Panda's Peculiar Thumb
||Incorporating Nature Magazine
Vol. LXXXVII No. 9
American Museum of Natural History
G. Goelet, President
Thomas D. Nicholson, Director
| This View
by Stephen Jay Gould
When nature uses a
Tinkertoy approach, evolution's role becomes more apparent
Few heroes lower their sights in the prime of their lives; triumph leads
inexorably on, often to destruction. Alexander wept because he had no new worlds
to conquer; Napoleon, overextended, sealed his doom in the depth of a Russian
winter. But Charles Darwin did not follow the Origin of Species (1859)
with a general defense of natural selection or with its evident extension to human
evolution (he waited until 1871 to publish The Descent of Man), but with
his most obscure work, a book entitled: On the Various Contrivances by Which
British and Foreign Orchids Are Fertilized by Insects (1862).
Darwin's many excursions into the minutiae of natural history--he
wrote a taxonomy of barnacles, a book on climbing plants, and a treatise on the
formation of vegetable mold by earthworms--won him an undeserved reputation
as an old-fashioned, somewhat doddering describer of curious plants and animals
who had one lucky insight at the right time. A rash of Darwinian scholarship has
laid this myth firmly to rest during the past twenty years. Before then, one prominent
scholar spoke for many ill-informed colleagues when he judged Darwin as a "poor
joiner of ideas . . . a man who does not belong with the great thinkers."
In fact, each of Darwin's books played its part in the
grand and coherent scheme of his life's work--demonstrating the fact of evolution
and defending natural selection as its primary mechanism. Darwin did not study
orchids solely for their own sake. Michael Ghiselin, a California biologist who
finally took the trouble to read all of Darwin's books (see his Triumph of
the Darwinian Method, University of California Press), has correctly identified
the treatise on orchids as an important battle in Darwin's campaign for evolution.
Darwin began with an important evolutionary premise:
continued self-fertilization is a poor strategy for long-term survival. With self-fertilization,
offspring carry only the genes of their single parent, and populations do not
maintain enough variation for evolutionary flexibility in the face of environmental
change. Thus plants bearing flowers with both male and female parts usually evolve
a mechanism to ensure cross-pollination. Orchids have formed an alliance with
insects. They have evolved an astonishing variety of "contrivances"
to attract insects, guarantee that their sticky pollen adheres to the visitor,
and ensure that the attached pollen comes in contact with female parts of the
next orchid the insect visits.
Darwin's book is a compendium
of these contrivances, the botanical equivalent of a bestiary. And like the medieval
bestiaries, it is designed to instruct. The message is paradoxical but profound.
Orchids manufacture their intricate devices from the common components of flowers,
parts usually fitted for very different functions. If God had designed a beautiful
machine to reflect his wisdom and power, surely he would not have used a collection
of parts generally fashioned for other purposes. Orchids were not made by an ideal
engineer; they are jury rigged from a limited set of available components. Thus,
they must have evolved from ordinary flowers.
paradox: Our text books like to illustrate evolution with examples of optimal
design--nearly perfect mimicry of a dead leaf by a butterfly or of a poisonous
species by a palatable relative: But ideal design is a lousy argument for evolution,
for it mimics the postulated action of an omnipotent creator. Odd arrangements
and funny solutions are the proof of evolution--paths that a sensible God
would never tread but that a natural process, constrained by history, follows
perforce. No one understood this better than Darwin. Ernst
Mayr has shown
how Darwin consistently turned to organic parts and geographic distributions that
make the least sense for his defense of evolution. Which brings me to the giant
panda and its "thumb."
Giant pandas are peculiar
bears, members of the order Carnivora. Conventional bears are the most omnivorous
representatives of their order, but pandas have restricted this catholicity of
taste in the other direction--they belie their order by subsisting entirely
on bamboo. They live in dense bamboo forests at high elevations in the mountains
of western China. There they sit, largely unthreatened by predators, munching
bamboo ten to twelve hours each day.
As a childhood fan of Andy
Panda, and a former owner of a stuffed toy won by some fluke when