Wesley R. Elsberry
Posts: 4991
Joined: May 2002
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| Quote (Soapy Sam @ Jan. 02 2013,04:50) | | Quote (Wesley R. Elsberry @ Jan. 01 2013,03:11) | | Go here, then search for "karyotype". |
I'm not so sure fission/fusion is a common mechanism of speciation per se. It's possible for both fission and fusion to spread entirely neutrally, with no mechanism to partition a population - in both instances, it's simply the presence or absence of a 'gap', and meiosis need have little problem aligning gap+ and gap- chromosomes in heterozygotes. You can end up with different numbers of centromeres, which can result in drive in female meiosis, as the polarity of division can reward greater or fewer as they avoid becoming polar bodies, but this is not a fitness effect. The main potential for fitness depression comes from a higher rate of aneuploid gametes, which might lower fitness in heterozygotes. This acts against the change while rare, but helps push it to fixation when common, if it can get there. But if hybrid fitness is significantly reduced, the likeliest result remains extinction, rather than reinforcement of isolating mechanisms to avoid hybridisation. In the tiny inbreeding population demanded in this circumstance, there is little variation to provide them, while the wider population is hardly likely to be troubled by occasional hybridisation.
If an effectively neutral change in chromosome number arises (say) every 1 in 10000 gametes, then fixation of a change in the species chromosome number will flip at the same rate. But if there is drive, this acts in tandem with drift to raise the rate, which can oppose a weak selective effect. If there is fitness depression in heterozygotes, fixation will occur less often; where it does there will be a nonlinear progression (slowed at first, accelerated later). But I think this mostly occurs after isolation, rather than driving it. |
Maybe change in karyotype is mostly incidental and secondary to other isolating mechanisms. If that's the case, what pattern of karyotype changes do you expect to see in taxa? Does it fit with the observed pattern in the taxa I mentioned of swine and peccaries?
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"You can't teach an old dogma new tricks." - Dorothy Parker
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