Evolution, 4th Edition

290 CHAPTER 11

correlated [17, 23]. But a population could nevertheless evolve to be specialized, for

several related reasons.

The strength of selection for an allele that is advantageous in a certain envi-

ronment depends on how much of the population experiences the environment.

Suppose some larvae in a moth population feed on an abundant plant species,

some feed on a rarer plant, and after they complete development, the adult moths

mix and mate at random. Imagine that a mutation at locus A increases fitness of

(A) (B)

baritula complex

albilatera complex

carbonaria complex

Lobelia laxiora

Ipomoea orizabensis

Leandra subseriata

lafresnayii complex

blue owerpiercers

Consumption rate (mg/s)

0

20

40

20

40

80

60

40

60

100

80

Beak hook length

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_11.17.ai Date 02-02-2017

FIGURE 11.17 Experimental demonstration of a functional

trade-off that makes a specialist more efficient than a generalist.

(A) Flowerpiercers (Diglossa) differ in the length of the maxillary

hook. Species with a pronounced hook feed mostly on nectar;

those with a less hooked bill feed mostly on fruit. (B) Bills were

experimentally modified (as shown by black profiles) in a species

with a pronounced hook, and the feeding rate was then assessed

when the birds were given fruit (top graph), flowers that required

them to use the hook in order to get nectar (middle), and flowers

from which they could obtain nectar without using the hook (bot-

tom). Comparing the top and middle graphs, note the trade-off

between fruit consumption and flower use. (After [52].)

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_11.18.ai Date 11-21-2016

Survival (%)

0

20

40

60

100

80

St. Lawrence (saline)

0 5 10 15 20 25 0 5 10 15 20 25

Salinity

Lake Michigan (fresh)

Hatching

Metamorphosis

Adulthood

FIGURE 11.18 urvival in relation to S

salinity in a saltwater population (St.

Lawrence Seaway) and a freshwater

population (Lake Michigan) of a cope-

pod (Eurytemora affinis). Each is more

highly adapted to its normal environment

than the other population is, and both

suffered higher mortality in the other

population’s environment. Other data

showed that the higher the survival of a

genotype in one environment, the lower

it was in the other, indicating a trade-off

within populations. (After [34].)

11_EVOL4E_CH11.indd 290 3/22/17 1:11 PM