Thread: The Variation of Chromosome Count

Hello everyone,

I'm new to the forum and this will be my first post. This is a very elementary question, and if you have any recommended reading for basic evolutionary biology, it would be appreciated!

My question concerns evolution and the variation of chromosome count. I can understand a benign random genetic mutation contributing to the overall strength of a particular species, but the emergence of entirely new species with a new number of chromosomes remains unclear.

While we see chromosomal abnormalities in humans fairly frequently, these abnormalities seem to be accompanied exclusively with significant disruptions in the expression of advantageous traits. Second, wouldn't an organism with an entirely new number of chromosomes require a mate with the same number of chromosomes to reproduce? In other words, wouldn't the same chromosomal abnormality have to occur simultaneously in at least two members of a given population in order for it to spread?

Originally Posted by robj37

Second, wouldn't an organism with an entirely new number of chromosomes require a mate with the same number of chromosomes to reproduce? In other words, wouldn't the same chromosomal abnormality have to occur simultaneously in at least two members of a given population in order for it to spread?

Not really so much of a problem in organisms that can self-fertilise, such as many plant species. In animals interbreding between siblings (may happen when population levels are low) is one way this problem can be overcome.


More generally, unequal chromosome numbers is not a complete barrier to successful breeding. Three examples that spring to mind are humans with Down's Syndrome (ignore the syndrome itself and consider instead the unequal chromosome counts), the house mouse Mus domesticus and the zebra-like Okapi (Okapia johnstoni). Chromosome number is not a complete barrier, but rather one that is leaky.

Originally Posted by Zwirko

Originally Posted by robj37

Second, wouldn't an organism with an entirely new number of chromosomes require a mate with the same number of chromosomes to reproduce? In other words, wouldn't the same chromosomal abnormality have to occur simultaneously in at least two members of a given population in order for it to spread?

Not really so much of a problem in organisms that can self-fertilise, such as many plant species. In animals interbreding between siblings (may happen when population levels are low) is one way this problem can be overcome.


More generally, unequal chromosome numbers is not a complete barrier to successful breeding. Three examples that spring to mind are humans with Down's Syndrome (ignore the syndrome itself and consider instead the unequal chromosome counts), the house mouse Mus domesticus and the zebra-like Okapi (Okapia johnstoni). Chromosome number is not a complete barrier, but rather one that is leaky.

What is the rate of mutation? Is mutation an error that's occurred or the introduction of something entirely new?