Evolution, 4th Edition

How To BE FiT 285

arrive, they shut down immune defenses and other physiological functions, and

expend all their energy in a frenzy of reproduction (FIGURE 11.11).

Iteroparity, however, can be advantageous for several reasons. For one, it increases

the chance of successful reproduction in fluctuating environments, when reproduc-

tion or offspring survival vary from one reproductive season to another. It is also

advantageous if adult mortality is low and greater fecundity can be achieved by

deferring reproduction to older age classes (FIGURE 11.12A). In this case, reproduc-

tive effort, in each reproductive episode, is expected to be lower in iteroparous than

in semelparous organisms [49]. This pattern has been found in comparisons among

species within several taxa. For example, inflorescences make up a lower proportion

of plant weight in perennial than in annual species of grasses (FIGURE 11.12B) [63].

FIGURE 11.11 A “big bang” life history. (A) Coho salmon (On-

corhynchus kisutch) swim from the ocean up raging rivers to

streams where they spawn. (B) Their physiological and physical

condition deteriorates as all their energy goes into reproductive

activities, and they die as poor semblances of their former selves.

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Evolution4e_11.12.ai Date 11-22-2016

(A) (B)

Number of species

0 0.3 0.6 0.9 1.2 1.5 1.8 2.1

Reproductive effort

5

0

10

0

600

800

1000

1200

400

200

30 40 50

Radius of rosette (cm)

Number of fruits

20 60 70

Annual species

Perennial species

Quaking grass,

perennial

Timothy,

annual

FIGURE 11.12 A) The number of fruits increases exponentially with plant size (measured by the (

radius of the rosette of leaves) in the terrestrial bromeliad Puya dasylirioides. Larger plants are older.

(B) Reproductive effort—measured here by the proportion of biomass allocated to inflorescences—is

greater in annual (semelparous) species of grasses, such as Phleum pratense, than in perennial (iterop-

arous) species, such as Briza media. (A after [3], photo by D. J. Futuyma; B after [63].)

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(A)

(B)

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