Evolution, 4th Edition

SPECiES And SPECiATion 231

offspring that are more likely to pass on her genes [12].) This is one reason why

the distinction between prezygotic and postzygotic mechanisms is important:

prezygotic mechanisms can evolve by reinforcement, but postzygotic mechanisms

generally cannot.

Wildflowers in the genus Phlox provide a clear example of reinforcement [37].

Allopatric populations of the two species Phlox drummondii and P. cuspidata both

have light blue flowers (FIGURE 9.17). Where their ranges overlap, however, P.

drummondii has evolved dark red flowers, a difference in color caused by changes at

two loci. Because the fertility of hybrids is up to 90 percent lower than that of non-

hybrids, the difference in color is strongly favored by selection: pollinators move

less pollen between flowers that have different colors, so P. dr umm o n dii produces

fewer low-fitness hybrids when it has dark red flowers rather than light blue flow-

ers in the zone of sympatry.

Selection for reinforcement can occur only when two species continue to inter-

breed after some postzygotic isolation (reduced fitness of hybrids) has already

evolved between them. If reinforcement is common, we would expect sympatric

pairs of species (which could potentially hybridize) to show greater prezygotic

isolation than allopatric pairs of species (which have no chance of hybridizing).

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_09.17.ai Date 11-22-2016

Light red

(iihh)

Allopatry Sympatry

Austin

Texas

100 km

P. drummondii

P. drummondii

P. cuspidata

Dark blue

(I_H_)

Dark red

(IIhh)

Light blue

(iiHH)

Fruit set

Light blue Light red Dark blue Dark red

10

0

30

20

40

50

60

Relative hybridization

Light blue Light red Dark blue Dark red

Flower color genotype

10

0

30

20

40

50

60

(A)

(B)

(C)

FIGURE 9.17 Reinforcement of reproductive isolation by flower color in Phlox. (A) The

geographic distributions of P. cuspidata and P. drummondii overlap in Texas. Allopatric

populations of both species are light blue, but populations of P. drummondii are dark

red where the species is sympatric. (B) The flower color difference in P. drummondii is

based on two loci. (C) Results of common-garden field experiments, in which all four

color types of P. drummondii were grown together with P. cuspidata. Both parental

types (light blue and dark red) and hybrid genotypes with light red and dark blue flow-

ers have equal fruit production (top graph), but differ in the proportion of their offspring

that are hybrids with P. cuspidata (bottom graph). (A and B from [37]; C after [37].)

09_EVOL4E_CH09.indd 231 3/23/17 9:36 AM