EVoluTioN iN SPACE 205
Dispersing individuals experience less competition than their siblings
that stay at home. Genes for dispersal can therefore benefit from kin
selection (a topic we will discuss more in Chapter 12).
Another kind of interaction between individuals can also favor
dispersal. Mating between closely related individuals often results
in offspring that suffer low fitness due to inbreeding depression (see
Chapter 10). Many species, including humans, are able to recognize
close relatives and avoid mating with them. But other species do not
have that ability. They can, however, decrease the risk of inbreeding
by dispersing far from where they were born [34].
These benefits to dispersal are offset by costs. Moving is often
dangerous. Passive dispersal can land an individual in hostile habitat
where there is little or no chance of survival. Active dispersal is also
dangerous when patches of good habitat are separated by regions of
bad habitat. Several species of salamanders inhabit isolated springs
in the hot and dry southwest of the United States (FIGURE 8.14). M il-
lions of years ago, the ancestor of these salamanders had a terrestrial
phase of the life cycle that could disperse between springs. As the
climate became warmer and drier, life on land became increasingly
hostile to these salamanders. In response, the salamanders lost the
terrestrial phase and are now no longer able to disperse between
springs. Ultimately, this may be a form of evolutionary suicide: a spe-
cies that is endemic to one spring can be driven to extinction by a
single catastrophe, either natural or human-caused.
Energetic trade-offs can also select against dispersal. Many species
of crickets have two morphs, one with functional wings and the other
without (FIGURE 8.15). Winged individuals are able to disperse by
flying. But they have lower fecundity than the wingless morph, probably because
of the large energetic investment required to develop functional flight muscles [39].
Habitats that change in time and space create mosaics of shifting selection pres-
sures, favoring the evolution of increased dispersal at some times and in some
places, and decreased dispersal elsewhere. The Glanville fritillary (Melitaea cinxia)
is a butterfly that lives in meadows that are surrounded by forest. When a newFutuyma Kirkpatrick Evolution, 4e
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Evolution4e_08.13.ai Date 11-17-2016Frequency of the better disperser
1 2 3 4 5
GenerationFrequent
extinctionNo extinction
No extinction,
no migration0.40.60.80.201FIGURE 8.13 Dispersal evolved in response to envi-
ronmental disturbance in a laboratory experiment with
the nematode Caenorhabditis elegans. Mixtures of
two genotypes that differ in their dispersal rates were
cultured on agar plates with two patches of the bacteria
on which the worms feed. In the experimental treatment,
every 3 days nematodes from a randomly chosen patch
were used to inoculate a new plate. The old plate was
discarded, which simulated extinction of the population
in the second patch. There were two control treatments: a
single patch with no extinction, and two patches with no
extinction. (After [10].)Futuyma Kirkpatrick Evolution, 4e
Sinauer Associates
Troutt Visual Services
Evolution4e_08.14.ai Date 11-17-2016 01-24-17FIGURE 8.14 The Barton Springs
salamander (Eurycea sosorum) has lost
the terrestrial phase of its life cycle. It
cannot survive on land and so is unable
to move to another spring. This species
is found only in the aquifer that feeds
a group of springs in Austin, Texas, that
provide water to a large swimming pool
(inset). The salamander’s entire range is
only a few square kilometers.08_EVOL4E_CH08.indd 205 3/23/17 9:12 AM