The Atheist Blogger

Popular atheist biology professor turned blogger PZ Myers has written a response to a question put to him in an email:

How did life evolve from one (I suspect) chromosome to… 64 in horses, or whatever organism you want to pick. How is it possible for a sexually reproducing population of organisms to change chromosome numbers over time?

Firstly: there would have to be some benefit to the replication probability of the organisms which carry the chromosomes. I don’t see how this would work. How is having more chromosomes of any extra benefit to an organism’s replicative success? Yes, perhaps if those chromosomes were full of useful information… but the chances of that happening are non existent and fly in the face of ’small adaptations over time’.

Secondly, the extra chromosomes need to come from somewhere. I’m not sure about this, but I believe chromosome number are not determined by genes, are they? There isn’t a set of genes which determines the number of chromosomes an organism has. So the number is fixed, determined by the sexually reproducing parents. Which leads me to believe that if the number does change, and by chance the organism is still alive and capable of sexual reproduction, that the number will start swinging back and forward, by 1 or 2, every generation, and never stabilising. The chances of this happening are also very very slim.

It’s an important question, and a lot of people (myself included) have wondered about it. I’ve often searched for a answer online but found nothing, so I’m glad PZ has swooped in with an easy to understand (albeit lengthy and descriptive) answer.

If you thought you knew a lot about Evolution, prepare to be amazed…

Basics: How can chromosome numbers change?

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