Heath Anderson

Trying to explain this to my brother - does anyone have a good website that may offer me some help. I'm horrendous at explaining things (hense he's training to be a teacher and I'm not ) This was his argument:

A population has a genome 2n (he is of course automatically assuming diploidy is universal - not necessarily so). If an organism comes along and has a mutation 2n + 1, (by mutation - I told him that most likely that would kill the organism, or severly retard it) it would have to mate with another organism of 2n +1, in order for the offspring to be 2(n+1), and thus create a new chromosome number and the starting point of a new species - something that is not likely to happen.

It tried to explain that it doesn't work like this, and that evolution is a bit more gradual and subtle than that, macromutation involving the use of entire new chromosomes doesn't normally produce viable/fertile offspring and that to change a genome of a population you need a lot more time, usually geographic isolation and all the rest of it. He still remains unconvinced. What I really need is a little bit of reference. The only one that came up in my use of MSN Search was <a href="http://www.grisda.org" target="_blank">www.grisda.org</a> - people who like to use the 'filter of revelation'

Can anyone provide me with some reference material/websites?

Grazie!

Jerry Smith

Heath, it may take a while, but I will do my best to find you a good explanation with examples. Someone will probably beat me to it, but either way: keep checking back here.

Jerry Smith

Oh, and here is a small start:

<a href="http://www.ndsu.nodak.edu/instruct/mcclean/plsc431/chromnumber/number7.htm" target="_blank">Cabbage & wheat</a>

RufusAtticus

Changes in chromosome number are due to either polyploid events, hybridizations, fusion or fission.

The problem with changing chromosome numbers is it's effects on meiosis. Most events screw up the pairing phase, resulting in inviable gametes. It's the rare ones that allow individuals to still be (somewhat) fertile.

~~RvFvS~~

MrDarwin

The situation is very different for plants and for animals. Most plants can tolerate tetraploidy, triploidy, aneuploidy, and sometimes even haploidy. In fact, plant species with different chromosome numbers can frequently be crossed, and the progeny are frequently fertile (although they generally exhibit reduced fertility). Animals (mammals, at least) tolerate much less chromosomal aberration, and aneuploidy seems to be the primary difference between related species with different chromosome numbers.

I don't have much time to explain right now but I hope somebody will talk about the example of chimps and humans. In this case, the different chromosome number has clearly come about by the fusion of two chromosomes into one. In such a case, since there are no "extra" or "missing" chromosomes, and there has been little (or no) loss of genetic information, proper chromosomal pairing may have been able to take place, and the first individual with the different number may have been fully or slightly fertile; or it may have had siblings with the same aberration with which to mate (in which case speciation may have been more or less instantaneous). Most animals do not have the taboos against incest that we do, and in small or isolated populations would be almost unavoidable.

MrDarwin

BTW 2n + 1 would be trisomy, which would almost certainly result in a sterile individual (and in animals is frequently the reason for non-viable embryos, or individuals with severe disabilities, like Down Syndrome). True aneuploidy in the evolutionary sense is either n + 1, or 2n + 2.

I'm willing to bet that in virtually all cases of aneuploidy, it results (as I described above) from the fusion or splitting of existing chromosomes, rather than the duplication (or loss) of an existing chromosome. It's the genes that are important; the chromosomes are simply how they're arranged and packaged.

theyeti

It's also worth noting that chromosome number often differs markedly within a species, and not just with plants. See for example <a href="http://www.ncbi.nlm.nih.gov:80/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=117372 77&dopt=Abstract" target="_blank">1: Heredity 2001 Sep;87(Pt 3):305-13</a>, The effects of Robertsonian fusions on chiasma frequency and distribution in the house mouse (Mus musculus domesticus) from a hybrid zone in northern Scotland. And see also the <a href="http://www.ncbi.nlm.nih.gov:80/entrez/query.fcgi?db=PubMed&cmd=Display&dopt=pubmed_pubme d&from_uid=11737277" target="_blank">related articles</a> page; this particular example has been well studied. There are dozens of different karyotypes based on a few fusion variants and lots of hybridization.

Similar chromosomal polymorphisms are found amongst a <a href="http://www.ncbi.nlm.nih.gov:80/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=118332 88&dopt=Abstract" target="_blank">grasshopper</a>, a <a href="http://www.ncbi.nlm.nih.gov:80/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=109371 94&dopt=Abstract" target="_blank">spider</a>, a <a href="http://www.ncbi.nlm.nih.gov:80/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=107918 13&dopt=Abstract" target="_blank">gall inducing insect</a>, a <a href="http://www.ncbi.nlm.nih.gov:80/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=111173 56&dopt=Abstract" target="_blank">fish</a>, and an <a href="http://www.ncbi.nlm.nih.gov:80/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=968851 3&dopt=Abstract." target="_blank">iguana</a>. These are just some of the more serious ones that turn up in a quick search; there are lots more.

As for humans and kin, check out the recent article: <a href="http://www.ncbi.nlm.nih.gov:80/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=118237 92&dopt=Abstract" target="_blank">Nat Rev Genet 2002 Jan;3(1):65-72</a>, Segmental duplications and the evolution of the primate genome.

theyeti