Evolution, 4th Edition

SPECiES And SPECiATion 221

elevation, pollinator fidelity alone is 98 percent effective. If a

flower receives both species’ pollen, the conspecific pollen (that

is, pollen from the same species) fertilizes the ovules at least

70 percent of the time. The germination of F 1 hybrid seeds is

reduced by 20 percent compared with nonhybrids, but a hybrid

seed that does germinate is just as viable. Hybrids produce

fewer seeds, however, and they produce much less viable pol-

len. But because isolation by elevation and pollinator behavior

is so great, the later barriers—reduced production, viability, and

fertility of hybrids—hardly come into play at all.

The Mimulus species illustrate some of the many kinds of

RIBs (TA B LE 9. 1). Prezygotic barriers reduce the likelihood

that hybrids are formed. These include such factors as separa-

tion of the species in different habitats, pollination by differ-

ent animals, mating at different seasons, mating preferentially

with conspecifics, and failure of gametes to unite even if mat-

ing occurs. Postzygotic barriers reduce gene exchange between

populations even if hybrid zygotes are produced. They consist

of reduced hybrid viability (survival) or reproduction (fertil-

ity). Both classes of barriers are often asymmetric: for example,

females of species A may be less inclined to mate with males of species B than

females of B are to mate with males of A [40], or F 1 hybrids between the species may

differ in viability, depending on the direction of the cross [104, 108].

Since prezygotic isolating mechanisms act before postzygotic mechanisms, they

have a greater opportunity to restrict gene flow. A second reason why the distinc-

tion between prezygotic and postzygotic mechanisms is useful is that different

kinds of selection act on them, as we will see shortly.

It is often difficult to tell which isolating barrier was the original cause of spe-

ciation. A character difference that contributes to reproductive isolation now may

have evolved partly in geographically segregated populations before they became

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

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Evolution4e_09.08.ai Date 11-18-2016

Elevation

Pollinator

Pollen precedence

Reproductive barrier

Hybrid germination

M. cardinalis

M. lewisii

Hybrid fertility

0 0.25 0.5

Contribution to isolation

FIGURE 9.8 Relative contributions of successively acting isolat-

ing mechanisms between the monkeyflowers Mimulus lewisii

and M. cardinalis. Elevational separation and pollinator isola-

tion account for almost all the reproductive isolation. In places

where both species occur, pollinators provided almost com-

plete reproductive isolation. (After [80]; photos courtesy of

D. W. Schemske and H. D. Bradshaw, Jr.)

TABLE 9.1 A classification of isolating barriers

i. Premating barriers: features that impede transfer of gametes to members of other species

A. Ecological isolation: potential mates do not meet

1. Temporal isolation: species breed at different seasons or times of day


2. Habitat isolation: species mate and breed in different habitats


3. immigrants between divergent populations do not survive long enough to interbreed
   B. Potential mates meet but do not mate


4. Sexual isolation in animals: individuals prefer mating with members of their own species


5. Pollinator isolation in plants: pollinators do not transfer pollen between species
   ii. Postmating prezygotic barriers: mating occurs, but zygotes are not formed
   A. Mechanical isolation: reproductive structures of the sexes do not fit
   B. Copulatory isolation: female is not stimulated by males of the other species
   C. Gametic isolation: failure of fertilization
   iii. Postzygotic barriers: hybrids are formed but have reduced fitness
   A. Extrinsic: hybrids have low fitness for environmental reasons


6. Ecological inviability: hybrids are poorly adapted to both of the parental habitats


7. Behavioral sterility: hybrids are less successful in obtaining mates
   B. intrinsic: low hybrid fitness is independent of environmental context


8. Hybrid inviability: reduced survival is due to genetic incompatibility


9. Hybrid sterility: reduced production of viable gamates
   Source: After [15], in part.

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