Re: 'Directed' evolution?

Tim Ikeda (
Mon, 21 Sep 1998 22:16:31 -0400

Howard Van Till writes:
>It may be the case that Denton's _Evolution: A Theory in Crisis_ was either
>misunderstood or misrepresented by many. Correct me if I am wrong on this,
>but I believe his criticisms there (whether valid or not is another
>question) were mostly about undirected Darwinian gradualism. He had no
>objections to the concept of evolutionary continuity, just to certain types
>of attempts to give a scientific account of it.

Howard's assessment seems correct.

After the publication of his second book, Denton mentioned that perhaps
a better title for his first book would have been "_Darwinism_: A theory
in crisis".

Re-read from that point of view, one can interpret his first book as
a tirade against Darwinism. However, he still did a hack-job in the
sections about molecular similarities. Considering the data available
at the time it was quite clear that he did a poor job there. Also, I
think it was far too easy to initially interpret that book as a
critique of evolution. Given that so many missed the point, I think
that most of that fault falls on the author. Fortunately, his later
book seems to have corrected many of those problems...
While generating some new ones, it appears.

Howard, since you've got Denton's book in front of you, let me ask
you how Denton's beliefs could be reconciled with gene recruitment
in the evolution of lens crystallins. I'm pulling this straight out
of Graeme Wistow's article, "Identification of lens crystallins: a
model system for gene recruitment" (Methods in Enzymology Vol 224,
Academic Press, 1993. pp563-575). Crystallins are soluble proteins
found in high concentrations in the lens of vertebrate and in-
vertebrate eyes which aid in the refraction of light. Many of the
taxon-specific crystallins identified are multifunctional. Not
only do they serve to refract light in the lens but many also
have distinct enzymatic activities used elsewhere in other tissues.
The diversity of these activities (& sequences) clearly indicate that
they had different origins and probably arose by the mechanism of gene

So in this example, we have a common function (light refraction in the
lens of an eye), that was generated in different organisms by different
proteins. Yet, Denton seems to suggest that the directed, "unfolding"
of life through evolution was deterministic and tightly controlled.
"Just the right thing for just the right place" might be a way of
paraphrasing this view. However, with lens crystallins, it appears
that there are many "right things" for the "right place". Many things
seem capable of working in the lens and it's hard to see how one protein
would be "just right" in one case while in another it would not. I think
this shows that the options available for evolutionary change might be
more open than Denton supposes.

Tim Ikeda (despam address before use)