Encyclopedia of Evolution.pdf - Online Reading Center

disjunct species
have happened? Something, or someone, might have carried
the seeds from Delmarva to the other locations. Neither wind
nor watershed patterns seem to suggest how this could have
occurred. Native Americans use alder bark as a medicine and
may have carried seeds or cuttings with them. The Georgia
population is within the former Cherokee Nation territory,
and the Cherokees are known to have moved south from
the Iroquoian lands of what is now the northeastern United
States. Subsequently the Cherokees were relocated by the U.S.
government to Oklahoma in 1838. Could Cherokees have carried
the plants with them, from the Northeast to Georgia to
Oklahoma, connecting all three of the existing populations?
This explanation is unlikely to be true, because the three populations
(subspecies) are different enough that they must have
been separated for a long time, much longer than the Cherokee
migrations of ca. 1000 and 1838 c.e.
In the case of the seaside alder, and in many other cases
as well, the disjunct populations are threatened with extinction.
Not only do small populations have a greater risk of
extinction, but disjunct populations are frequently not in the
habitats most suitable for them. Oklahoma seaside alders have
been planted, and are surviving well, in Iowa, while they are
barely surviving in Oklahoma, where they can withstand the
summer heat only if the soil is continually wet. This is because
the climatic changes that isolated the disjunct populations in
the first place are now threatening them with extinction. The
Georgia and Delmarva populations are similarly threatened.
In other cases, a species may have a large, central population
and several disjunct populations at some distance away
from the central population. The least tern, for example, is
a shorebird that is abundant in many areas, but the interior
subspecies of the least tern (which migrated up the Mississippi
and Arkansas Rivers into Oklahoma) is rare. The white
spruce (Picea glauca) is abundant in the subalpine forests of
the mountains of central North America and the boreal forests
of Canada. The limber pine (Pinus flexilis) is fairly common
in the Rocky Mountains. In the Black Hills of South Dakota,
relictual populations of both the white spruce and the limber
pine survive in conditions that are not cool enough for their
optimal growth. They became stranded in the Black Hills after
the most recent Ice Age and have declined in abundance since
then. The spruce remains abundant in the Black Hills because
it grows well in cool, moist, shaded areas of the northern hills.
In contrast, the limber pine has nearly become extinct in the
Black Hills, because it requires cool, dry, sunny conditions,
which are now rare in the Black Hills. The few remaining limber
pines are being crowded and shaded out by the extremely
abundant ponderosa pines (see figure on page 126).
Disjunct populations are frequently the failed dead ends
of evolution. They can also be wellsprings of evolutionary
novelty. Because they are disjunct, these populations are
reproductively isolated from the larger population, which
allows them to evolve in a different direction. If they have
dispersed from a large, central population, they may also by
chance be genetically different from the original population
(see founder effect). Evolution can occur more rapidly in
small populations (see natural selection). If the disjunct
L
P
This is one of the last limber pines in the Black Hills of South Dakota.
Pinus flexilis (labeled L) is a widespread pine species of high mountain
areas in North America, but this population in South Dakota, a remnant
from the most recent Ice Age, is disjunct, small, and heading toward
extinction. Behind the pine is a spruce, Picea glauca (labeled S),
which is also a remnant of the most recent Ice Age, and which is
part of a relatively abundant population. A ponderosa pine is labeled
P. (Photograph by Stanley A. Rice)
population evolves rapidly, it can produce a new species
which will appear on the scene in what amounts to a geological
instant. Small peripheral (sometimes disjunct) populations
may be largely responsible for the speciation events in the
punctuated equilibria model.
Further Reading
Schrader, James A., and William R. Graves. “Infraspecific systematics
of Alnus maritima (Betulaceae) from three widely disjunct provenances.”
Castanea 67 (2002): 380–401.
———, Stanley A. Rice, and J. Phil Gibson. “Differences in shade tolerance
help explain varying success in two sympatric Alnus species.”
International Journal of Plant Sciences, 167 (2006) 979– 989.
S