jeannot
Posts: 1201
Joined: Jan. 2006
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I read the paper. It's rather well written (ie, easily understandable for a person like me, whose English is not the first language) and his observations regarding the origin of germ cells are interesting. I guess that's why he got published in this journal, not in rivista di biologia.
However, I certainly disagree with his view that, implicit to Darwin, "sexual reproduction is the mechanism producing the differences that natural selection acts upon". Actually, Darwin's model of natural selection is mostly independent of sexual reproduction, unless in the cases of sexual selection and avoidance of inbreeding, which provide selecting pressures. Of course, he never considers recombination, since the field of genetics didn't exist. In the end, Darwin never dealt with actual speciation. However, it is of common knowledge that recombination can accelerate the evolution of a lineage, because independent mutations can be combined in the same genomes.
As JAD notes, It's clear that sexual reproduction uses different mechanisms in distantly related vertebrates, however, it doesn't necessary imply that sexual reproduction appeared independently from a pre-sexual ancestor. One could easily imagine that a lineage can evolve different sexual mechanisms (sexual chromosomes, environmental determinism) without reverting to asexuality between those. For instance, opisthoconta (animals and fungi) share common structures in their spermatozoids, and I'm not even considering these homologies within vertebrates. So it seems that sexual reproduction has a single origin in this phylum. This would constitute a powerful evidence against JAD's hypothesis that semi-meiosis was common in vertebrates and underlay most of their macro-evolutionary history. In the end, his semi-meiosis hypothesis is interesting, however this mechanism may not have been widespread in eukaryotes, if it ever existed.
By the way, it’s clear to me that JAD supports common descent, at least between vertebrates. He relies on this principle to posit semi-meiosis as a pre-sexual state, the root of different mechanisms of sexual reproduction between different taxa.
Genomic rearrangements in the evolution of vertebrates and plants are well know, so are their underlying physiological mechanisms. But it is also well known that adaptation and speciation doesn't need such rearrangements, most of the time then don’t involve them. I would conclude that semi-meiosis may be an evolutionary mechanism to consider, however it's not less "Darwinian" than, say, allopolyploidy, stasipatric speciation, etc. And I certainly don’t see what it has to do with intelligent design (I know JAD doesn’t claim that in the paper)
And contrarily to JAD's words, his hypothesis doesn’t sounds more testable that "NeoDarwinism" is, in his own definition of testability: being able to observe a new genus appear.
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