Chromosomes, Drift, and Demes

One of the characteristics of evolution is change in chromosome number and organization. These large-scale changes are often associated with speciation events although it would be a mistake to assume that there's a causal relationship.

One particular chromosomal rearrangement has been getting a lot of press recently because it has been featured on blogs and in some recent trade books on evolution.

Humans (H) have only 23 pairs of chromosomes while most other apes, such as the chimpanzee (C), have 24 pairs. Evidence for a fusion of two of these ancestral chromosomes into a single chromosome 2 in humans has been well supported by genome sequence data. Our fusion chromosome contains remnants of telomeres at the fusion point and it has another centromere-like region at just the right position.

Intelligent design proponents have a hard time explaining this event. They don't propose an explanation based on their concept of intelligent design—that would be too ridiculous—instead they concentrate on raising questions about evolutionary explanations. One of the common objections is that the new fusion chromosome would screw up mitosis and meiosis because it would initially have two centromeres. According to them, the chromosomal rearrangement would be detrimental and could never be fixed by natural selection.

As it turns out, the latter part of this statement is correct. Natural selection is not responsible for this kind of evolution. But no serious scientist would suggest otherwise.

The first part of the statement isn't as serious as the IDiots would like to think. Rearrangements of this sort aren't much of a problem. Many species are heterozygous for such rearrangements and we can see that it has little effect on the viability of dividing cells. Nevertheless, a newly rearranged chromosome is unlikely to be completely neutral. It's probably slightly deleterious with respect to the ancestral chromosome(s).

So, how does a slightly deleterious mutation become fixed in a population? The answer, of course, is random genetic drift. But in order to understand the importance of random genetic drift you have to understand the substructure of species. Species are usually subdivided into many smaller, locally inbreeding, populations or "demes." Slightly deleterious (nearly neutral) mutations can easily become fixed in a deme by accident.

Over at Panda's Thumb, Dave Wisker Arthur Hunt has been writing about chromosomal rearrangements and how they become fixed in a species. His latest essay talks about how fixation within a deme can lead to fixation by random genetic drift in the entire population: The Rise of Human Chromosome 2: Beyond the Deme.

This is a nice, short, explanation of a very important mechanism of evolution. I urge everyone to get on over to Panda's Thumb and read it right now.

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[Image Credit: This drawing is from: Chromosome Fusion: Chance or Design?. I don't know the original source.