Brian
D. Farrell (1) provides a clear illustration of the utility of phylogenetic
reasoning in evaluating the evolutionary history of ecological or behavioral
traits, or both. With the use of morphological and molecular evidence, Farrell
demonstrated that phytophagous beetle lineages feeding on angiosperms had
considerably higher rates of diversification than their gymnosperm-feeding
sister taxa (table 1 in the report). These results support Ehrlich and Raven's
hypothesis that insect diversity is intimately tied to the rise and
diversification of the angiosperms in the Cretaceous (2). Farrell argues that
the pattern he observed in beetles is of broad significance and should be
reflected in other lineages of insects showing close ecological associations
with angiosperms.
Bees
[Series Apiformes, Superfamily Apoidea (3)] provide support for this
hypothesis, but have often been overlooked in this context (4), because,
instead of feeding internally or externally on plant vascular tissues, they
consume angiosperm pollen and provide a direct benefit to plants through their
activities. Like many beetle lineages, bees form a speciose group, and, like
beetles, many species of bees have intimate and often specialized (oligolectic)
ecological associations with specific plant genera or species (5). Like
angiosperm-feeding beetles, bees may have arisen in the early Cretaceous (6),
and diversified along with the early angiosperms.
Recent
phylogenetic evidence indicates that bees form a monophyletic lineage that has
arisen from within the spheciform wasps [Series Spheciformes, Superfamily
Apoidea (3), or “hunting wasps”], thus rendering the hunting wasps paraphyletic
(7,8). However the precise position of the bees within the Spheciformes is
unclear. Recent studies indicate that bees either arise basally within this
lineage as sister to all Spheciformes except the families Sphecidae and
Ampulicidae (including ∼7000 species) (7), or as sister to the family Philanthidae
(including ∼1100 species) (8). The vast majority of hunting wasps are
predatory on insects and other arthropods (9). Association with angiosperms in
this case has led to enhanced rates of speciation: bees include ∼20,000 species, or 3 to 18 times the number of species
included in their putative sister clade.
However,
not all aculeate Hymenoptera support Farrell's hypothesis. The vespid subfamily
Masarinae includes angiosperm-associated wasps [also called pollen wasps (10)],
which are ecologically very similar to bees, despite their independent origin.
Like bees, masarines arose in the Cretaceous (11) from within a lineage of
predatory wasps (the family Vespidae). Masarine wasps include ∼300 species (10) and form the sister group to a clade of
over 3500 species of predatory solitary and social wasps (12). In this case the
angiosperm-associated lineage shows lower diversity than its non-herbivorous
sister clade.
Phylogenetic
reasoning of this kind can provide an extremely powerful tool for assessing the
implications of angiosperm associations in insects. However, before fully
accepting the hypothesis that angiosperms have had a direct impact on insect
diversity, we should seek additional evidence from other speciose phytophagous
insect orders, including Hemiptera, Thysanoptera, Orthoptera, Phasmida,
Lepidoptera, and Hymenoptera (especially sawflies, bees, and pollen wasps).
Finally, we should distinguish between herbivores that feed on plant vascular
tissues and pollenivores, and ask whether these two groups exhibit similar
patterns of coevolution with angiosperms.
By.......
- Bryan N. Danforth and
- J. Ascher
0 comments:
Post a Comment