482 CHAPTER 18D iSPERSAl liMiTATionS The border of a species’ geographic range is some-
times set by utterly unfavorable conditions, as when the distribution of a ter-
restrial organism stops at the ocean’s edge. This may be a nonequilibrial (i.e.,
temporary) border, for the species might thrive beyond the barrier if individuals
eventually manage to cross it. The plants with trans-Atlantic distributions show
that this can happen. In some cases, obtaining mates is a problem. Plant species
that reproduce by self-fertilization have consistently broader latitudinal ranges
than congeneric outcrossing species (FIGURE 18.15), probably because they do
not need another plant’s pollen [15]. This suggests that the outcrossing species
might still be slowly spreading, but are retarded in their progress.EColoGiCAl niCHES A species can persist only if the organism can tolerate
each of several environmental conditions, such as the range of temperatures, the
amount of available water, and the availability of suitable food items. That is,
both abiotic and biotic aspects of the environment can affect the species’ distri-
bution. G. Evelyn Hutchinson [18], a leading ecologist, defined the fundamental
ecological niche of a population as the set of all those environmental conditions
in which a species can have positive population growth. A particular locality falls
within the species’ fundamental niche if all the relevant environmental factors
fall within the organism’s tolerance limits, but will fall outside the niche if any
one variable, such as lowest winter temperature, falls outside these limits.
Even in a potentially habitable locality, competitors or predators may further
restrict a species’ distribution. The competitive exclusion principle holds that spe-
cies that are too similar in their use of food or other limiting resources cannot coex-
ist indefinitely. Accordingly, ecologists have described many examples in which one
species occupies a broader range of elevation or habitat where a related species is
absent than where it is present [3]. Competition can affect whether or not two spe-
cies that have formed by allopatric speciation can spread into each other’s range
(see Chapter 9). Species of tropical American woodcreepers and ovenbirds that have
originated by allopatric speciation become sympatric much faster if they differ mor-
phologically and forage in different ways for different prey (FIGURE 18.16) [32].
The distributions of many species are correlated with climate variables (espe-
cially aspects of temperature and rainfall). Some species have shifted their
geographic or elevational ranges in recent decades, apparently in response toFutuyma Kirkpatrick Evolution, 4e
Sinauer Associates
Troutt Visual Services
Evolution4e_18.15.ai Date 02-02-2017Relative range size of selfer48210.5Outcrossing Sel ng
FIGURE 18.15 Self-fertilizing plants (“selfers”)
have larger geographic ranges than outcrossers.
The histogram shows the relative range sizes of
selfers compared with outcrossers in 20 clades
of flowering plants. Each column that extends
above the horizontal axis shows a clade in which
selfers have larger ranges than outcrossers. In all
but one clade, selfers have larger ranges. (Clades
are arranged in descending order of relative
range size of the selfers.) Inset: The flowers of
outcrossing and selfing individuals of a single
species of water hyacinth (Eichhornia panicula-
ta). (After [15]; photo courtesy of S. C. H. Barrett.)18_EVOL4E_CH18.indd 482 3/22/17 1:39 PM