>>This is the question I posed in my post of 22 Sep 2000 13:51:34 +0200 (asa-digest V1 #1804, 23 Sep 2000 09:20:01 -0000): how many specific amino acid substitutions can we reasonably expect in a random mutational walk before selection for a newly emerging activity sets in? Up to now, no one on this list has tried to deal with it.<<
The minimum number of amino acid substitutions required before selection operates on a new function is zero. The crystallins form important structural components in the eye lens in vertebrates and cephalopods. Some of them also function as enzymes and heat-shock proteins with the exact same gene and amino acid sequence. Other crystallins are very similar to corresponding enzymes but have diverged somewhat, apparently reflecting gene duplication followed by divergent evolution specializing for the different functions.
In general, the vast majority of new genes seem to be produced from manipulation of existing genes-mixing and matching parts, duplicating and then modifying, etc. Although this does not address the question of where the first gene came from, it does challenge the irreducibility of several systems. Many complex systems include genes that show signs of being duplicated from each other at some point. Obviously, the organism in which the duplication occurred had survived just fine with only a partial system.. Some of these duplications occurred very early in evolution, before the differentiation of Eubacteria, Archaea, and Eucaryota, including tRNAs, elongation factors, and aminoacyl-tRNA synthetases.
Dr. David Campbell
"Old Seashells"
Biology Department
Saint Mary's College of Maryland
18952 E. Fisher Road
St. Mary's City, MD 20686-3001 USA
dcampbell@osprey.smcm.edu, 301 862-0372 Fax: 301 862-0996
"Mollusks murmured 'Morning!'. And salmon chanted 'Evening!'."-Frank Muir, Oh My Word!
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