihood that random mutations would create two different, and completely complimentary reproduction systems? And what is the likelihood that this would happen at the same time? Apparently chance is just really lucky.
A couple of new Darwinian explanations have been offered to solve this long-standing Darwinian conundrum. The post explores these explanations and shows how they fall short of explaining what needs to be explained. Check it out.
Thinking to Believe 
July 23, 2011 at 6:35 am
Claim CB350:
Sex is too complex for its origin to be explained by evolution. Males and females would have to evolve independently, and any incompatibility in any of the physical, chemical, or behavioral components would have caused extinction. Furthermore, evolutionary theory predicts that asexual reproduction would be favored because asexual species can reproduce faster.
Response:
1. The variety of life cycles is very great. It is not simply a matter of being sexual or asexual. There are many intermediate stages. A gradual origin, with each step favored by natural selection, is possible (Kondrashov 1997). The earliest steps involve single-celled organisms exchanging genetic information; they need not be distinct sexes. Males and females most emphatically would not evolve independently. Sex, by definition, depends on both male and female acting together. As sex evolved, there would have been some incompatibilities causing sterility (just as there are today), but these would affect individuals, not whole populations, and the genes that cause such incompatibility would rapidly be selected against.
2. Many hypotheses have been proposed for the evolutionary advantage of sex (Barton and Charlesworth 1998). There is good experimental support for some of these, including resistance to deleterious mutation load (Davies et al. 1999; Paland and Lynch 2006) and more rapid adaptation in a rapidly changing environment, especially to acquire resistance to parasites (Sá Martins 2000).
http://www.talkorigins.org/indexcc/CB/CB350.html
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