Biology Reference
In-Depth Information
Rapid Evolution of Enhanced Dispersal
The introduction of species to new geographical areas, coupled with
global climate change that is altering the locations of climatically suitable
habitats, means that selection for enhanced dispersal capability is acting on
many species. Modeling studies show that in many cases selection should
favor higher dispersal tendencies in populations at expanding edges of
their range (Travis and Dytham 2002). However, for species showing a
strong Allee effect—reduced population growth effects at densities below
some optimum—selection for increased dispersal capability may be low
(Lewis and Kareiva 1993).
A few studies have examined rapid evolution of characteristics related
to the actual dispersal process. As we saw at the chapter outset for annual
plants that colonized islands in Barkley Sound, British Columbia, such
characteristics can be subject to intense selective pressures (Cody and
Overton 1996). Cwynar and MacDonald (1987) found that seed mass of
lodgepole pine (
Pinus contorta
) was smallest in the northernmost and most
recently established stands in the Canadian Rocky Mountains. Seed mass
increased significantly with estimated time since founding of stands as
lodgepole pine migrated north following glacial retreat. Similar selection
pressures are especially likely to act on species colonizing any sort of frag-
mented habitat. Roff (1990) concluded, for example, that among insects,
the loss of flight capability was lowest in species that use patchy habitats.
Inasmuch as habitat fragmentation is a major feature of global environ-
mental change, alien species in particular will experience strong selection
for the ability to colonize such habitats (Barrett 2000).
Similar selection has acted on animal species, such as the speckled
wood butterfly (
Pararge aegeria
), a widespread species in the western parts
of Europe and North Africa. In the United Kingdom, the species greatly
expanded its range during the latter part of the twentieth century. Some-
time between 1976 and 1985, it also colonized the island of Madeira,
about 540 km west of Morocco. Hill et al. (1999) compared the body size
and morphology of recent colonist populations with those where the
species had long been resident. Colonists tended to be larger in body size,
and in the case of the subspecies in the United Kingdom, to have larger
thoraxes, where wing muscles are located, and broader wings. These dif-
ferences were suggestive that selection favored particular adaptations for
flight in source populations of colonist butterflies.
In England, two species of bush crickets have been expanding their
ranges northward from areas along the southern coast (Thomas et al.
