While the fact that cross-over during meiosis does in fact contribute to increased genetic diversity leading to resistance to disease, parasites, and other adaptational advantages, one still needs to answer why did sexual reproduction evolve at all? Bacteria and protists are quite good at promoting genetic diversity through horizontal gene transfer, and viral infection with natural DNA replication error rates provide enough sources of "random" genetic variation to keep a diverse gene pool. One can think of competition to share genetic material as a primordial form of sexual selection. Getting from this kind of competition to orchestrated sharing and cross over of large genomes is a huge jump. The real question may be, did increasing organismal complexity necessitate a different type of reproduction to maintain genetic diversity? The answer to this may be yes, but alternative, less sexy hypotheses have to be tested as well. Another question to answer would be – What evolved first: multicellularity or sexual reproduction?
vegggdor|11 years ago
well, that's the kind of thing the GP wants to put in numbers and a mathematical system, to provide substantial arguments.