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Yesterday (16 VII 97) Nick Longrich commented about the convergence
in down feathers and various plant seeds. Indeed, this is convergence.
In the airlifted seeds a sail/parachute develops in down feathers there
are spaces to trap the air for insulation.
By the same token there are many 'feather-shaped' forms in nature; leaves
and especially many ferns. Again this is convergence on a shape. Perhaps for
a variety of functions. A structure like that is a good one to produce a large
surface area at minimal weight. But it is convergence.
Further the branching pattern in plants is produced in a completely
different way than in feather (eg growth and assembly are very different).
Certainly the material as about as different as possible.
The problem again is one of langauge. We cannot ask if "the branching
of feathers evolved for aerodymanics, not insulation...". Evolution does not
entertain goals of this sort, no matter how attractive may be the adaptation
of the structure for its function. It is, however, informative to ask what
the common ancestor of feathers might have looked like, or how it evolved
(eg what changes were necessary to move from structure A to subsequent
strcutures B,C, and D.). Certainly there were functional differences that
emerge from developmental sequence changes, incorporation of new
regulatory or structural genes, etc.
It is not surprizing to find things in nature that are 'feather-like'
or 'down-like". These are the similies and metaphors of communication.
They have little to do with science. "Hair-like' was used, for example,
to describe the strcutures on Sinosauropteryx. But, they cannot be hair in
the restricted defination a scientist uses. There are lots of 'hair-like'
structures in both the natural world and in man-made products. Unfortunately,
everyone is not always clear on these, perhaps finer, distinctions. But
because something in 'down-like' says nothing about its internal structure and
its history. When it comes to understanding fossil material, or many other
structure, it is paramont to use the proper term. In fact, there are even
hair-like structures on birds (modified feathers, eye lashes) but they are