Evolution__3rd_Edition

622 PART 5 / Macroevolution
One study suggests angiosperms
did not promote insect
diversification
Another study found that biotic
pollination promotes angiosperm
diversity
22.3.4 Coevolution between plants and insects may explain the grand
pattern of diversification in the two taxa
Terrestrial life on Earth today is dominated, among animals and plants, by insects and
flowering plants. It is a plausible hypothesis that the two taxa have promoted each
other’s diversification, by coevolutionary mechanisms such as specialized pollinator
relations. Here we can look at two tests of this hypothesis.
If the two taxa promoted each other’s diversity, then they are predicted to have
diversified at much the same time in the fossil record. Flowering plants diversified in
the Cretaceous (Figure 18.8, p. 539 a though the molecular clock suggests an earlier
origin for the angiosperms (Section 18.5, p. 538)). Insects were also diversifying, but
the question is whether their diversification accelerated in the Cretaceous while the
angiosperms were evolving.
Labandeira & Sepkoski (1993) counted the number of insect families through geological
time, from 250 million years ago to the recent past. The number of families
steadily increases, on a logarithmic scale, from about 100 in the Triassic, to 300 in the
early Cretaceous, to around 400 in the early Tertiary, and 700 at the end of the Tertiary.
The number of families went up as a (logarithmically) straight line, with no acceleration
in the Cretaceous as the angiosperms proliferated.
At least superficially, Labandeira and Sepkoski’s result contradicts the hypothesis
that insects and flowering plants promoted each other’s diversity. However, Grimaldi
(1999) argued that diversification could have taken place within each insect family.
Pollinating forms evolved within many insect groups independently, and the pollinating
insects seem to have diversified at about the same time as the angiosperms in the
fossil record. Thus, the total number of large insects groups, such as families, may have
shown little or no increase, but the diversity of insects could still have increased.
Therefore, Labandeira and Sepkoski’s test may be inappropriate. But Grimaldi’s
argument has not yet been tested quantitatively, and no one has yet shown that insect
diversity was promoted by the rise of flowering plants.
A second kind of test looks at the numbers of species within angiosperm taxa today.
Some angiosperms are pollinated biotically (usually by insects), others by abiotic
means such as the wind or water. If insect pollination promoted the diversity of
angiosperms, we would expect insect-pollinated groups of angiosperms to be more
diverse than comparable abiotically pollinated groups of angiosperms. The exact form
of the test is to compare related branches of the angiosperm phylogeny, where one
branch has biotic and the other abiotic pollination (Figure 22.6). Dodd et al. (1999)
identified 11 such comparisons, and the evidence as a whole strongly supported the
hypothesis that biotic pollination is associated with increased angiosperm species
diversity. Other tests of the same sort also suggest that insect, and particularly beetle,
diversity is enhanced in groups that are associated with angiosperms rather than
gymnosperms (Farrell 1998). Therefore, this second kind of test using phylogenetic
comparisons within modern plants and insects, suggests that the two taxa have promoted
each other’s diversity.
Insects and flowering plants have likely influenced each other’s evolution in many
detailed ways, and research on particular insect–plant relationships seeks to understand
..