Re: AAT Questions...
Elaine Morgan (Elaine@desco.demon.co.uk)
Tue, 01 Aug 1995 15:50:42 GMT
Convergence and the palinosts.
Convergent evolution is a respectable scientific concept. The examples
most commonly given are the way the dolphin acquired a fish-like
silhouette, and the way the aardvark and the pangolin became
look-alikes. It is usually a suite of changes in behaviour and
morphology which is so described, and there is a fixed and perfectly
natural expectation that an animal will be currently occupyin the niche
for which the convergence has specialised it.
People have registered irritation with the way I use the word.
Maybe because (a) I use it to account for single enigmatic features
rather than overall morphology, and (b) I suggest there may be signs
of convergent evolution having specialised an animal for a niche which
it no longer occupies.
I would like to argue that these processes are known to take place,
and if "convergent evolution" (sensu stricto) is held not to be
applicable to them, then we need a term that is applicable and I
would welcome suggestions.
In resepect of (a) White coats in winter are a single feature of
Arctic animals which do not in other respects grow to resemble ona
another. Would this be accepetd as convergent evolution? I ask
because I am now in genuine doubt. It is at least an illustration of
the fact that a single feature in the environment can cause unrelated
animals to adopt a common strategy as a way of coping with it. (as I
suggested seagulls and hominids may have adopted a common strategy for
coping with excess salt.)
In respect of (b) it seems to me we are not short of animals which show
signs of having become specialised by convergent evolution for niches
they no longer occupy. One group of reptiles moved underground and
grew to resemble worms in their general outline. Can we no longer call
that convergent evolution because they later returned to the surface
and became known as snakes (or in a later similar case slow-worms)?
These creatures are palinosts- they came back to the kind of
environment they formerly occupied (term supplied by an Oxford
professor of Greek : palin as in palindrome, nost as in nostalgia)
but they had made some changes which were irreversible. Snakes had to
evolve a new and unique mode of locomotion. I believe we are palinosts.
Some creatures move on to a third environment. Like sea-snakes, (which
will never get to look like fish, it's too late). If you look at the
general outline of the limbs of a platypus, everything but its tail
proclaims it to be a marsupial mole. Those shovel-shaped forelimbs
never evolved for either running or swimming.
It may seem more parsimonious to opt for a hominid scenario involving
only one move (trees to ground) just because the shortest distance
beween two points is a straight line. But many primates followed that
straight line. There are far too many anomalous featured in Homo which
none of them share, and which the straight line fails to account for.
Elaine.
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