Biology Reference
In-Depth Information
Studies of the evolutionary history of land birds in the West Indies
support the concept of the taxon cycle, in which taxa invade and expand
through new geographical areas, undergo evolutionary modification, and
eventually become adapted to specialized or restricted habitats (Wilson
1961; Ricklefs and Cox 1972; Ricklefs and Bermingham 2002).Taxa late
in this sequence may also undergo secondary or repeated expansions. In
the West Indian land bird fauna, the youngest colonizing lineages of the
Lesser Antilles appear to have entered the islands from historical time to
750,000 yr BP (Ricklefs and Bermingham 2002). Many other lineages
entered between 750,000 and 10 million yr BP. The oldest colonist may
date from 9 to 12 million yr BP. Secondary expansions of taxa within the
Lesser Antilles appear to have occurred 6.5-8.6 million yr BP or more
recently. Taxon cycles are most easily recognized in insular regions, but
also occur in continental taxa (Cox 1985).
Taxon cycles are postulated to be driven by the changing coevolution-
ary balance between a taxon and its competitors, predators, and parasites
(Ricklefs and Cox 1972). Invading species succeed in large measure by
virtue of their escape from such biotic constraints. In time, pressure from
counteradaptation by members of the native biota and by later invaders
forces the invading taxon to adapt to local conditions and may cause the
extinction of some local populations.
Host-parasite coevolutionary interactions may play a significant role in
the taxon cycle process (Apanius et al. 2000; Ricklefs and Bermingham
2002). In Lesser Antillean land birds, for example, the relationship of host
species and malaria parasites tends to coevolve independently on different
islands (Fallon et al. 2003). These patterns seem to reflect interactions
between parasite virulence and host resistance mediated by mutation-
driven evolutionary change.
Coevolution and Biodiversity
Processes of integration of invading species into new biotic assemblages
lead not only to coevolutionary specialization, but often to speciation.
Counteracting the tendency of alien species to cause the extinction of
native species with which they interact is the potential for origin of new
species in the longer term. In the next chapter, we shall examine the
extent to which speciation is stimulated as the result of alien invasions.
