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ATTENTION EVERYONE! ..."Trends" in Dinosaur evolution [was: Re: Dinosaurs to birds]
PLEASE, everyone take a good long look at the following quote
recently put forth by the dinolists own George Olshevsky. Know this quote,
love this quote. This is a good point:
>Trends in evolution are an illusion fostered by your selecting a particular
>lineage from the tree of life from among all the billions of other lineages.
>Select a crocodile, and suddenly you see trends to ever better
>"crocodileness"; select a bird, and suddenly you see trends to ever better
>flight, etc. Select Homo sapiens, and suddenly you see trends toward greater
>intelligence, bipedality, etc. There is no way to predict which lineages will
>survive and diversify and which will become extinct. It is a random process.
It seems to me that the idea of "trends" results from the a priori
assumption that evolutionary rates are slow, and that we should be able to
see the process of natural selection at work in the fossil record. Critters
from time A would be assumed not to be as well adapted for what they are
doing as critters at subsequent time B.
George's point above is a good one philosophically, in that mutation
is random, and no "lineage"'s genome is deliberately "trending" towards any
culminating state. It is also good in pointing out the subjectivity of
trends we observe. However, why do paleontologists (notably Sereno) still
study "trends" in the fossil record?
Wish I had a good answer for you. My guess is, if we accept that the
trends we observe are only "trends" a posteriori, and have no existance as
an independant process, they still may be useful to look at. Tetanuran
theropods exhibit a trend towards tail-shortening on the way to birds (and,
therefore, through geologic time, although how much time depends a lot on
our biostratigraphic data). There may be one compelling selection pressure
for this observed relationship, or there may be many, but I believe we can
safely say that this does occur. So, even if this is not an "inherent"
trend, we are obliged to attempt to explain it what is going on in each
case. Perhaps it has to do with a more active role of the tail for balance
rather than as a muscle anchor. This could have then been exapted for
steering, then exapted for flight.
There doesn't have to be one explanation. Indeed, in some cases, in,
say (and this is JUST an example) ornithomimids, they may have shorter tails
than earlier theropods (I think they do... don't they?) simply because their
ancestors were already adapted to shorter tails and there was no selection
pressure to re-evolve longer ones. The ornithomimids ancestors were not
thinking about shortening the tail to make it a better flight organ, and the
ornithomimid doesn't care what cousin dromaeosaur is doing with his tail,
"as long as he doesn't do it near me!"
Any thoughts from y'all on this trend thing?
Jonathan R. Wagner, Dept. of Geosciences, TTU, Lubbock, TX 79409-1053
"Only those whose life is short can truly believe that love is forever"-Lorien