Different numbers of chromosomes?

Hi Ryan,There are a number of possibilities - chromosome fusion being one of them. Polyploidy (chromosome doubling) can also happen, although it's waaay more common in plants than in animals. As long as the mutation/doubling doesn't present a reproductive barrier, then the organisms are usually still considered the same species. The point is "species" is something of a squishy term - most are composed of discrete populations (called demes on the genetic level, or clines on the population level), that blend into each other where these populations overlap.If you'd like to look up further references, and have access to a decent library, here are three articles that you might find interesting from the journal Heredity:Castiglia, R and Capann, E wrote a series of three articles for the journal Heredity describing this observation:
Contact zones between chromosomal races of Mus musculus domesticus. 1. Temporal analysis of a hybrid zone between the CD chromosomal race (2n=22) and populations with the standard karyotype.
Heredity. 1999 Sep; 83:319-26Contact zone between chromosomal races of Mus musculus domesticus. 2. Fertility and segregation in laboratory-reared and wild mice heterozygous for multiple robertsonian rearrangements.
Heredity. 2000 Aug; 85:147-56Contact zones between chromosomal races of Mus musculus domesticus. 3. Molecular and chromosomal evidence of restricted gene flow between the CD race (2n = 22) and the ACR race (2n = 24).
Heredity. 2002 Sep; 89:219-24.If you're lazy (like me ) I posted links to several more articles discussing the genetics of incipient speciation in this post, although not all are dealing specifically with chromosome (karyotype) changes.If you have any further questions, let us know. Happy reading!

Taz:A certain not to mentioned temporarily suspended hairy proboscidean asked me to tell you: [This message has been edited by Quetzal, 04-03-2003]

Hi judge,Although schraf is the horse expert here, I am aware that they have produced fertile hybrids from crosses between 2n=64 domestic horses (Equus caballos) and 2n=66 Przewalski's horse (E. przewalski). The hybrids do show the 2n=65 haplotype. However, this does not contradict the species concept. Remember, post-zygotic barriers that differentiate true "species" may not appear in the first hybrid generation (I posted this in some other thread which I've now lost track of). The fact is that the hybrid line doesn't breed true: the hybrids breed back to 2n=64 the next generation. Apparently, the modern haplotype is dominant.This makes a bit more sense if the people who study equids are right and Przewalski's represents an ancestral (if not direct ancestor) to caballos. Hope this answers your question.(edited to add: And besides, the biological species concept has the caveat "in the wild". Since Przewalski's has been extinct in the wild for quite a while, any hybrid produced as part of a captive breeding program with modern horsese doesn't invalidate the concept.)[This message has been edited by Quetzal, 04-04-2003]