Evolution__3rd_Edition

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Phylogenetic tests for sympatric
speciation have recently been
devised
CHAPTER 14 / Speciation 413
How general a process is sympatric speciation by host shifts? A definite answer cannot
be given as it has not even been confirmed that sympatric speciation ever does take
place by host shifts. But there are interesting hints that the process might be important
(Section 22.3.3, p. 620). Several phytophagous insect taxa have undergone extensive
phylogenetic radiations on plant host taxa. There are, for example, about 750 species of
fig wasps, and each breeds on its own species of fig; in Britain alone there are 300 species
of leaf miners in the dipteran family Agromyzidae, and 70% of them each feed on only
one plant species. It is easy to imagine how these groups could have radiated from a single
common ancestor, as successive new species arose by host shifts like the one taking
place in the apple maggot fly in the USA. If phytophagous insect species consisted of an
occasional odd species scattered through the phylogeny of insects, and feeding on unrelated
kinds of food plants, the process would probably have not been operating; but the
existence of whole large taxa of host plant-specific phytophages does suggest that speciation
by host shifts could have contributed to their diversification.
14.10.3 Phylogenies can be used to test whether speciation has been
sympatric or allopatric
Direct attempts to test the theory of sympatric speciation, such as in Rhagoletis
pomonella, are only one way to test whether sympatric speciation occurs. Recently, a
new kind of evidence has been put forward for sympatric speciation. The evidence suggests
that sympatric speciation occurs, but tells us nothing about how it occurs. The
evidence comes from the shape of phylogenetic trees, and was first obtained for cichlid
fish in African lakes (Schliewen et al. 1994). As we saw (Section 13.3.3, p. 357), many
species of cichlid fish have evolved in the East African lakes. Did they originate by sympatric,
or allopatric, speciation?
Figure 14.14 shows the argument. If a new species arises by allopatric speciation, its
nearest relative will usually live in a different geographic area, such as in a nearby lake or
river. If the species evolved sympatrically, the nearest related species will usually live in
the same lake. In the case of a number of fish species, including the African cichlids, the
phylogenetic evidence supports sympatric speciation. Similar studies for other taxa
usually suggest allopatric speciation (Barraclough & Vogler 2001).
In conclusion, few biologists would rule out non-allopatric mechanisms of
speciation. Speciation probably occurs non-allopatrically, though it may only be rare.
Sympatric and parapatric speciation are more controversial theories than allopatric
speciation, except for special cases such as hybrid speciation in plants, because they are
not supported by such an impressive range of evidence.
14.11 The influence of sexual selection in speciation is one
current trend in research
We can finish this chapter by looking briefly at two big themes in current, and possibly
future, research on speciation. One is the possibility that sexual selection is important.