Re: [asa] sea urchins

From: David Campbell <pleuronaia@gmail.com>
Date: Mon Sep 22 2008 - 18:12:14 EDT

I also note that the idea of convergent evolution is not apparently
understood by the author of these notes. It's an issue with the sea
urchin bit, but pretty much the entire substance of the entry on
placental-type embryonic nourishment. In addition to the examples
given, there is placental-like feeding of developing young by the
mother in certain sharks and even in some snails and other
invertebrates. Typically, there are close relatives with less
developed systems such as ovovivipary (i.e., eggs are retained in the
female until they hatch, resulting in birth of live young, but there
is no direct feeding of the young with maternally derived tissue).
The evidence strongly supports multiple separate evolutionary origins
of placental vivipary from somewhat simpler systems.

Convergent evolution is an obvious expectation in any evolutionary
system and not merely ad hoc, though of course reasonable
justification for the claim is needed. All organisms deal with the
same laws of physics; common ancestry al;so means that they have the
same basic genetic system and some of the same basic genes, and
general structural similarities. Thus similar solutions to the same
problem will occur numerous times, just as my great grandfather
invented automobile ignition (i.e., starting it with a key instead of
turning a crank out front) independently of the fellow who got in a
patent just before him; presumably both wanted to sit in the car and
start it instead of being outside and then running around to the
driver's seat. A good argument for convergent evolution would include
both a demonstration that the features are useful and that they have
separate origins.

-- 
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 18:12:52 2008

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