Counting Chromosomes

Question from Ken:
I’m agnostic. There’s one thing I’ve always wanted to understand about evolution for a while, and I hope this is the place to get it answered.

When I learnt about it, in school and university, they expressed the idea of increment changes (whether modifications of traits that were already there, or new traits through mutation) over time, passed on through generations, “chosen” by the ability to procreate and pass on genes. That, I think I get for the most part.

But what I never really understood is how the number of chromosomes could change over time. How can chromosome numbers increase and decrease? I mean… let’s say an individual somehow has it happen, shouldn’t he or she be unable to produce viable (or fertile?) offspring with others of the species because of the wrong numbers of chromosomes? How does chromosome number change at all? The only way I know is through issues that happen in meiosis, the kind of stuff that causes Down’s Syndrome, and the XXY and X gender chromosomal abnormalities that cause problems in humans.

So, my question is: How can chromosome number change in to produce a viable, fertile individual for it to be able to spread to an entire species?

Answer by SmartLX:
It’s a good question because a sudden whole extra chromosome full of junk, or a whole one gone missing, can indeed cause serious defects. That said, it helps to remember that a chromosome is merely a container of genes, and the number of chromosomes has very little to do with the amount of genetic information in each.

The addition of a chromosome is the more complex process, so I’m linking to an explanation of one mechanism by PZ Myers. Essentially, one chromosome’s worth of genes ends up being shared by two, and at first it can interact just fine with the old combined chromosome because the total sequence is the same. This does introduce a higher rate of error until individuals with the split chromosome start mating with each other, at which point there’s no longer a downside. Once the new number of chromosomes is settled, each chromosome is free to mutate independently and add new genetic information in the usual ways.

As for a reduction in chromosomes, we need look no further than our own genome. Our #2 chromosome pair is equivalent to two separate chromosome pairs in our closest ape relatives, but fused together. You can tell because there are two end markers (telomeres) right in the middle of it. No genetic information was lost, it was merely repackaged. This, importantly, is a very clear example of the kind of predictions one is able to make using evolutionary theory: the number of chromosomes in our genome compared to our ancestors’ (i.e. one less) tells us that exactly one fusion must have occurred, and we can then check the genome for only one extra set of markers.