question about evolution theory

Jeff Lawrence lawrence at bioscience.utah.edu
Mon Apr 18 15:11:23 EST 1994


In article <sticknbd-160494113419 at 160.129.125.9>,
sticknbd at ctrvax.vanderbilt.edu (Henry) wrote:

> I am a molecular biologist, but not one who
> studies molecular evolution, so I ask the following
> question of the folks here.  In reading a review, I 
> came across the following statement:  "The issue of 
> the evolution of the [mammalian] metalloproteinase
> family can eventually be resolved by characterizing 
> the MMPs in more primitive species."  I do not doubt
> that we can learn valuable lessons from such analysis.
> However, because of the lack of linear evolution from 
> "more primitive" extant species to so-called "higher
> species," I am not convinced that the issue of evolution
> (per se) of a gene family can be resolved in this manner.
> That is, these genes have been evolving for millions of 
> years in all species; therefore, we cannot conclude that
> gene A in humans derived from gene B in, say, fish.  That
> they evolved from a common ancestor may be probable,
> but the linear relation implied by the quote above
> disturbs my sensibilities.  Can somebody with a better
> evolution background either confirm or deny my suspicions 
> that the quote is more than a little specious?
> sticknbd at miranda.cc.vanderbilt.edu

you're right in that gene A in humans cannot have evolved from gene B in
fish, and that, at most, we may say they evolved from a common ancestor. 
The original statement was, perhaps, poorly worded.  The point of looking
at gene B in fish, among other taxa, is to identify features *shared* by
multiple derived forms, and concluded that these forms were in the common
ancestor.  for example, mice, humans, and cows all have feet.  therefore,
we conclude that the ancestor of those three lineages, ancestor A, had
feet.  all fish do not have feet.  therefore, ancestor A evolved feet after
the divergence of fish.  this is not as simple as it sounds, because
someone sooner or later points out that whales don't have feet either. 
therefore, characters may come and go (like feet), or evolve to a similar
state by more than one means (like wings in birds and bees).  the job of
evolutionary biologists is to sort the meaningful patterns from the
background noise.  this is especially difficult in molecular evolution, as
one can imagine, in that the ancestry of homologous characters may be
difficult to ascertain (hard to line up sequences to identify base position
that are identical by descent), and there are only four character states
(G,A,T,C).  Despite these difficulties, molecular changes can be sorted out
to identify gene families and trace their lineages (for example, globin
genes among mammals or transposase genes among Eukarya and Bacteria).

jeff

--
jeff lawrence
university of utah
dept of biology
lawrence at bioscience.utah.edu



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