194 CHAPTER 8and other marine animals that are sedentary as adults have planktonic larvae that
are dispersed hundreds or even thousands of kilometers by ocean currents.
In other animals, dispersal is active. Desert locusts (Schistocerca gregaria) are
grasshoppers that typically live solitary lives. But when they are crowded and
resources are scarce, changes in gene expression and hormone profiles radically
transform their behavior and morphology [37]. They form vast swarms of hundreds
of millions that fly long distances in active search of food (FIGURE 8.4). The devasta-
tion that locust swarms wreak on crops has been feared by farmers for millennia.
Evolutionary biologists often use the word “migration” as a synonym for “gene
flow”—both refer to the mixing of genes between populations. In other contexts,
“migration” means something quite different. Many species of salmon are born in
streams, then swim to the ocean where they live for several years. When they are
ready to reproduce, salmon have the astonishing ability to find their way back to the
very stream in which they were born. To a behavioral biologist, the salmon migrate
twice in their lives, once to the sea and again a second time when they return to
fresh water. But to an evolutionary biologist, there is very little migration here:
almost all salmon return to breed in the very population where they were born, and
there is hardly any mixing of genes between populations in different streams.How is gene flow measured?
To understand how gene flow acts, we need a way to measure it. The appropriate
measure depends on whether the environment is divided into discrete patches
(like islands, lakes, or mountaintops) or is spatially continuous (like a grassland
or an ocean). With discrete patches, gene flow is quantified with the migration
rate. This rate, symbolized by m, is the fraction of individuals in a population that
arrives from another population in each generation. If 120 individuals in a popula-
tion of size 1000 are immigrants, then the migration rate is m = 0.12.
The migration rate tells us how quickly gene flow erodes genetic differences
between populations. The difference in an allele’s frequency before and after
migration in a given population is given by the equation Δp = m (pm – p) (8.1)Futuyma Kirkpatrick Evolution, 4e
Sinauer Associates
Troutt Visual Services
Evolution4e_08.04.ai Date 11-17-2016GregariousSolitaryFIGURE 8.4 The solitary and gregarious
forms of the desert locust (Schistocerca
gregaria, shown at left) differ in color,
morphology, and behavior. High population
densities cause individuals to develop into
the gregarious form, which gathers in huge
swarms that disperse long distances (right).08_EVOL4E_CH08.indd 194 3/23/17 9:12 AM