Evolution, 4th Edition

220 CHAPTER 9

Reproductive isolation

Gene flow between biological species is prevented by biological differences called

reproductive isolating barriers (RIBs), also referred to as isolating mechanisms.

Under the biological species concept, speciation is the evolution of biological bar-

riers to gene flow, and so understanding the evolution of reproductive isolating

barriers tells us how new species evolve.

The total degree of reproductive isolation between two species may result from

several RIBs that act in sequence—and some potential RIBs may not come into

play. For example, the monkeyflower Mimulus lewisii is distributed in the Sierra

Nevada of California at higher elevations than its close relative Mimulus cardinalis,

although they both occur at intermediate elevations [92]. Mimulus lewisii has pink

flowers with a wide corolla and is pollinated by bees, whereas M. cardinalis has

a narrow, red, tubular corolla and is pollinated by hummingbirds (FIGURE 9.7).

Although almost no hybrids are found where the species occur together, the spe-

cies can be readily crossed, and they produce viable, fertile hybrids.

To understand the roles played by different isolating barriers between these spe-

cies, Douglas Schemske, Toby Bradshaw, and their colleagues performed a massive

field experiment. They bred a large number of F 2 hybrids and planted them in an

area where the two species coexist. The F 2 s have far greater phenotypic variation

than the parental species, and they have novel combinations of traits. By amplify-

ing the variation this way, the researchers were able to determine which of 12 floral

traits that distinguish the parental species are important to reproductive isolation.

They went further by using a quantitative trait loci (QTL) study (see Chapter 6) to

reveal the genes underlying those traits [8]. At least four traits affect the type of

pollinator that is attracted to a flower, which in turn determines which individu-

als exchange genes. The difference between the species in some of these traits is

based on as few as one to as many as six QTL, so a change to one or a few genes

can greatly affect reproductive isolation.

The investigators were able to quantify the contribution that different mecha-

nisms make to reproductive isolation, in sequence (FIGURE 9.8). Separation by ele-

vation alone reduces gene exchange by 59 percent. Among plants living at the same

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(A) M. lewisii (B) M. cardinalis

(C)

FIGURE 9.7 Pollinator isolation in monkeyflowers. (A) Mimulus

lewisii has the broadly splayed petals characteristic of many bee-

pollinated flowers. (B) M. cardinalis has the red coloration and

narrow, tubular form that have evolved independently in many

bird-pollinated flowers. (C) Some F 2 hybrids, showing the varia-

tion that Schemske and Bradshaw used to analyze the genetic

basis of differences between these two species. (From [92].)

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