Convergent evolution

From: Dr. David Campbell <amblema@bama.ua.edu>
Date: Mon Nov 21 2005 - 18:24:29 EST

Cleaning out old messages, I ran across a question about convergent
evolution but did not find much of an answer. I've seen it invoked as
a problem for evolution in various sources, but in fact convergence is
expected to be common in any evolutionary model.

Different types of convergence may be in view. The classic type is
convergence on a similar solution to the same problem. This
convergence is driven by the shared selective force, and possibly also
by shared genetic heritage and/or strutural constraints. For example,
squid, arrow worms, tuna, mako sharks, ichthyosaurs, dolphins,
penguins, and other rapid swimmers have fairly similar shapes evolved
independently in response to the same hydrodynamic forces. Despite
the external similarities in shape, internal anatomy, DNA, and other
features differ greatly. The support for and configuration of the
tails provides one example. This relates to the disparate methods of
propulsion-water jet in the squid, lateral body flexing in the arrow
worm, shark, and tuna, vertical body flexing in the dolphin, and limbs
in the penguin. Some molecular convergence falls under this, such as
relatively high GC/AT ratios in thermophiles.

There's also the convergence that occurs because of the limited number
of options relative to the number of organisms. The random
convergence of DNA, as in long-branch attraction, is a classic
example. There are only 4 DNA bases (a deletion could count as a 5th
option), so random or practically randomized (relative to each other
for the aspect of interest) sequences will have a 1 in 4 chance of
similarity at any one spot. Another example would be the apparently
limited number of basic body plan types. Roger Thomas and co-authors
have done some work on this. Making an efficient forelimb for rapid
seizure of prey, starting with arthropod appendages, gives limited
options. The bent forelimbs of mantis, mantis flies, and mantis
shrimp account for the common name commonalities, but the rest of the
body is quite different.

Convergence on the same result, despite a large design space, is very
likely when common descent is taken into account. Organisms
ultimately starting from the exact same point in design space have a
pretty good chance of ending up not too far apart in design space.
For example, some of the similarities of cephalopod and vertebrate
eyes reflect independent development of good lenses and other features
that the laws of physics dictate to be useful for seeing images.
Howver, many of the same fundamental genes are involved. Some of
these genes may have functioned in a very primitive eye in the common
ancestor (probably no more than detecting light versus dark); others
may have had other functions but were co-opted evolutionarily as they
turned out to be useful. Examples of parallel evolution, such as the
sequence of microstructure changes in mollusks, provide additional
examples of this.

-- 
Dr. David Campbell
425 Scientific Collections Building
Department of Biological Sciences
Biodiversity and Systematics
University of Alabama, Box 870345
Tuscaloosa AL 35487-0345  USA
Received on Mon Nov 21 18:26:38 2005

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