Evolution, 4th Edition

THE GENETiCAl THEoRy of NATuRAl SElECTioN 123

change depending on their own frequencies. In some cases, an allele gets a fit-

ness advantage when it is rare, a situation called negative frequency dependence.

This can maintain polymorphism because when one allele becomes rare, selec-

tion causes its frequency to increase. In the alpine elderflower orchid (Dactylo-

rhiza sambucina), about half the plants have purple flowers and about half have

yellow flowers (FIGURE 5.21). Ten experimental arrays of 50 plants, with vary-

ing proportions of the two colors, were put out into the field. Results showed

that pollinating bumblebees visited flowers of the rare color more frequently,

which increased the number of their seeds that were fertilized and the amount

of pollen taken from them and carried to other flowers. Why did the bees prefer

the rare color? Like many orchids, Dactylorhiza does not reward pollinators with

nectar or pollen. The bees may have learned more quickly to avoid the orchids

with the common flower color.

A third form of balancing selection results when different genotypes special-

ize on different ecological niches. In effect, each genotype is partly shielded from

competition with other genotypes, and so has its own ecological carrying capacity.

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_05.21.ai Date 12-28-2016

(B)

(A)

Relative female reproductive success0.6 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

0.7

0.8

1.0

0.9

1.1

1.2

1.4

1.5

1.3

1.6

Frequency of yellow morph

Pollinia

deposition

Fruit set

Relative male reproductive success

(pollinia removal)

0.6

0.7

0.8

1.0

0.9

1.1

1.2

1.4

1.5

1.3

1.6

Dactylorhiza sambucina color morphs

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Deposition of a pollen mass

on a stigma represents

reproductive success via

female function, as

demonstrated by fruit set.

Removal of a pollen mass is

the rst step in reproductive

success via male function.

FIGURE 5.21 Negative frequency-dependent selection maintains a polymorphism for

flower color in the alpine elderflower orchid (Dactylorhiza sambucina), which is poly-

morphic for yellow and purple flowers (inset). Experimental gardens were planted with

different proportions of the two colors. (A) Yellow flowers have declining relative fitness

through male function (the amount of pollen removed) as yellow becomes more com-

mon in the population. (B) Yellow flowers also have declining relative fitness through

female function (the number of fruits produced) as the frequency of yellow increases.

Fitness in (A) and (B) is measured relative to purple flowers. The dashed lines indicate

where yellow and purple flowers have equal fitness. (After [16].)

05_EVOL4E_CH05.indd 123 3/23/17 9:01 AM