Re: How do species evolve different numbers of chromosomes?

Hello there!

It is true that different species have different number of chromosomes. Throughout evolutionary history, organisms have changed the numbers and organization of their chromosomes in a variety of ways. For instance, humans have 46 chromosomes while our close relative the chimpanzees, gorillas, and oragutans each have 48. This suggests that sometime in our the ancestry of our human race, two pairs of chromosomes fused together. One could imagine that events like this in other evolutionary scenarios might involve chromosomes duplicating or breaking into two. However, it is important to realize that each individual chromosome is very important to an organism's survival, and a new species probably could do without almost all of it's "parent" species chromosomes.

Such observations are made even more exciting by the popular theory that chromosomal rearrangments play an important role in the creation of new species. When these rare random breaking/fusing/duplicating events occur, they are random and occur in the gametes (sex cells) of a parent. When that individual reproduces with one of its fellow species, the offspring will carry the "normal" chromosome from the unaffected parent, and the fused/broken/rearranged chromosme from the other parent. We call these types of organisms heterozygotes. This mis-match of chromosomes is probably unhealthy, because recombinants between the two chromosomes would yield even more unusual chromosomes, and that animal would be relatively unfit for survival. In light of this, it might seem that it would be very difficult for this rare heterozygote to proliferate and form a new species. So the other requirement for this new species to emerge from the chromosomal rearrangement is that heterozygote brothers and sisters from the parent with the unusual chromosome must mate (inbreeding). Through inbreeding, the rare chromosomal rearrangement may come to exist in both affected chromosomes of one of the next offspring (homozygote). Now the homozygotes have the potential to mate among themselves and eventually generate a new species that only has the rearraned chromosome and not the orginal one. In this way, rearrangements may eventually result in a new species as long as inbreeding among siblings can occur. Interestingly, there has been a study that suggests that species that exist in social groups, that facilitate inbreeding, have a higher rate of speciation.

I hope this helps to answer at least part of your question. The source that I consulted to answer your question was the textbook EVOLUTION, by Mark Ridley, Blackwell Scientific Pulications, c. 1993, pp. 440-442. If you would like further clarification, you might consider finding a copy of this book. Best wishes!