Geography Reference
In-Depth Information
2. Vicariance ( figs. 2.6d and 2.6e ) : When organisms have occupied all avail-
able geographic or ecological space, their distribution may stabilize. This
allows the isolation of populations in different sectors of the area (subspe-
cies, races, or varieties) and the differentiation of new species through the
appearance of geographic barriers.
Considering dispersal and vicariance as alternative processes has en-
genderedtheimpressionthattheyareinopposition(e.g.,Avise2004;Cowie
and Holland 2006; MacDonald 2003; Santos 2007; Voelker 1999; Whittaker
1998). According to Parenti and Ebach (personal communication, 2007), the
terms vicariance and dispersal are both explanatory and descriptive, and
this is the cause of much confusion in determining which is the most likely
explanation for a particular disjunct distribution. Descriptive vicariance rep-
resents a way to identify disjunct distributions, showing where biotic com-
ponents vicariated, not when the isolation occurred. It may function as a
general statement of geographic distribution (Humphries and Ebach 2004).
Descriptive dispersal identifies the extent to which organisms are able to
move within their distributional area. Explanatory dispersal and vicariance
are ad hoc mechanisms that are invoked to explain some disjunction, using
different models that are particular to each taxon. In evolutionary biogeo-
graphy, identification and testing of biotic components rely basically on de-
scriptive vicariance.
Biotic Components and Cenocrons
Areas of endemism traditionally have been considered the basic biogeo-
graphic units (Humphries and Parenti 1999; Nelson and Platnick 1981;
Platnick 1991); however, Henderson (1991) and Andersson (1996) found
that focusing on areas instead of biotas was reductionist, and panbiogeo-
graphers (Craw et al. 1999; Croizat 1958b, 1964) considered areas of
endemism to be artificial units, preferring instead to recognize generalized
tracks. Hausdorf (2002) proposed that biotic elements are more appropriate
biogeographic units. Units proposed by other authors include lineages
(Jeannel 1942; Ringuelet 1957, 1961), horofaunas and cenocrons (Reig
1962, 1981), chorotypes (Baroni-Urbani et al. 1978; Zunino 2005), and dis-
persal or distributional patterns (Halffter 1978, 1987). All these concepts ba-
sicallyrefertotwodifferententities: bioticcomponentsandcenocrons.Biotic
components are sets of spatiotemporally integrated taxa that coexist in giv-
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