Evolution, 4th Edition

284 CHAPTER 11

guppies in Trinidad [6, 48]. In some streams, the cichlid fish Crenicichla preys heavily

on guppies. In other streams, or above waterfalls, Crenicichla is absent and there is

much less predation; as a result, the density of the guppy population is higher and

there is stronger competition for food. The low-predation (LP) guppy populations

are therefore more K-selected than are the high-predation (HP) populations that

coexist with Crenicichla. When guppies from both situations are reared in a common

environment, HP populations mature earlier, have more offspring, and devote more

resources to larger offspring than do LP populations: features expected to evolve

under r-selection (FIGURE 11.10A). Moreover, experimental populations of guppies

moved from HP to previously guppyless LP environments rapidly evolve life history

characteristics typical of LP populations. When experimental populations of HP and

LP guppies are subjected to different density levels, the growth rate of LP popula-

tions is less depressed by high density, showing that these fishes are more adapted

to contend with high-density conditions (FIGURE 11.10B).

Is it better to reproduce only once or repeatedly? In theory, a semelparous, or

“big bang,” life history may be favored by selection if the probability of survival

increases with body mass and if there is an exponential relationship between body

mass and reproductive output [38, 51]. These conditions have been documented in

many species of semelparous plants (see Figure 11.8) [38]. Bamboos, agaves, cab-

bage palms, and other species reproduce only after many years, produce massive

numbers of seeds, and then die. Compared with iteroparous species of trees in

an Amazonian rainforest, a semelparous tree (Tachigali vasquezii) has a very rapid

growth rate, which it achieves by producing low-density wood. The rapid growth

reduces the risk of dying before maturity, when the tree produces a very high num-

ber of seeds. These results together compensate for not reproducing repeatedly

[46]. A “big bang” life history is also advantageous if reproduction is so stressful or

risky that an individual is unlikely to reproduce more than once. The most famous

example is migratory salmon that expend enormous energy and face great hazards

in swimming upstream from the ocean in order to spawn in streams. Once they Futuyma Kirkpatrick Evolution, 4e

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Low

predation

High

predation

Male age at maturity (days)

Low High

Predation

50

Female age at rst birth (days)

Low High

Predation

70

60 80

(A) (B)

Population growth rate

Low High

Density

2.0

1.8

1.6

1.4

1.2

FIGURE 11.10 Life history variation in guppies in Trinidad, where waterfalls separate

guppies into two populations: below the waterfall, they are in a high-predation (HP)

environment with large predatory fishes; above the waterfall is a low-predation (LP)

environment, where guppies reach higher density and there is strong competition for

food. (A) Both sexes of guppies from the HP environment start reproducing earlier in

life than those from the LP environment. (B) An experiment shows that life history differ-

ences between guppies from the HP and LP environments are genetic. When reared in

a controlled environment, populations founded with guppies from the LP environment

are less sensitive to the effects of density than those from the HP environment: the LP fish

are adapted to high density. The population growth rate was estimated from survival and

fecundity values measured over a 28-day interval. (A data from [48]; B after [6].)

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