Identification of Biotic Components - Evolutionary Biogeography - page 78

Individual Tracks
An individual track is the basic unit of panbiogeography. It can be defined as
the primary spatial coordinates of a species or supraspecific taxon (Crisci et
al. 2000). Operationally, an individual track is a line graph drawn on a map
that connects the different localities or distributional areas of a taxon accord-
ing to their geographic proximity. According to Craw (1988a), the concept
of individual tracks is not original to Croizat but comes from van Steenis
(1934-1935).
From the topological viewpoint, an individual track is a minimum-span-
ning tree that for n localities contains n - 1 connections (Page 1987). When
a track is drawn, the criterion for connecting the different localities of a spe-
cies is simple. When any locality is chosen, the nearest locality to it is found,
and they are connected by a line; then, this pair of localities is connected
with the nearest locality to any of them; the nearest locality to any of the
three is united, and so on ( fig. 4.2 ) . The result is an unrooted cladogram,
where the sum of the segments connecting the localities is minimal, follow-
ing a sort of geographic parsimony. An alternative formalization, based on
minimal Steiner trees (where extra localities are added in order to reduce
the length of the tree), is provided by Zunino et al. (1996).
How can we interpret individual tracks? Each taxon has a distributional
area or range, namely, the area where it is distributed. In order to study geo-
graphic distributions, biogeographers need some sort of representation or
abstraction (Gaston 2003; Rapoport 1975). Dot maps ( fig. 4.3a ) plot points
in the localities where the taxon has been recorded, and for some biogeo-
graphers they convey accurately the known records. Traditionally, biogeo-
graphers have enclosed the points with a free-form line around the peripher-
al localities, obtaining an outline map ( fig. 4.3b ). However, outline maps may
be so generalized that important localities are not sufficiently highlighted
(Heads 1994). Rapoport (1975) developed the mean propinquity method,
which consists of connecting the points on a map by means of arcs, then
establishing their mean distance, and finally surrounding every point with a
circle whose ratio equals the obtained mean distances ( fig. 4.3c ) . Individu-
al tracks are another representation of the geographic range of a taxon ( fig.
4.3d )inwhichspacegeometryisinterpretedasanexplicitcomponent(Craw
et al. 1999; Grehan 2001c).
Individual tracks can be oriented. Orienting an individual track consists
of formulating a hypothesis on the sequence of the disjunctions implied in
it. The most common way to orient a track is designating a baseline ( fig.