Geography Reference
In-Depth Information
The center of origin is the key concept of Darwinian dispersalism. For Dar-
win, ancestral species occupy restricted geographic areas, from which they
are constantly evolving and dispersing out to other parts of the earth by us-
ing different means of dispersal. The dispersal abilities are then basic mech-
anisms that allow the organisms to reach distant areas. Barriers provide the
isolation needed for evolutionary differentiation. During the history of disper-
salism, authors have proposed several criteria for identifying centers of ori-
gin. Cain (1944) analyzed critically thirteen of them, finding inconsistencies
and contradictions. Some of these criteria are as follows (Cain 1944; Mor-
rone, Espinosa Organista, et al. 1996):
• The area with the greatest taxonomic or ecological diversification of
the taxon. It assumes that diversity increases through time, so the
most ancient area—the center where the taxon appeared—harbors
more species. Although the reasoning may seem to be correct, it lacks
empirical support, and there are numerous examples of areas with
great species richness that correspond to secondary centers of di-
versification, such as the Hawaiian islands for the genus
Drosophila
(Diptera: Drosophilidae).
• The area where the species has the greatest abundance of organisms.
It assumes that because the center of origin has the optimal ecological
conditions, it is the place where the organisms of the species will have
the best development, whereas in the periphery, where the conditions
are different, there will be fewer individuals. This criterion also lacks
theoretical and empirical support, and there are numerous examples
of species introduced to some area where they are so successful that
they have become plagues.
• The area where the most recent or apomorphic species of the taxon
analyzed is distributed. This criterion supposes a priori that new spe-
cies arise in the centers of origin and push the most primitive to the
periphery. This criterion assumes that no appreciable climatic changes
or extinctions have occurred, which is erroneous. Furthermore, islands
can harbor young species, and this does not imply that they constitute
the centers of origin of the taxa to which they belong.
• The area where the oldest fossil of the taxon analyzed is found. It as-
sumesthatthefossilrecordisexhaustive,butweknowthatitisincom-
plete, so there is no way to know whether a fossil is the oldest of the