My limited understanding of evolution is based on the notion of genetic mutations where those mutations which confer some survival advantage are naturally selected over time.
But species have different numbers of chromosomes. A frequently cited example is that humans have 46 and chimpanzees have 48. To me, a change in the number of chromosomes is hugely different than a change in an individual gene.
How does the theory of evolution explain the descent of a species with a different chromosome number than its ancestors?
Basically, two chromosome pairs didn’t copy correctly and ended up fusing into one.
From here:
Animals with different numbers of chromosomes can sometimes reproduce and produce fertile offspring, if the remaining chromosomes are similar enough. You aren’t going to get something with 12 chromosome pairs breeding with something that has 32, but 23 and 24 are pretty close.
ETA:
This article has more details, and even has a nice pic showing the evidence that two fused into one.
In the specific example cited, Humans and Chimps, oddly enough there are two chromosomes in humans which are quite similar to one in Chimps, but seem to be joined into a single one, that one having a region in the middle very similar to the termination sequences of the Chimp chromosomes.
Species gain and loose whole chromosomes by individual members experiencing genetic accidents, like non-disjunctions, or breaks. Mostly those individuals are never born. Of those that are born the overwhelming majority die very young. Of that tiny surviving percentage, almost all fail to have offspring. That extraordinarily small number of individual members pass on their abnormal genes, and almost all of their progeny also die. Among the survivors, many do not pass on the abnormal genetic information. Another extremely small percentage of such cases actually have multi-generational descendants who have a small chance of finding a niche in the habitat they live in that allows them to survive. Wait a thousand years, and in most cases, the particular resultant mutation is less common in that area, and if it is isolated from members of the original species, it has begun the long process of becoming a separate species. It will probably die out sooner or later, most species do.
That’s not really true. There are numerous triploid or tetraploid animals that retain fertility with the diploid form. In plants the case is even more extreme, and organisms with anything up to 16 copies of each chromosome retain fertility. Chromosome count doesn’t actually appear to be a major barrier to fertility provided that the chromosomes all retain some homology in both partners.
As to the question of why differences in chromosome number occur, the activity and action of genes can vary depending on where they are on a chromosome, and which other genes they are near. So that once a chromosomal division or fusion occurs, it can be selected for or against just like other genetic variations.
I was about to start a new thread, but searched first, and this is pretty close.
However, to broaden the question:
Is it common for speciation to involve differing numbers of chromosomes? It doesn’t seem like it would be.
What about increasing number of chromosomes? Two chromosomes fusing kind of makes sense to me, and I could see how it would happen even if only one parent had the lesser number of chromosomes (with one twice as big). How would it go in the other direction? How could one parent with a chromosome that ‘broke in two’ produce offspring like that?
Sometimes an error causes the production of an extra pair of chromosomes. Usually the effect is just a genetic illness, but sometimes it presumably helps the organism and like any other successful mutation it begins to spread.
Column A
Usually if a chromosome breaks the fragment not attached to a centromere is lost. Sometimes that centromere-lacking fragment attaches itself to a completely different chromosome instead.
Column B
Sometimes non-disjunction results in an extra copy of a chromosome.
Take Column A + Column B and hypothetically a trisomic individual could have one of the extra chromosomes break, leaving a small fragment with little more than a centromere.
Over generations that centromere fragment gets passed down and could accumulate translocations (per Column A process).
It works on paper. I have not had the time to research to see if there is evolutionary evidence of this process.