Basic Concepts - Evolutionary Biogeography - page 20

it does not form patterns. Different taxa may actually be biogeographically
congruentbutnotappearsoinacladisticbiogeographicanalysisbecauseof
pruning caused by extinction (Lieberman 2004). Simulation studies conduc-
ted by Lieberman (2002) have shown that cladistic biogeographic studies of
extant taxa that ignore extinct taxa may be predisposed to artificial biogeo-
graphic incongruence. Wiley and Mayden (1985) have also shown the con-
founding effects of extinctions in the study of extant biotas. Two other pos-
sibilities may be invoked to explain the absence of a taxon in an area: prim-
itive absence (it never lived in the area) or pseudoabsence (it lives or lived
there but has not been discovered yet).
It has been debated extensively whether dispersal or vicariance rep-
resents the most relevant process to explain biogeographic patterns
(Humphries and Parenti 1999; Nelson 1978a; Platnick and Nelson 1978). In
the nineteenth century and the first decades of the twentieth century, dis-
persalism emphasized dispersal through a stable geography from centers of
origin to explain the distribution of organisms (Darwin 1859; Matthew 1915;
Wallace 1876). In the second half of the twentieth century, usually asso-
ciated with the acceptance of plate tectonics, vicariance arose as a more
appropriate explanation than dispersal (Croizat 1964; Croizat et al. 1974).
However, intraspecific phylogeography (Avise 2000) shows that dispersal
continues to be relevant in explaining the distribution of organisms.
In the mid-nineteenth century, Hooker (1844-1860) discovered that,
quite paradoxically, both dispersal and vicariance could explain the same
disjunctions. For example, a species may inhabit two disjunct areas, which
could be due to a widespread ancestral distribution when both areas were
united(vicarianceexplanation)ortoevolutioninoneareaandthendispersal
to the other (dispersal explanation). Can one choose between them? The
solution to the vicariance-dispersal opposition consists not of choosing one
process or the other but of adopting a different reasoning where vicariance
includes dispersal, although the latter occurs before the geographic barrier
appears (Andersen 1982; Brooks and McLennan 2001; Colacino 1997; Cro-
izat 1958b, 1964; Grehan 1991; Morrone 2004a; Savage 1982). According
to this dispersal-vicariance model, geographic distributions evolve in two
steps ( fig. 2.6 ) :
1. Dispersal ( figs. 2.6a - 2.6c ) : When climatic and geographic factors are fa-
vorable, organisms actively expand their geographic distribution according
to their dispersal capabilities or vagility, thus acquiring their ancestral dis-
tribution or primitive cosmopolitism.