From: bivalve (bivalve@mail.davidson.alumlink.com)
Date: Mon Apr 28 2003 - 19:49:15 EDT
>How does evolution change the number of chromosomes? I have no problem with adaptations between species with the same number of Chromosomes. But how does the number get increased?<
There are three general categories of chromosome number change. On the one hand, division or fusion of chromosomes can change the number of chromosomes present. For example, there are races of mice in isolated valleys in the Alps that have chromosome fusions relative to standard mouse genomes. Likewise, a single chromosome fusion accounts for the chromosomal difference between us and apes.
Secondly, chromosome number can change through duplication or deletion of chromosomes.
Finally, polyploidy or aneupoidy resulting from atypical fertilization or errors in mitosis or meiosis can result in changes in chromosome number. Downs syndrome is an example of having an extra chromosome in humans. Wheat is a successful example of extra chromosomes through polyploidy. It has three sets of chromosomes and evidently originated through two hybridization events.
>Advanced creatures have many more than primitive. Absolutely necessary functions are in completely different places.<
Advanced and primitive are somewhat problematic terms, but high chromosome numbers occur here and there among many types of organisms. For example, some tiny clams have close to two hundred chromosomes. As parthenogenetic polyploid hybrids, they do not have to sort the chromosomes for meiosis. Likewise, the total amount of DNA does not directly relate to evolutionary complexity; some things just have a whole lot of DNA without apparently doing much with it. The total number of distinct genes is a better proxy for the organismal complexity. In this regard, we, and other vertebrates, rank fairly high.
The puzzle for groups in which chromosome number is highly variable is why, not how. Changing chromosome number is not that hard, though it may be difficult to reproduce successfully if self-fertilization or asexuar reproduction are not options. Why certain taxa vary widely whereas others show hardly any differences is not known to me.
Dr. David Campbell
Old Seashells
University of Alabama
Biodiversity & Systematics
Dept. Biological Sciences
Box 870345
Tuscaloosa, AL 35487-0345 USA
bivalve@mail.davidson.alumlink.com
That is Uncle Joe, taken in the masonic regalia of a Grand Exalted Periwinkle of the Mystic Order of Whelks-P.G. Wodehouse, Romance at Droitgate Spa
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