Evolution__3rd_Edition

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Some critics suggest that the initial
stages of a character would be
disadvantageous
CHAPTER 10 / Adaptive Explanation 263
advantageous in their final form, could not have been when in a rudimentary form:
“What is the use of half a wing?” is a familiar example. The anatomist St George Jackson
Mivart particularly stressed this argument in his The Genesis of Species (1871). The
Darwinian reply has been to suggest ways in which the character could have been
advantageous in its rudimentary form. In the case of the wing, partial wings might have
broken the force of a fall from a tree, or protowinged birds might have glided from cliff
tops or between trees a as many animals, such as flying foxes, do now. These early
stages would not have required all the muscular back up of a full, final wing. The concept
of preadaptation (see below) provides another solution to the problem.
Evolutionary biologists are sometimes challenged with arguments about functionless
rudimentary stages or the impossibility of complex adaptive evolution. It is impossible
to imagine, someone will insist, how such-and-such a character could have evolved
in small, advantageous steps. In reply, the evolutionary biologist may offer a possible
series of stages by which the character might have evolved. We need to keep in mind the
status of the evolutionary biologist’s argument here. The series of stages may in some
cases not be particularly plausible, or well supported by evidence, but the argument is
put forward solely to refute the suggestion that we cannot imagine how the character
could have evolved.
A second order of critics may latch on to the argument at this point and accuse evolutionists
of making speculative, even fanciful, suggestions about the stages through
which individual adaptations could have evolved. But the critics overlook the original
point of the discussion. The speculations are not the prize specimens of evolutionary
analysis. It is not being claimed that the series is particularly profound or realistic, or
even very probable. The long evolutionary history that precedes any complex modern
adaptation will appear, with hindsight, to be an improbable series of accidents: the
same point is as true for human history as evolution. Given the state of our knowledge
at any one time, for some characters we can reconstruct their evolutionary stages with
some rigor (Chapter 15), but for others we cannot a and for these it is only possible to
make guesses to illustrate possibilities, not conduct a careful scientific investigation.
It is fair to conclude that there are no known adaptations that definitely could not
have evolved by natural selection. Or (if the double negative is confusing!), we can conclude
that all known adaptations are in principle explicable by natural selection.
10.4 New adaptations evolve in continuous stages from
pre-existing adaptations, but the continuity takes
various forms
10.4.1 In Darwin’s theory, no special process produces
evolutionary novelties
We saw in the previous section that Darwin’s theory of adaptation is “gradualist.”
New adaptations evolve in small stages from pre-existing organs, behavior patterns,
cells, or molecules. Another way of saying the same thing is to say that there is
continuity between all the forms of adaptation that we see in the world today. This
view of continuity contrasts with, for example, a creationist view of life in which the