empirical studies typically examines short-term evolutionary change among
extant genotypes (Brommeret al., 2002). Therefore, against the theoretical argu-
ment that invasibility (the ability to invade a population) is a more realistic
measure of fitness thanror LRS, is the practical one that invasibility is not
readily measured in natural populations, especially compared with LRS.
Moreover, Stearns ( 2000 ) highlighted a number of the predictive successes of
classical life-history theory, and suggested that these successes occurred
because the theory focused on influences, such as age- and size-specific impacts
on mortality and production, in an assumed stable, asexual population, rather
than influences ignored by optimality theory, which include those central to
adaptive dynamics (explicit population dynamics and frequency dependence). A
general conclusion is that increased realism (e.g. adaptive dynamics) is likely to
improve predictions, but lack of knowledge of actual dynamics, logistic prob-
lems in routinely measuring invasibility, and the established predictive suc-
cesses of optimality theory, should prolong the use of the classical life-history
approach and of easily measured fitness estimates such as LRS.
Predicting life-history plasticity
Life-history theory not only predicts the mean value of a trait, but has been
developed to predict the plastic response of a phenotypic trait in different
environmental conditions (Stearns & Koella, 1986 ). Phenotypic plasticity occurs
when a single genotype produces different phenotypes, including life histories.
The magnitude of a trait’s plastic response to changes in an environmental
variable is known as a reaction norm (Stearns & Koella, 1986 ; Stearns, 2000 ).
A life history involves many developmental decisions that are made on the
basis of physiological and environmental cues (the organism’s ‘state’), and life-
history theory has incorporated dynamic optimization models (Houston &
McNamara, 1999 ) to cope with sequential decisions.
Reaction norms are important because they capture both the history of an
individual’s development in a particular environment, and the evolutionary
history of selection which has produced a set of adaptive responses to informa-
tion about environmental conditions and physiological state. Thus a theory of
reaction norms has to deal with two effects of the environment on a reaction
norm. First is the proximate effect, in which environmental resources, stressors
and information induce a change in life history. Second is the ultimate effect, in
which the environment selects only those individuals whose life history
response to environmentalinformationleads to enhanced fitness.
To illustrate this, life-history theory predicts that additional mortality on
large individuals selects for smaller size at maturity, such as in species of
Cladocera whose larger individuals are most at risk from fish predation
(Brooks & Dodson, 1965 ). Thus, adaptive plasticity would involve reducing size
at maturity in the presence of fish cues. Many studies have shown that, even in
LIFE HISTORIES AND BODY SIZE 37