Speciation and Chromosome Counts
January 20, 2006 12:43 PM   Subscribe

How do species come to have different numbers of chromosomes?

From highschool biology I have a decent understanding of speciation. I know something splits up a population (geographically, or by prohibiting mating, etc), and the two populations then no longer have gene flow with each other and can adapt seperately. What I've never had explained to me properly is how the number of chromosomes shifts. Unlike most changes I can't see how it could be a gradual thing, as I assume a mutant born with the wrong number for the population wouldn't be fertile (or bear viable offspring). I assume there's an explination, or at least several heated possibilities that I'm just not aware of, or else this would be huge ammunition for the ID crowd.
posted by luftmensch to Science & Nature (14 answers total)
It's important to remember that a species isn't homogenous. There is a lot of variation in a given population and speciation really just reflects a divergence between disjoint groups.

The number of chromosomes probably changes when the embryo is in it's very earliest stages of life. Remember that variations in chromosomes happen all the time. Trisomy 21 results in a non-fertile organism, but not all mutations result in sterility.
posted by bshort at 12:55 PM on January 20, 2006

This sequence of events must happen:

1. Gross chromosomal reorganization in the germline of a parent

2. Mutated gametes resulting in at least two mutated offspring

3. Inbreeding between the (compatibly) mutated offspring

posted by rxrfrx at 12:56 PM on January 20, 2006

I am not a biologist and my understanding of biology is laughable (as in everyone I've ever explained biology to has laughed at me!). But...

It seems obvious how this would work with asexual repreduction. But with sexual reproduction, is it possible that since the extra-chromosome (or chromosome deficient) potentail parent would only pass along half their chromosomes to any given child, that if the gamete fertilizes was the mutation-free half, then it would be no problem.

Of course, this would create a child without the mutation....so I'm not sure how that would result in eventually creating a new species. Anyone who actually knows bio want to comment on whether or not the half-the-chromosomes nature of the gamete could be part of the answer?
posted by duck at 1:05 PM on January 20, 2006

Best answer: Same question answered elsewhere. Inbreeding is indeed the answer.
posted by beagle at 1:10 PM on January 20, 2006

There are a number of possible ways this can happen. Off the top of my head:

1. Fusion of two chromosomes to form a single big one. Example: The second and third chromosomes of Drosophila melanogaster are both large chromosomes with two long arms. Other species of Drosophila have more, smaller, chromosomes. It is easy to see that chromosomes 2 and 3 of D. melanogaster were each formed from fusion of two of the smaller ones.

2. A chromosome might break into two to give two smaller ones. Like chromosome fusion, there is no great change in gene dosage or the amount of DNA present so this kind of stuff probably happens all the time.

3. Whole genome duplication. Sounds crazy but it happened at least twice during vertebrate evolution. The probability of any embryo surviving something like this is low, but with trillions of embryos over millions of years it happens now and again. Land vertebrates and higher fish basically quadrupled the whole genome of some primitive ancestral fish. This provides large quantities of redundant material for evolution, and much of the excess material can subsequently be lost. Google "genome duplication" for more info.

4. Presumably, if whole genome duplications can survive sometimes, then so could the occasional trisomy. Then you can have complicated things like duplications of part of one chromosome that get inserted into another and whatnot. Anything can happen. The point is that you are right; most of the time the offspring will be inviable or infertile, but rarely it'll survive and contribute its new chromosomes to the gene pool. Most of its offspring will probably die, but some will be OK, perhaps because of complementary defects acquired independently, and a new species is born. Remember: millions of years and billions of critters breeding in each one.

5. Newfangled and man-made. Google "mammalian artificial chromosome".
posted by nowonmai at 1:24 PM on January 20, 2006

Best answer: There was quite recently a great post on Pharyngula regarding this, specifically in regard to the fact that other apes all have 24 pairs of chromosomes, while humans have 23 pairs, and rebutting the attempt of an IDer to use this as evidence against evolution.

I strongly urge you to read the whole thing, but the short version is that human chromosome 2 is the result of the fusion of two chromosomes. This is not wild speculation, but is borne out by comparing the sequence of human chromosome 2 with the genetic sequences of other apes: the genes found on human chromosome 2 are the genes found on two different ape chromosomes. There are even remnants of telomeric sequences--in most cases found only at the ends of chromosomes--in the middle of chromosome 2.

As for the question of how this fusion gets fixed in a population, scroll down about 1/3 of the way in the post I linked to the part where he talks about Robertsonian translocation.
posted by DevilsAdvocate at 2:19 PM on January 20, 2006

I can't say much about the mechanism of changes in chromosome number during evolution, but I know that chromosomal changes do occur pretty frequently, although they usually result in nondevelopment or miscarriage of the individuals bearing them. My wife is involved in prenatal genetic testing and they see all sorts of things going on at the chromosomal level somewhere between gametogenesis and embryogenesis. About a quarter or so of the eggs that are isolated and screened for IVF have chromosomal problems. So, there is pretty steady source of chromosomal variety for whatever mechanism it is that allows them to get selected for later on.
posted by shoos at 2:34 PM on January 20, 2006

Since this question seems to have been answered, I'd just like to mention that the Bengal Cat breed involves mating two small cats (domestic + Asian leopard cat) with different numbers of chromosomes. The first few generations of males produced tend to be sterile; a proper Bengal is 15/16ths domestic.
posted by Aknaton at 5:15 PM on January 20, 2006

There are also cases where one organism absorbs the genome of another whole, adding it to it's own. Extremely common in single cell organisms and the origin of our own mitochondria (although these did not get absorbed into the nucleus).

Genome duplication mentioned above is pretty common in plants, you get haploid (one copy) and diploid (two) versions of a lot of plants.
posted by fshgrl at 6:19 PM on January 20, 2006

For examples of an animal absorbing another genome: some sea slugs have incorporated algal cells into their bodies and get their energy from photosynthesis.
posted by fshgrl at 6:21 PM on January 20, 2006

It should be noted, in regards to plants, that they are much more capable of coping with changes in chromosome numbers. Hell, some species come in varieties with 2x, 4x, 8x the "normal" number of chromosomes.
posted by Jimbob at 7:29 PM on January 20, 2006

Response by poster: Inbreeding is indeed the answer.

Ah, no wonder the creationists are against evolution! It's forbidden!

"Cursed be he that lieth with his sister, the daughter of his father, or the daughter of this mother..."
Deuteronomy 27:22

Thanks everyone. :)
posted by luftmensch at 9:34 PM on January 20, 2006

Ah, no wonder the creationists are against evolution! It's forbidden!

"Cursed be he that lieth with his sister, the daughter of his father, or the daughter of this mother..."
Deuteronomy 27:22

Well, interbreeding is as much a problem for creationism - Augustine claimed it only became forbidden in the third generation & beyond - Adam & Eve's kids had to screw each other, and then the next generation had to mate with their cousins... For him, of course, it's a question of morality, not pragmatic concerns about recessive genes, so as long as god said it was okay at the beginning, it was, and it's only once the population has spread that the later rule kicked in.

Of course, there's always this solution!
posted by mdn at 8:04 AM on January 21, 2006

And just as a PS: Pharyngula = awesome. Since relocated to Science Blogs but a great resource for anyone teaching biology, especially when teaching developmental.
posted by caution live frogs at 9:15 AM on January 21, 2006

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