I'm back. Pretty much. The last couple of months have been -- well, not conducive to writing posts about science. But we're ready for the next installment of "Earth: A Biography."
Last time, or actually time before last, we were talking about the three groups of protists -- Archaeans, Bacteria, and Eukaryotes -- and their relationships, which are still somewhat unclear. One thing that's very important here is that somewhere along the line they discovered sex -- that is, reproduction not as a more-or-less automatic function of a single-celled organism, accomplished by the simple expedient of the organism duplicating itself, but as a rather more involved process in which genetic material is shared between two organisms.
(A sidebar: at one point, far in the misty past, single-celled organisms did share genetic material more or less randomly, to the extent that a concept such as "species" was meaningless. Somewhere along the line (and at this point I haven't found a source that even addresses it) there developed a sort of regulatory gene that would no longer allow that: the only genes that could be shared were those between like organisms. This is probably a precursor for reproduction, but that took a while.)
So, for sexual reproduction to "take over," so to speak, as the main form of reproduction in complex organisms, there has to be an advantage, or a complex of advantages that, from an evolutionary perspective, makes it more desirable than nonsexual reproduction.
As it turns out, there are a number of advantages: increased resistance to parasites and diseases, removal of deleterious genes, and genetic variation. This last is going to be very important: genetic variation gives rise, in turn, to novel genotypes -- new species, in other words. We're going to see the of this as we enter the Cambrian Era, in what is known as the "Cambrian Explosion."
(The Wikipedia entry is fairly exhaustive and heavily documented, and is worth reading if you want more detail on this.)
Last time, or actually time before last, we were talking about the three groups of protists -- Archaeans, Bacteria, and Eukaryotes -- and their relationships, which are still somewhat unclear. One thing that's very important here is that somewhere along the line they discovered sex -- that is, reproduction not as a more-or-less automatic function of a single-celled organism, accomplished by the simple expedient of the organism duplicating itself, but as a rather more involved process in which genetic material is shared between two organisms.
(A sidebar: at one point, far in the misty past, single-celled organisms did share genetic material more or less randomly, to the extent that a concept such as "species" was meaningless. Somewhere along the line (and at this point I haven't found a source that even addresses it) there developed a sort of regulatory gene that would no longer allow that: the only genes that could be shared were those between like organisms. This is probably a precursor for reproduction, but that took a while.)
So, for sexual reproduction to "take over," so to speak, as the main form of reproduction in complex organisms, there has to be an advantage, or a complex of advantages that, from an evolutionary perspective, makes it more desirable than nonsexual reproduction.
As it turns out, there are a number of advantages: increased resistance to parasites and diseases, removal of deleterious genes, and genetic variation. This last is going to be very important: genetic variation gives rise, in turn, to novel genotypes -- new species, in other words. We're going to see the of this as we enter the Cambrian Era, in what is known as the "Cambrian Explosion."
(The Wikipedia entry is fairly exhaustive and heavily documented, and is worth reading if you want more detail on this.)
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