204 CHAPTER 8the phenotypic and genomic levels. In humans, the E PA S1 locus, which contributes
to adaptation to high elevations in Tibetans (see F i g u r e 7. 2 3), was also discovered
by scanning the genome for regions of high divergence.
Testing the hypothesis of local adaptation in quantitative traits presents new
challenges. In body size clines such as that in moose (see Figure 8.2), the evidence
for local adaptation is clear: the correlation between body size and latitude in mam-
mals is repeated across many species and has a simple functional interpretation. In
other cases, however, the evidence is less clear. One strategy is to compare variation
among populations in a quantitative trait of interest with estimates of FST based on
neutral genes. We can then use FST to predict the amount of variation in the quan-
titative trait under the null hypothesis of no local adaptation [33, 38]. An excess of
variation in the trait between populations suggests local adaptation is at play.The Evolution of Dispersal
Gene flow is caused by dispersal. But why do individuals disperse? There are often
benefits to movement, for example to find food or a mate. But there are costs as
well—dispersing can be risky business.
An important evolutionary advantage to dispersal is that it allows individuals
to find habitat that is better now or that will be in the future. When the environ-
ment changes in time and space, natural selection favors dispersal if individuals
that move have higher fitness on average than do those that do not move [22]. If
environmental changes wipe out local populations from time to time, any geno-
type that does not disperse will leave no progeny to the evolutionary future. Weeds
such as dandelions live in ephemeral patches of open habitat that are created by
disturbances such as fire, grazing, and human activity. These patches are eventu-
ally overgrown by larger plants (or paved over by people), driving the population
of dandelions living there to extinction. Only dandelions that disperse out of a
patch will leave descendants to the distant future. This has selected for seeds that
disperse well in the ancestors of dandelions. They evolved seeds with a parachute
(technically called a “pappus”) that carries seeds to newly opened patches.
The evolution of increased dispersal in response to environmental disturbances
has been studied using experimental evolution [10]. Replicated microcosms were
established of the nematode Caenorhabditis elegans living on agar in petri dishes.
Each dish had two patches of food for the worms. The worms could either feed in
their current patch or move between patches. This setup allowed the investigator to
manipulate environmental disturbance. A single patch in each plate was inoculated
with a small number of worms. After the worms had the opportunity to disperse
and reproduce, a sample of the offspring was transferred to a new plate. Disturbance
was simulated by transferring worms from only one randomly chosen patch, which
is analogous to extinction of the other patch. In control treatments, there was no
selection favoring dispersal. The experiment was run with a mixture of two worm
genotypes that differ in their propensity to disperse. The high-dispersal genotype
had lower fecundity than the low-dispersal genotype. The experiment’s outcome
fits the theoretical expectation (FIGURE 8.13). Despite the fecundity disadvantage, in
just five generations the high-dispersal genotype had become much more frequent
in the treatment that simulated patch extinction. In the control treatments without
extinction, it became more rare.
A second factor that can select for dispersal is competition with relatives. To see
this, consider a plant that lives in patches of suitable habitat that are so small that
each patch is only big enough for a single individual. If a plant drops all its seeds
within its own patch, when it dies, many seeds may germinate but only a single
offspring will survive and inherit the patch. But if instead the plant disperses its
seeds to other patches, it has a chance of leaving more than one surviving offspring.08_EVOL4E_CH08.indd 204 3/23/17 9:12 AM