> Subject: Evolvability of new functions
David Campbell, "bivalve" <bivalve@mail.davidson.alumlink.com> wrote:
> Date: Fri, 20 Oct 2000 11:44:02 -0400
>
> >>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.
>
This is not evolution of a novel function, but cooptation of an existing
enzyme for another function, as its concentrated solution happens to
form a transparent gel, usable immediately as a crystallin. Of course,
selection can operate at once in such a case.
> 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.
>
The origin of a new protein by exon shuffling may also be considered a
cooptation of a set of preexisting functionalities. This also applies to
duplicates of genes happening to already possess an initial minimal
activity of a new kind. In such cases, selection is possible from the
start. This is microevolution and does not pose any informational
problems.
But to assume that ALL functionalities emerged in such a manner, without
any non-selectable intermediates, is entirely speculative. How do you
know this is "the vast majority" of genes? You yourself concede that the
origin of "the first gene" is not dealt with. There are an estimated
1000 different protein folds (each grouping a series of protein families
or superfamilies) in the biosphere, considering the globular,
water-soluble proteins only (Y.I.Wolf, N.V.Grishin, E.V.Koonin,
"Estimating the number of protein folds and families from complete
genome data", J.Mol.Biol. 299 (2000), 897-905). Almost by definition,
these 1000 folds are not related to each other by exon shuffling and
gene duplication. Each one of them had to originate somewhere at least
once during the past 3.8 billion years. Thus, it would be more realistic
to talk about "the first 1000 genes" whose emergence cannot be accounted
for at present. These are the cases I am considering when I talk about a
mutational random walk without intermediate selection until a minimal
selectable activity happens to be produced. These are cases I consider
macroevolutionary steps posing considerable informational problems
deserving careful attempts at estimating their probability and at
possibly finding more realistic evolutionary scenarios than merely
assuming that "it must have happened somehow" through selectable
intermediates. You may call these the most elementary cases of Behe's
"irreducibly complex systems" - whose non-existence has not yet been
made plausible.
Peter Ruest
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