Okay, so natural selection, I’m all down with that, makes total sense.
Speciation by separation of initially same species populations, sure that makes sense. You move to california and your siblings move to texas and conversation afterwards just never seems to work. Same basic idea.
But how on earth does the number of chromosomes change? That seems like an abrupt step, that would be discrete in nature, and I would thus expect very little variation in the number of chromosomes. I don’t understand how it would work unless chromosomes aren’t really so separate, like the various phases of mitosis, a human imposed deliniation on what is naturally continuous.
So, bio geeks, educate me.
well, there are a number of ways I know of.
One is most common in plants – where it often causes instant speciation, because plants are weird like that. It’s called “polyploidy” or “a polyploid event” and what happens is that you have bad gamete formation and end up replicating all your chromosomes. So suddenly instead of 6 chromosomal pairs you have 12, and on you go with your little plant life. It happens in animals too, but with only rare exceptions, it’s fatal. (Ferns, which have been around a very long time indeed, can have thousands of chromosome pairs because of repeated polyploid events over the history of the whole fern clan.)
You can have non-disjunction, also usually fatal in animals, in which newly formed chromosomes fail to fully detach from their parents. This tends to result in trisomy (one common non-fatal trisomy is Down’s Syndrome) but I don’t see why you couldn’t end up with FOUR copies of a chromosome by this method (and therefore, an extra pair).
And I think you can have wacky stuff happen with translocation, which is when genes copy themselves to different chromosomes; iirc what happens is that in some cases, you’ll end up essentially combining two chromosomes into one (almost everything gets swapped to one of the chromosomes, and the wee remnant one gets “lost”), and then you’ll have something with half a chromosome. Half-chromosomes aren’t that uncommon – mules have them, and I think tiger/lion hybrids do, too. Mostly, half-chromosomed critters don’t reproduce, but some of them do. Also, what can happen to one can happen to a pair, so I don’t see why you couldn’t lose a whole pair that way, particularly as you’re already having a major replication error.
I think that has covered most of the bases. Translocation can do it, by the way, big ol’ chunks snapping off and floating about… if there are still sections of the broken off bits that can pair with sections on intact chromosomes, things can still work.
Chromosome numbers actually vary widely, even within species.
You’ve got to figure that things like this are going to happen given the rather diffuse nature of chromatin prior to the big wind-up necessary for mitosis and meiosis… also given the fact that what we really need is sections of genes to line up, not necessarily “chromosomes” as we conceive of them… if that makes any sense. There’s some stuff in Talk Origins about this topic I think from a few different sources.
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particularly, even if you assume there is a way for one instance of a species to have a different number of chromosomes, it still never made sense to me how that propogated throughout the whole species. or even how they could breed at all – aren’t we taught that creatures with different #s of chromosomes can’t breed?
i’m guessing this isn’t strictly true, like, oh, just about everything we learn in high school of this calibre.. 🙂
Creatures with different numbers of chromosomes breed all the time. Often (as with mules), the offspring are sterile. That’s not always true, however; some are fertile (equines are interesting this way – there are some consistently fertile hybrids where the parent species have different chromosome numbers).
as i suspected… 🙂 i didn’t realize it was commonplace – nor, actually, that donkeys and horses had a different # of chromosomes…
thanks, it all makes more sense now 😉
*grin* Yeah, the thing with creatures with different numbers of chromosomes is that they are capable of breeding so long as the requisite sections of chromosome (those parts that are necessary, which as we know are not actually all that many) can line up and do their thang. Again, there are many examples of this happening, none of which do I have citations for at this moment… now I’m going to have to find them though. Darnit.
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