Evolution, 4th Edition

282 CHAPTER 11

population, causing the population to evolve a higher rate of increase. But this

potential for population growth is often limited, as resources become depleted

or as predation or disease become more common. These factors cause population

growth to be density-dependent, and constrain population size. In the simplest

ecological models, the per capita growth rate of the population (r) declines in pro-

portion to the population’s size (FIGURE 11.6). If we measure population growth

per unit of time, r declines from its maximum possible value, written as rm, which

in most species occurs when density is very low. The reduction of population

growth causes population size to reach a stable equilibrium number that is called

the carrying capacity, symbolized by K.

When a population is near or at carrying capacity, natural selection favors

alleles that affect characteristics that increase K [11]. These will often be alleles that

increase the ability of individuals to compete with others for limited resources: as

the population density approaches equilibrium, a more competitive genotype may

sustain positive population growth while inferior competitors decline in density.

The more competitive genotype is likely to achieve a higher equilibrium density

(K). Experimental Drosophila populations, maintained for a long time, evolve higher

population densities (FIGURE 11.7). Species that are well adapted to crowded condi-

tions near carrying capacity are said to be K-selected. Those genotypes, however,

may have pleiotropic trade-offs that decrease the population’s maximum growth

rate when population size is far below the carrying capacity. That is, the population

may evolve a lower maximum potential rate of per capita increase (rm).

As we already noted, however, populations of some species are frequently in a

state of rapid, exponential increase (as illustrated in Figure 3.7), so genotypes with

higher r have higher fitness. These species are said to be r-selected. Life history

characteristics that increase r include higher fecundity (mx), especially at young

ages. Genotypes that reproduce at an early age have a shorter generation time, and

so a higher rate of increase per unit of time, than do genotypes that defer repro-

duction to later ages. These characteristics often have trade-off effects that make

r-selected species poor competitors. During ecological succession, for example, soil

that is newly exposed (e.g., landslides or abandoned crop land) is colonized by rap-

idly growing, fast-reproducing weeds that are later replaced by more slowly grow-

ing, K-selected trees that start to reproduce at a later age but have a long reproduc-

tive life span.

Diverse life histories

Some species conform to the “reproduce early, die young” scenario that we have

described. These include some semelparous species, such as annual plants and Aus-

tralian “marsupial mice” (Antechinus) that grow fast, reproduce, and die within a

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KA

rA

rB

KB

rm,A

rm,B

0

Rate of increase (r)

Population density (N)

FIGURE 11.6 A model of density-depen-

dent selection of rates of increase. Assume

that a population contains two different

genotypes, A and B. Their instantaneous

per capita rates of increase are rA and rB,

both of which decline as population den-

sity (N) increases. The maximum intrinsic

rate of increase for genotype B (rm,B)—its

growth rate at very low density—is lower

than the maximum rate of increase for

genotype A (rm,A). Genotype B has a selec-

tive advantage at high density, however,

and it attains a higher equilibrium density

(K) than genotype A: KB is greater than KA.

(After [50].)

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Number of ies (thousands)

10 20 30 40 50 60 70

Weeks

1

0

2

3

4

FIGURE 11.7 Experimental evolution of population density

in Drosophila serrata. The densities of two experimental

populations (red and blue dots) increased over 70 genera-

tions, implying adaptation to high densities and improved

conversion of food (supplied at a constant rate) into flies.

The potential rate of increase of this species is so great that

without evolutionary change, the population would have

reached carrying capacity in fewer than 10 weeks, and

would have remained fairly constant in size. (After [5].)

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