Agriculture Reference
In-Depth Information
*
other Clitellata
FIGURE 3.1
Simplified phylogenetic tree of major groups of earthworms. The node with the asterisk is the
polytomy discussed in detail in the text. (After Jamieson et al. 2002.)
To provide an example with biogeographic relevance, the Glossoscolecidae (South America)
are clearly a sister group of the Eudrilidae (Africa) rather than allied to the Microchaetidae (Africa),
as presented by Bouch (1983) and Omodeo (1998). The Lumbricina of Omodeo and similar
concepts of other groups emerged relatively intact but with significant modifications, particularly
the possible inclusion of the Eudrilidae. This remains unclear because a probable trichotomy of
Megascolecoidea (M), Lumbricoidea (L), and Glossoscolecidae plus Eudrilidae (G) was unresolved
by Jamieson et al. (2002) and could be resolved in three different ways: (M, (L, G)); ((M, L),G);
or ((M, G), L). Like Omodeo (2000), one could resort to paleogeography to support a phylogenetic
theory, choosing the last of the three because this leaves the Gondwanan taxa sharing a more recent
common ancestor than with the primarily Laurasian Lumbricoidea. However, this was not the
conclusion reached by Omodeo (1998), who placed the Glossoscolecidae with the Lumbricoidea,
nor is it consistent with the work of Bouch (1983), who placed the Glossoscolecidae with the
Microchaetidae, Kynotidae, and Almidae as the Glossoscolecoidea, leaving the Eudrilidae with the
Megascolecoidea. What is needed is to expand the taxon sampling (see
Zwickl and Hillis
2002)
of Jamieson et al. (2002) to include more Lumbricoidea, Glossoscolecidae, Ocnerdrilidae, and
Eudrilidae to resolve the polytomy and to address more definitively OmodeoÔs (1998) polyphyletic
model of earthworm evolution. This should also include some species of Alluroididae, because that
family is proposed as the source of two independent ancestors of major trunks of the Crassiclitellata
tree (Omodeo 1998).
Another problem is that some versions of the distribution of Lumbricoidea have members (the
Microchaetidae) located in sub-Saharan Africa. Can this indicate that earthworms have a significant
pre-Pangaea history, such that their biogeography can be understood only with reference to two
cycles of continental fragmentation? Clearly, this issue cannot be settled until the proposed phy-
logeny can be stabilized, but some pre-Pangaea reconstructions link continental geologic units that
are now far apart, such as northeastern North America and South Africa. Although geological
models of earth evolution can provide some corroboration of phylogenetic hypotheses, the main
burden of gathering evidence lies with biologists. I now consider the contributions of biological
data to understanding geological events.
