EVoluTioN iN SPACE 207
The evolution of dispersal rates and distances has a host of downstream effects.
Dispersers take their genes with them, and so evolutionary changes to dispersal
also affect local adaptation and patterns of geographic variation. Deleterious muta-
tions can become fixed by drift in isolated populations, which can cause immi-
grants that arrive from populations where they are not fixed to have high-fitness
offspring (see Figure 7.15). This effect amplifies the genetic impact of migration [9]
and can rescue local populations from extinction [30]. Paleontologists have found
that marine snails with planktonic larvae capable of dispersing long distances had
larger geographic ranges and survived longer in the geological record than did
those without planktonic larvae [14].The Evolution of Species’ Ranges
Humans live over a greater expanse of Earth’s surface than any other species. At
the other extreme, the world’s entire population of the Barton Springs salaman-
der is restricted to just a few square kilometers (see Figure 8.14). The geographic
range of a species can evolve, which raises the question of why some species have
evolved large ranges and others small ones. In some cases, barriers prevent a spe-
cies from moving elsewhere. The Barton Springs salamander is unable to live out
of water, and so does not have the ability to colonize other springs. On a larger
scale, the ranges of many marine species are defined by the edges of continents,
and the ranges of many terrestrial species by the edges of oceans.
In many cases, however, there is no obvious barrier that limits a species’ range.
From an evolutionary perspective, these situations are more difficult to under-
stand. If a tropical plant cannot survive far from the equator because winters are
too cold, why don’t populations that are at the northern and southern limits of the
range evolve greater cold tolerance, and so cause the range to expand outward? If a
barnacle cannot live higher in the tidal zone because it reaches its tolerance limits
for heat and desiccation there, why doesn’t it evolve greater ability to withstand
those stresses? If a bird cannot live in the same habitat as another species that is a
better competitor, why doesn’t it evolve to feed on different foods?
What limits species’ ranges is one of the most puzzling questions in evolutionary
biology. Several general kinds of explanations have been proposed [32]. Populations
may simply lack genetic variation in a trait necessary for adapting to a new envi-
ronment. For example, populations of two species of rainforest-dwelling Drosophila
have no detectable genetic variation for desiccation tolerance, which might prevent
them from expanding into drier habitats [15, 16]. A second possibility is that gene
swamping caused by migration from other parts of a species’ range can prevent local
adaptation to the extreme conditions at the range edge and prevent the species from
expanding outward [17, 19]. Consistent with that idea, cold resistance in the fly Dro-
sophila birchii is lower along a steep mountain slope than it is along a shallow slope
at the same altitude. Along the steeper slope, populations living at high altitudes
are closer to warm-adapted populations at low altitudes, so they may receive more
gene flow that prevents them from adapting to cold [4]. Third, biotic interactions
can set range boundaries where a species encounters a new competitor, predator, or
pathogen.
Global climate change provides a very large (if uncontrolled) experiment that
gives insights on how species ranges respond to environmental change. Species
might respond in two ways: by changing where they live and by adapting to the
new conditions (FIGURE 8.17). Reviews of ranges for which there are historical data
have found that many (perhaps more than half) shifted in the direction expected
from climate change. In the Northern Hemisphere, many northern range limits
have expanded farther north, while southern limits are contracting toward the range08_EVOL4E_CH08.indd 207 3/23/17 9:12 AM