Evolution__3rd_Edition

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Abiotic pollination
Biotic pollination
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CHAPTER 22 / Coevolution 623
these influences. In this section we have looked at some grander comparisons. Grand
comparisons are rarely conclusive, because so many factors can come into play. The
hypothesis has been tested using evidence from fossils and from the shapes of modern
phylogenies. The fossil test remains inconclusive, but the test with modern phylogenies
is supportive. The evidence as a whole suggests that coevolution partly drove the insects
and plants to cover the modern globe.
We have concentrated here on insect–plant relations, and on tests using phylogenetic
methods. However, other methods are being used, and other taxa besides insects
have evolutionary relations with plants. Box 22.1 looks at an example.
22.4 Coevolutionary relations will often be diffuse
The clearest examples of coevolution come from ecologically coupled pairs of species.
In practice each species will experience, and exert, selective pressures on many other
species. The evolution of a species will be an aggregate response to all its mutualists and
competitors, and any evolutionary change may not be easy to explain in terms of any
one competitor. The process is called diffuse coevolution. It undoubtedly operates in
nature; indeed, it may be the main force shaping the evolution of communities of
species. But it is difficult to study, and its importance is consequently controversial.
22.5 Parasite–host coevolution
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Figure 22.6
Testing the effect of biotic as opposed to abiotic pollination
on species diversity, using phylogenetically independent trials.
The test starts with a phylogeny of angiosperms, distinguishing
whether each species is pollinated by biotic or abiotic means.
We then identify nodes in the tree where two sister branches have
contrasting means of pollination, and count the species in the two
branches. Each such node contributes one trial to the final test.
(The nodes are statisitically independent. Statistical problems
arise if we simply count the numbers of species with biotic and
with abiotic pollination in the tree as a whole.) In this diagram,
two nodes provide independent contrasts. In both nodes, the
evidence supports the hypothesis that species diversity is higher
in the branch with biotic pollination. The species in the middle
branch are ignored in the test. Dodd et al. (1999) performed a test
of this general form with real data, and found higher species
diversity in branches with biotic pollination.
Step-by-step coevolution is particularly likely to take place between parasites and their
hosts. They can have specific and close relations, and it is easy to imagine how a change
in a parasite, which improves its ability to penetrate its host, will reciprocally set up