Marsh Epipactis,
lower sepals removed
Runway
of labellum depressed
after insect lands |
Runway
of labellum raised
after insect crawls into cup below |
| D. L. Cramer |
that the bigger radial sesamoid may have been fashioned by a very simple genetic
change, perhaps a single mutation affecting the timing and rate of growth.
In a panda's foot, the counterpart of the radial sesamoid, called the tibial
sesamoid, is also enlarged, although not so much as the radial sesamoid. Yet the
tibial sesamoid supports no new digit, and its increased size confers no advantage,
so far as we know. Davis argues that the coordinated increase of both bones, in
response to natural selection upon one alone, probably reflects a simple kind
of genetic change. Repeated parts of the body are not fashioned by the action
of individual genes--there is no gene "for" your thumb, another
for your big toe, or a third for your pinky. Repeated parts are coordinated in
development; selection for a change in one element causes a corresponding modification
in others. It may be genetically more complex to enlarge a thumb and not
to modify a big toe, than to increase both together. (In the first case, a general
coordination must be broken, the thumb favored separately, and correlated increase
of related structures suppressed. In the second, a single gene may increase the
growth rate in a field regulating the development of digits.)
The panda's thumb provides an elegant zoological counterpart to Darwin's orchids.
An engineer's best solution is debarred by history. The panda's true thumb is
committed to another role, too specialized for a different function to become
an opposable, manipulating digit. So the panda must use parts on hand and settle
for an enlarged wrist bone and a somewhat clumsy, but quite workable, solution.
The radial thumb is, to use Michael Ghiselin's phrase, a contraption, not a lovely
contrivance. But it does its job and excites our imagination all the more because
it builds on such improbable foundations. Darwin's orchid
book is filled with similar illustrations. The marsh Epipactis, for example,
uses its labellum--an enlarged petal--as a trap. The labellum is divided
into two parts. One, near the flower's base, forms a large cup filled with nectar--the
object of an insect's visit. The other, near the flower's edge, forms a sort of
landing stage. An insect alighting on this runway depresses it and thus gains
entrance to the nectar cup beyond. It enters the cup, but the runway is so elastic
that it instantly springs up, trapping the insect within the nectar cup. The insect
must then back out through the only available exit--a path that forces it
to brush against the pollen masses. A remarkable machine but all developed from
a conventional petal, a part readily available in an orchid's ancestor.
Darwin then shows how the same labellum in other orchids evolves into a series
of ingenious devices to ensure cross-fertilization. It may develop a complex fold
that forces an insect to detour its proboscis around and past the pollen masses
in order to reach nectar. It may contain deep channels or guiding ridges that
lead insects to both nectar and pollen. The channels some-times form a tunnel,
producing a tubular flower. All these from a part that began as a conventional
petal in some ancestral form. Yet nature can do so much with so little that it
displays, in Darwin's words, "a prodigality of resources for gaining the
very same end, namely, the fertilization of one flower by pollen from another
plant." Darwin's metaphor for organic form reflects
his sense of wonder that evolution could fashion such a world of diversity and
adequate design with such limited raw material: Although
an organ may not have been originally formed for some special purpose, if it now
serves for this end we are justified in saying that it is specially contrived
for it. On the same principle, if a man were to make a machine for some special
purpose, but were to use old wheels. springs, and pulleys, only slightly altered,
the whole machine, with all its parts, might be said to be specially contrived
for that purpose. Thus throughout nature almost every part of each living being
has probably served, in a slightly modified condition, for diverse purposes, and
has acted in the living machinery of many ancient and distinct specific forms.
We may not be flattered by the metaphor of refurbished wheels
and pulleys, but consider how well we work. Nature is, in biologist Francois Jacob's
words, an excellent tinkerer, not a divine artificer. And who shall sit in judgment
between these exemplary skills? Stephen Jay Gould teaches
biology, geology, and the history of science at Harvard University.
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