Evolution, 4th Edition

EVoluTioN iN SPACE 201

Equation 8.4 shows that the width of a cline can be used to estimate the strength

of selection. Alleles for insecticide resistance in the mosquito Culex pipiens form

clines in southern France at the boundary between regions that are treated with

insecticide and those that are not. Combining estimates for the cline width (wc)

and the migration variance (σm^2 ) with a theoretical prediction similar to that in

Equation 8.4 suggests that the insecticide generates very strong selection for the

resistance alleles, with values of s up to 0.33 [20].

Clines also develop when the transition between two types of habitats is grad-

ual. In that case, the shape of the cline is typically similar to those in Figure 8.11,

even though the clines in that figure result from an abrupt change in selection.

The shape of a cline therefore does not tell us much about whether selection varies

abruptly or in a smooth gradient.

As we saw with body mass in moose, quantitative traits also have clines. Many

quantitative traits experience stabilizing selection with an optimum that varies in

space. In that situation, the mean value for a trait evolves to a compromise between

what local selection pressures favor and the homogenizing effect of gene flow. The

effect of gene flow is again determined by the migration rate (in habitats made of

patches) or the migration variance (in a continuous habitat).

Another common situation is when a patch of one type of habitat is embedded

in a landscape of another type. As we saw with the mine in Wales and the lava

flows in the southwestern United States, these patches can select for alleles that

are disadvantageous elsewhere. If the size of the patch is too small, however, gene

swamping occurs and the allele favored inside the patch will be driven to extinc-

tion. A locally favored allele will be lost in patches that are much smaller than the

cline width, wc, given by Equation 8.4. This sets a limit to the spatial resolution of

adaptation. Just as your eyes are unable to pick out details that are too small, selec-

tion is unable to cause beneficial alleles to spread if the region in which they are

advantageous is too small. The size of the minimal area to which a population can

adapt is once again set by the relative strengths of dispersal and selection.

Tension zones

We’ve just seen that selection can maintain differences between populations that

are connected by gene flow when fitness varies in space. While it may sound

counterintuitive, in some cases selection can also maintain differences even when

it acts the same way everywhere.

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_08.11.ai Date 11-17-2016 01-24-17

Frequency of

A

2

(A) Allele A 2 favored Allele A 1 favored (B)

–3 –2 –1 0 1 2 3

West Distance East

0.01

0.25

1

0.4 4

0.6

0.8

0.2

0

1

0

0.4

0.6

0.8

0.2

1

σm^2 s

Frequency of

A

2

wc

Distance

FIGURE 8.11 A) Clines in allele frequen-(

cies predicted by a mathematical model.

The horizontal axis is distance along a tran-

sect. The vertical axis is the frequency of

allele A 2 , which has a relative fitness of 1 – s

to the left (west) of x = 0, and fitness 1 + s

to the right (east). The four curves show the

clines for different values of the ratio of the

migration variance to the selection coef-

ficient, σm^2 /s. The clines become flatter as

the strength of gene flow increases relative

to selection. (B) The cline width, wc, is the

distance over which the allele frequency

changes from 0.1 to 0.9.

08_EVOL4E_CH08.indd 201 3/23/17 9:12 AM