Re: [asa] sea urchins

From: David Campbell <pleuronaia@gmail.com>
Date: Mon Sep 22 2008 - 14:20:48 EDT

"Sea urchins appear in the fossil record at the beginning of the
Cambrian Explosion about 540 million years ago"

Not hardly. Echinoderms turn up pretty early, but actual ancestral
urchins come a bit later, and things that look reasonably like a
modern sea urchin later still. It's not actually important to their
argument, but it does reflect not knowing what they're talking about.

Basic problems:
The false assumption that the urchin does not use the genes in
question. They do have rudimentary light sensing. The claims are too
vague for me to determine just what genes they are talking
about-relating to light sensing? genes used in building an eye? etc.
 Different components of the system have different alternative uses.
Likewise, urchins do have some sort of immune capacities. The fact
that simpler or just different systems can function with fairly
similar genes is an argument against irreducible complexity, not
evidence in support of ID.

The gene tally is not especially meaningful on two counts
Urchins do a bunch of things we don't, e.g. have radically different
larval and adult body plans, grow spines, make mutable connective
tissue, make a water vascular system. They doubtless have a bunch of
genes very different from ours and vice versa.
Gene regulation is as important if not more important in complexity
than gene number. Vertebrates have some fancy gene regulation
techniques.

Echinoderms are part of the sister clade to Chordata. As such, they
may be expected to have a lot of similar genes.

The specific sort of model they cite for an ancestral metazoan has
been pushed too far by some. Genes shared between more distantly
related organisms (such as humans and fruit flies or nematodes or sea
anemones) probably had their origins in the ancestor of the entire
group. However, just because related genes function in eyes of say,
vertebrates and flies, doesn't mean that gene was functioning in
making eyes in the common ancestor-you need to check out all that it
does.

There actually are ways to model how a gene might have functioned in
the past (through comparison of sequences from different descendants)
and test it by constructing the protein in question and seeing how it
functions in vitro or in vivo.

-- 
Dr. David Campbell
425 Scientific Collections
University of Alabama
"I think of my happy condition, surrounded by acres of clams"
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Received on Mon Sep 22 14:21:18 2008

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