A few words on the nature of gradualism may be in order,as it has
had wholly negative effects on studies of language evolution.Support for
gradualism derives from the fact that many,probably most,evolutionary
processes are indeed gradual and incremental,and for very good reasons.
Although radical mutations do occur in nature,they are almost always
destructive—legs appear where wings ought to be,or vice versa—and
it is easy to see why this should be so.If you have a complex organism
whose sustainability depends on very fine adjustments among all the
organs that compose it,any radical change,even if it brought a substan-
tial advantage in one area,is all too likely to be paid for with devastat-
ing handicaps in other areas.For that matter,many minor alterations may
prove dysfunctional,but there is less chance that a useful change will
necessarily have to pay an excessive price in the disruption of unrelated
functions.Such alterations may,if the advantage they convey is substan-
tial enough,spread through a population and serve as the ground from
which successive favorable changes may take off.If the changes are
cumulative in effect,they may eventually yield organisms very different
from,and better adapted than,those in which the long sequence of minor
changes was initiated.
However,it is one thing to believe that gradual processes predominate
in nature and quite another to hold that all evolutionary processes must
be gradual.The issue is,after all,simply an empirical one:even if no
nongradual changes were ever witnessed,one could never exclude the
possibility that the next evolutionary process to be uncovered might
be nongradual.But in fact,more than one nongradual type of change is
already known.Sudden changes in the environment,such as droughts,
floods,and iceages,especially if they radically modify the ecosystem,
may release a cascade of associated changes that can radically modify a
species in a space of time that is,by evolutionary standards,extremely
short,mere thousands or a few tens of thousands of years.More striking
still,one finds what have been called preadaptations or exaptations
(Gould and Vrba 1982).These occur when a trait originally adapted for
one purpose is switched to another.For example,the original insects
were exclusively terrestrial,but some had fanlike structures that served
as cooling mechanisms.These were selected for and become more and
more efficient until they were large and long enough to lift the insects
off the ground.Once insects could fly,and flight proved advantageous
for avoiding ground predators,increasing foraging range,and so on,
traits that would enhance flying capacity were obviously selected for.
However,the original act of flying was not specifically selected for as it
rested on a quite different capacity that hadbeen selected for.Were this
not the case,evolution would be impossible,and after four billion years
single-celled creatures would still populate the earth.160 Derek Bickerton