■■Most species show geographic variation in allele
frequencies and the means of phenotypic traits.
Clines, which are smooth changes in an allele
frequency or trait mean, are a very common
pattern.
■■Clines and other patterns can result from local
adaptation, which results when selection varies
in space.
■■Gene flow is the mixing of alleles from differ-
ent populations, eroding differences caused by
selection and drift. it results from the dispersal
of individuals and their gametes. Gene flow
is measured by the migration rate (m) when
populations are discrete or patchy, and by the
migration variance (σm^2 ) when populations are
continuously distributed in space.
■■FST is a statistic commonly used to describe ge-
netic divergence between two or more popula-
tions. in many species with broad geographic
ranges, FST increases with the distance between
two populations, a pattern called isolation-by-
distance. FST varies across the genome, and
genomic regions with high FST can be used to
find loci that are locally adapted.
■■When both gene flow and local selection are at
work, allele frequencies evolve toward a com-
promise between them. if gene flow is weak rel-
ative to selection, allele frequencies will evolve
to what selection favors at each location. if gene
flow is relatively strong, allele frequencies will
be equalized. Strong gene flow can cause gene
swamping, which is when a locally favored al-
lele is lost because migration overwhelms local
selection.■■i n continuous habitats, the widths of clines are
determined by the ratio of the migration vari-
ance to the strength of local selection. When
there is a patch of habitat that selects for a dif-
ferent allele than that favored outside the patch,
the locally adapted allele will be lost by gene
swamping if the size of the patch is smaller than a
critical size determined by the relative strengths
of migration and selection.
■■Tension zones are clines in allele frequencies
that result from selection against heterozygotes
(underdominance) that acts uniformly in space.
■■Drift can cause allele frequencies at selectively
neutral loci to diverge between populations.
Very small rates of migration prevent divergence
at neutral loci. The amount of divergence can be
used to estimate the amount of gene flow.
■■Dispersal rates evolve. Higher dispersal is fa-
vored by habitat disturbance that causes extinc-
tion of local populations, competition between
related individuals, and inbreeding. lower
dispersal is favored because movement is often
risky and energetically expensive. in a species
that is expanding its range, there is an automatic
increase at the range’s edge of alleles that en-
hance dispersal.
■■ Species ranges evolve. factors that prevent
ranges from expanding outward include dis-
persal barriers, genetic constraints and gene
flow that prevents adaptation to more extreme
environments, and competition with other spe-
cies that have adjacent ranges. Global climate
change is causing shifts in the ranges of many
species, but there is little evidence that species
can generally avoid extinction by adapting to
the new conditions.TERMS AND CoNCEPTS
cline
dispersal
gene flowgene swamping
isolation-by-
distancelocal adaptation
migration rate
migration variancetension zoneSuGGESTioNS foR fuRTHER READiNG
l ocal adaptation has been studied using a va-
riety of approaches, including geographic
surveys, genetic analyses, and mathematical
modeling. Reviews by T. lenormand (“Gene
flow and the limits to natural selection,”
Trends Ecol. Evol. 17: 183–189, 2002) and by T.
J. Kawecki and D. Ebert (“Conceptual issues
in local adaptation,” Ecol. Lett. 7: 1225–1241,2004) give excellent perspectives on this rich
literature.
The evolution of dispersal involves a fascinating
but complex web of evolutionary forces that
are discussed in the review “How does it feel
to be like a rolling stone? Ten questions about
dispersal evolution” by o. Ronce (Ann. Rev.
Ecol. Evol. Syst. 38: 231–253, 2007).SuMMARy
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