Evolution__3rd_Edition

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Fitness
Fitness
(a) Time 1
(c) Time 3
X
Species 1
X
Character
Species 1 Species 2
Character
Adaptive topography
for species 1
Adaptive topography
for species 2
Species could have non-adaptive
differences
Kangaroos and gazelles may be an
example
Species 2
Fitness
(b) Time 2
CHAPTER 10 / Adaptive Explanation 283
Species 1 Species 2
X X
Character
Figure 10.13
Different starting conditions lead to two species occupying
different, but equivalent, adaptive peaks. (a) The adaptive
topographies for two species differ, and each evolves to its own
peak. (b) The adaptive topographies now change, until (c) they
become identical for the two species; but each species remains on
its own peak. At stage (b) the species difference was adaptive, thus
it was better for species 1 to be on its peak, and species 2 on its. By
(c) the species difference is non-adaptive as either species would
be equally well adapted on either peak.
the global peak, and it is therefore recognizably an imperfect adaptation. If there were
several peaks of similar height, one would not be recognizably inferior to the others.
Imagine now that the ancestors of a number of different populations started out near
different future peaks. If they then experienced the same external force of selection,
each one would still evolve to its nearest peak. The different populations would then
evolve different adaptations. But they have evolved different adaptations because of
their different starting conditions, not because they have adapted to different environments
(Figure 10.13).
Kangaroos and placental herbivores such as gazelles are possible examples. The two
forms are ecologically analogous, but have different methods of locomotion. Kangaroo
hopping is no better or worse for moving than running on four legs. The lineage leading
to kangaroos improved one method of moving, while that leading to gazelles concentrated
on another. The difference is probably mainly a historic accident. If the
argument is right, the distant ancestors of kangaroos faced different selective conditions
from those of gazelles. The adaptations fixed in those ancestors then influenced
subsequent evolution such that now, even though kangaroos and gazelles occupy similar
ecological niches, the mutations influencing locomotion that are favored in the
two groups are completely different.
The example illustrates a different idea from the giraffe’s recurrent laryngeal nerve.
Neither kangaroo nor gazelle is claimed to be imperfectly adapted; it is only the difference
between the two that may be a historic accident. In the giraffe lineage, a similar
kind of historic accident has generated an actual imperfection in its laryngeal nerve.