Path: utzoo!attcan!uunet!mcvax!ukc!dcl-cs!bath63!bs_wab
From: bs_wab@ux63.bath.ac.uk (Bains)
Newsgroups: sci.bio
Subject: Re: Evolution
Keywords: evolution
Message-ID: <2761@bath63.ux63.bath.ac.uk>
Date: 5 Jul 88 12:31:37 GMT
References: <5944@bloom-beacon.MIT.EDU>
Reply-To: bs_wab@ux63.bath.ac.uk (Bains)
Distribution: sci.bio
Organization: University of Bath, England
Lines: 38

In article <5944@bloom-beacon.MIT.EDU> orwant@athena.mit.edu (Jonathan L Orwant) writes:
>I'm a bit confused about how special-purpose anatomical parts can evolve, e.g.
>the evolution of a bird's wings from an unwinged predecessor.
>
>If it happens gradually, then why isn't the 2% of a wing along the way an
>evolutionary detriment?   

This example has been studied in some depth. The answer is that 2% of
a vertebrate wing is a major advantage. If a pre-bird ancestor is
insectivorous, and wants to feed on flying insects (as the ground-hugging
ones are already spoken for) , it has to catch the insects while they are
on the wing. To do so it can chase
them and hope to jump up and catch them in its mouth. Any specialisation
which helps it to do this will therefore be advantageous. (Few animals
nowadays use this route, as the birds and bats have cornered the market in
airborn insects). Aerodynamic studies show that, for a lizard which runs
on its hind legs (itself very useful as it gets your head further off
the ground) any minor modification of the fore-limbs which makes them
able to give some lift increases the volume of air from which the lizard
is able to sweep its food, because it increases the height and duration
of short hops. This is an additive effect all the way from a slightly
wing-shaped alteration of the scales on the arm to full wings.

>Another explanation I have heard is that specialized master-genes turn on
>entire sequences of genes so that once a mutation triggers the proper master
>gene, an entire wing appears.  If this is the case, where is the information
>of how to grow a wing stored in an animal whose ancestors were wingless?
>-Jon Orwant (orwant@wheaties.ai.mit.edu)

This is an insuperable objection to developing wings. However it might be a
way of developing multiple wings, legs or whatever. Ask a passing
geneticist about the Drosophile (fruit-fly) Bithorax complex of genes,
which show just this sort of behaviour. By mutating the key Bithorax
gene you can turn a fly with one pair of wings into a fly with two, a
radical change involving many body structures.

William Bains, Department of Biochemistry (although not for long)
University of Bath, Bath, UK bs_wab@uk.ac.bath.ux63