Disparity in Tree Species Richness among Biomes 33between range size and speciation rate. On the
other hand, if barriers are small relative to the
largest ranges and less likely to subdivide them
completely, then the probability of speciation
will decrease with range size. These models are
not mutually exclusive: the former may operate
at smaller range sizes, and the latter at larger
range sizes, which may result in the probability
of allopatric speciation peaking at intermediate
range sizes (Rosenzweig 1995).
Intrinsic effects of range size on population
genetic processes affecting speciation have gener-
ally been proposed in the context of larger ranges
being commonly associated with greater disper-
sal ability and higher population abundances
(see Chown 1997). The effect of geographic area
on speciation may thus depend on the extent to
which large areas promote selection for life-history
traits such as high vagility, short generation time,
and good colonization ability, as these enable
species to increase their range (Marzluff and Dial
1991).
Some modes of speciation are thought to be
mor ecommon if rang esiz eis larg e. For exampl e,
peripatric speciation (isolation of small popula-
tions at the periphery of a range) will be accel-
erated if peripheral populations experience local
selective regimes that differ from those across
th er est of th erang e(Mayr 1954). This is a
mor elik ely circumstanc ewithin larg erang es,
which usually encompass more habitats than
small ranges. This hypothesis predicts a faster
rate of evolution and divergence in small periph-
eral isolates. In centrifugal speciation (Brown
1957), isolation between a smaller peripheral
population and its parent is driven by acceler-
ated evolution in the parent. Because centrifugal
speciation is dependent on population size, and
population siz eincr eas es with rang esiz e(Gaston
1996), centrifugal speciation also supports a pos-
itive relationship between range size and the
probability of peripatric speciation (Rosenzweig
1995).
Th er ear etwo ways that larg erang esiz e
may impede speciation. First, if high vagility
increases gene flow in addition to range size,
population cohesion will be maintained and
result in a lower probability of speciation (Mayr
1963, Stanley 1979). However, genetic evidence
in marine species shows that dispersal is not
always associated with gene flow (Palumbi 1992).
Second, higher local population densities and
dispersal rates characterize large ranges, pre-
venting stochastic divergence. Gavriletset al.
(2000) conducted simulations in one- and two-
dimensional systems to study how range size,
population density, mutation rate, and migra-
tion influ enc eth etiming and mod eof sp eciation.
Their results supported the centrifugal model if
new genetic variation was the limiting factor,
and conversely supported the “centripetal” model
(rapid evolution and isolation at the periphery)
if genetic variation was sufficient but gene
flow between populations was low. They con-
clud ed that larg erang es ar enot mor elik ely to
undergo speciation. The question of whether a
large range promotes speciation remains largely
unresolved, as there appear to be viable the-
oretical arguments that predict both higher
and lower rates of speciation in large-ranged
species.
In contrast to speciation, the expected negative
relationship between range size and probability of
extinction is less controversial, and is supported by
empirical studies of the fossil record (e.g., Gaston
and Blackburn 2000, p. 120). A common view is
that species with large ranges are more likely to
have broader habitat tolerances and more genetic
variation than species with small ranges, and are
thus more resilient to stochastic extinction caused
by changing environments (Rosenzweig 1995).
As a result, even if species with large ranges have
lower speciation rates over the short term, over
the long term they may generate a disproportion-
ate number of new species merely by persisting
longer than small-ranged species (Chown and
Gaston 2000) and thus incr eas eth ediv ersity of
larg ear eas lik eth etropics. Th e exp ectation that
long-persisting, widespread species are significant
in situsources of diversity in large areas high-
lights how being a museum may, in fact, also
mean being a cradle; the two models are not
mutually exclusive.
Study of the evolutionary dynamics underlying
the signal of geographic area in the latitudinal
diversity gradient is challenging because several
relevant parameters (range size, speciation rate
and mode, extinction, mutation rate, dispersal)