Evolution, 4th Edition

SUMMARY

■■Adaptations such as reproductive rates and

longevity can best be understood from the per-

spective of individual selection. Life history traits

are components of the fitness of individual gen-

otypes, which is the basis for natural selection.

■■The major components of fitness are the age-

specific values of survival, female fecundity,

and male mating success. Natural selection on

morphological and other phenotypic characters

results chiefly from the effects of those charac-

ters on these life history traits.

■■An organism allocates energy and resources

among several functions, such as reproduction

and survival. The trade-off between reproduc-

tion and survival, or cost of reproduction, pre-

vents organisms from evolving indefinitely long

life spans and infinite fecundity.

■■The effect on fitness of changes in survival (lx) or

fecundity (mx) depends on the age at which such

changes are expressed and declines with age.

Hence selection for reproduction and survival at

advanced ages is weak.

■■Consequently, senescence (physiological aging)

evolves. Senescence appears to be a result, in

part, of the negative pleiotropic effects on later

age classes of genes that have advantageous

effects on earlier age classes. in addition, more

deleterious alleles are expressed at later ages.

■■Reproduction at a later age may maximize fitness

if juveniles have high mortality, adults have high

survival, and large body size greatly increases

fecundity. Under these conditions, there can be

selection for long life. in populations that are

frequently growing in number, selection favors

early reproduction and a short generation time.

The life history of many species lies on a fast–

slow continuum, ranging from rapid maturation,

short life, and numerous small offspring to de-

layed maturation, long life, and fewer but larger

offspring.

■■The optimal number of offspring is affected by

a trade-off between number and the size (mass)

of each offspring, and by the optimal reproduc-

tive effort at that age—the parent’s allocation to

reproduction versus continued survival.

■■Because lower fecundity and delayed reproduc-

tion can evolve, the intrinsic rate of population

increase—the maximum rate of increase, which

occurs at low density, may evolve to be lower.

These features often evolve, especially in stable

populations that are limited by resources and are

not increasing anyway.

■■i n addition to survival and reproduction, the

life history of a species includes its ecological

niche. Species vary in niche width—the range of

conditions they tolerate or resources they use.

Broad tolerance, often enabled by phenotypic

plasticity, has some costs, such as lowered effi-

ciency because of trade-offs between functions.

Specialization may evolve because it increases

efficiency or because of relaxed selection for

fitness in a relatively rare environment or habitat.

Mutations that disable features adapted to rare

environments may increase by genetic drift.

TERMS AND CoNCEPTS

antagonistic

pleiotropy

carrying capacity

cost of reproduction

density-dependent

population

growth

ecological niche

evolutionary

ecology

intrinsic rate of

increase

iteroparous

K-selection

life table

lifetime

reproductive

success

mutation

accumulation

r-selection

reproductive effort

semelparous

senescence

trade-off

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