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Re: Homology - Ripe for burning?
In a message dated 9/8/98 10:26:17 PM, John V Jackson wrote:
<<In the latest "Nature" (3 Sept 98, pp17-18, V395) Diethard Tautz writes a
piece on the difficulties ? and possible pitfalls ? of interpreting
homologies, which will be of interest to many list members.
He stresses that homologues at the molecular level are [] confusing, but
says that with biotechnology companies getting involved, ". . . it will
eventually be investors and shareholders who have to be convinced that
invoking homology is valid". At last! People for whom the right answer is
vital will start to get involved!>>
Wishful thinking, I'm afraid. Beguiling investors is standard procedure for
biotech companies.
<<There could be dormant genes or ?latent homologies? that are not expressed
in a stem species, but that have been regained after further speciation and
give the erroneus impression of convergence." This is the mechanism sometimes
used to account for the "mosaic" distribution of characters amongst Archie ?
Troodon ? droms.>>
This is reasonable, but so far untested, given the lack of Dino DNA. On the
other hand, as discussed on another thread, the key endothermy gene for a
heat-generating protein has been found in both humans and the weed,
Arabidopsis. Latency seems likely within the intervening half-billion years
worth of speciation.
<<"In tetrapods, digits 3 and 4 are the first to develop and are also the ones
that are most stable against perturbations. In [newts and salamanders] the
same seems to be the case for digits 1 and 2. So if 3 and 4 are homologous to
1 and 2, digit 3 would be equivalent to digit 5 in [other] tetrapods and
digits 4 and 5 in [newts etc] would be novel structures. . . . Closer study of
developmental characteristics and comparative analysis of Hox gene expression
seem to confirm this view." I?m surprised ? but the chick digits thing always
was going to have a surprising solution one way or another ? not that I?m
saying this is the solution.>>
I'm not surprised. I have always regarded Feduccia's bird-dino digit ravings
to be unfounded. Have you ever seen a six-toed cat? Evolution of new digits by
accident seems a relatively quick sort of event. Humans with inherited
polydactyly usually do it differently from the cat, having an added pinky,
where the cat goes for an extra thumb. So digits come and digits go. Heck,
Feduccia might even be RIGHT about birds having a different set of digits than
theropods, and STILL be wrong about his contention that birds did not arise
from dinos.
<<He ends with: "Homology, then, is an idealised principle that works under
idealised conditions, but such conditions almost never apply.>>
However, in the fullness of time, the vast genome sequences will provide
another element that Tautz chooses to ignore: redundancy. Once we can compare
a billion or so bases of sequence data, rather than the hundreds that are
typically used for current homology arguments, the answers will no longer be
lost in the vageries of statistics. They will become clear.
<<One lesson is clear however ? homologies are a minefield, and we will
usually be wrong if we approach them simplistically ? and mechanically.>>
Amen. Several that I have published have caused me later anguish. I think I'll
wait about ten years until human, chicken and about a dozen other genomes are
entirely sequenced. And then I won't have to make arguments regarding the
relationship of digit I between hen and alligator. Someone else will be
reading it off, clear as a bell.
Tom Hopp