B. Charlesworth and colleagues review the ef-
fects of gene flow and other evolutionary
forces on patterns of neutral variation in DNA
in “The effects of genetic and geographic
structure on neutral variation” (Ann. Rev. Ecol.
Evol. Syst. 34: 99–125, 2003). DNA sequences
are widely used to study the genetic structure
of populations; see “inference of population
structure using multilocus genotype data:
linked loci and correlated allele frequencies”
by D. falush and colleagues (Genetics 164:
1567–1587, 2003).There is a growing literature on how species
ranges evolve, and specifically how they are
responding to climate change. Two authori-
tative overviews are “Ecological and evolu-
tionary responses to recent climate change”
by C. Parmesan (Ann. Rev. Ecol. Evol. Syst.
37: 637–669, 2006) and “Evolution and ecol-
ogy of species range limits” by J. P. Sexton
and colleagues (Ann. Rev. Ecol. Evol. Syst. 40:
415–436, 2009).PRoBlEMS AND DiSCuSSioN ToPiCS
- Suppose that in generation 0, the frequency of
allele A 1 in a population of armadillos is 0.4. in
each generation, 10 percent of the individuals
in that population are migrants from another
population that has an allele frequency of 0.6.
a. Calculate the frequency of A 1 in each of the
next two generations (generations 1 and 2).
b. is the change in allele frequency in genera-
tion 2 greater than, less than, or equal to the
change in generation 1? How can you explain
that answer?
c. What will the allele frequency become in this
population after many generations? - Consider a cricket that has recently colonized a
remote oceanic island from a source population
on a continent. How do you expect the average
size of wings in the island population to compare
with the average size on the continent? How
do you expect wing size in the island popula-
tion to evolve over the next several hundred
generations? - Equation 8.4 gives the equilibrium value of FST
between two populations for a neutrally evolv-
ing locus when the populations are of equal size
and are exchanging equal numbers of migrants.
When there is symmetrical migration among a
large number of populations, a different equa-
tion holds: FST = 1 / (1 + 4 Ne m). Suppose you
sample individuals from two populations, but
you do not know whether these populations
exchange migrants only with one another, or
whether they are part of a group of many popu-
lations that exchange migrants. you genotype
the individuals in your samples at several loci
and find that the average FST between the two
populations is 0.25. using the equation given
above and Equation 8.4, determine the range of
plausible values for the number of migrants that
arrive in each population in each generation.- Clines in body size have been observed in many
species, such as the latitudinal cline in moose
shown in figure 8.2.
a. Does a cline in body size necessarily result
from variation in allele frequencies at loci that
affect body size? Why or why not?
b. How might you determine whether a cline
in body size was caused by clines in allele
frequencies?
c. Say there is strong evidence that a latitudinal
cline in body size in a squirrel is caused by
variation in allele frequencies. Do you think
that data showing how rapidly the average
body size changes with latitude could by
themselves be used to determine how selec-
tion varies in space? Why or why not? - A species that has a high rate of long-distance
dispersal is more likely to colonize new habitat.
But that species may also be less likely to adapt
to local conditions, because migration will be
stronger than local selection pressures for many
loci. in light of those considerations, when do
you expect that increasing dispersal might result
in the evolution of a larger geographic range,
and when might it not? - it is now common to score many thousands of
SNPs in numerous individuals sampled from sev-
eral populations. (See, for example, the results
from stickleback fishes shown in figure 8.8).
Many of these SNPs are neutral and therefore
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