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(Continued from page
13)
A quantitative criterion
more acceptable to biologists is fitness
-- success in life. Fitness can only be measured
relatively: one organism would be twice as fit as
another if twice as many of its descendants have
survived after a certain period. But increasing
fitness turns out not to provide an overall trend
for evolution, since fitness is measured not only
relative to specific competitors, but also within
a specific environment. Natural selection works
to adapt organisms to survive and breed more and
more successfully in their existing
circumstances. However, circumstances --
environments -- change, and the long-term changes
in evolution involve the discovery of entirely
new circumstances, throwing up entirely new
problems in life.
A trout is superbly adapted to life in the river,
a hedgehog is ideally suited for hunting for
slugs on the river bank, and a toad is not
terribly good at either life-style. Yet the river
may dry up or the land may flood, and only the
toad will survive both emergencies. Which of the
three is furthest on in the process of evolution?
Julian Huxley's answer to this question is to
notice a sequence in the `dominance' of groups of
animals. We read of the Age of Trilobites, the
Age of Fishes, of Reptiles, of Mammals; these are
the groups regarded as dominant in each epoch,
and we might infer that each supplanted its
predecessor owing to some superiority, and so is
more advanced, perhaps more `fit'. However, the
story is probably not so simple. It now seems
likely, for example, that the mammals replaced
the dinosaurs not by out-competing them in any
sense, but because they happened to be small
enough to survive an environmental catastrophe
that wiped out the majority of animal species
then living, including the dinosaurs. In any
case, several groups are often dominant at any
one time, each in its own sphere: are we now in
an age of mammals, of birds, or of insects -- or
of humans? There must be several lineages of
dominance. (Where different types are competing,
the prize probably goes to the one that can be
most active. This means the type with the highest
metabolic rate, which might therefore be used as
a measure of progress.)
Perhaps an increasing ability to adapt to a wider
range of circumstances represents a real trend.
Have organisms become more adaptable over
evolutionary time? The answer is yes and no: in
every epoch, there have been some highly
successful specialists, and some adaptable
generalist species. It is usually said that the
specialists enjoy a heady period of success until
time deprives them of the special conditions they
exploit so well. Then they die out, unable to
adapt, and the new world is colonised by
descendants of the generalists, many of which
evolve into new specialists in new environments.
The mammals perhaps survived the dinosaurs in
this way.
If we look only at the best generalists in each
period, we may be justified in saying that
adaptability does indeed increase in each
succeeding period. (The point about choosing the
best in each period -- the leading edge of
evolution -- seems to apply to any potential
trend. There appears to be no trend in evolution
which draws all organisms in its wake. Some
species are highly conservative; they found a
cosy role in the living world millions of years
ago, and there is nothing to impel them to risk
advancement. Blue-green algae scums on modern
estuary muds, for example, are almost
indistinguishable from fossils 2,500 million
years old. We can only seek directional changes
at a leading edge of evolution.)
(Continued on
page 15)
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