We investigated three aspects of adaptation to variable environments
in Daphnia pulex (Cladocera:
Crustacea): (1) effects of temporal variation on the evolution of
phenotypic plasticity ; (2) plasticity
in sexual versus asexual lineages; (3) maintenance of genetic variation
in variable environments.
We performed a 72-day quasi-natural selection experiment comparing three
patterns of variation:
constant temperatures, varying but predictable temperature change, and
unpredictable temperature
change. All populations were begun with an identical array of 34 clones.
During selection clonal
variation declined in all populations and different patterns of
environmental variation had little
effect on amounts of genetic variation. Sexual and asexual lineages
differed in size and growth rate,
but did not differ in amounts of plasticity or in adaptation to
variable environments. The primary
target of selection was the Malthusian parameter (r) and
life history traits of development time,
offspring size and offspring number. The heritability of plasticity
was generally lower than trait
heritability. Because of this difference, the selection response on the
mean of the traits
overwhelmed the selection response on plasticity. Lower heritabilities
of plasticity are very typical,
suggesting that our results will be typical of responses to selection
in nature. Our results suggest
that selection will act mostly on trait means within environments and
that plasticity will evolve
often as a correlated trait. Because selection on plasticity is based on
its across-deme, global
fitness, this process will usually be slow. Comparative studies
need to shift from closely related,
local population differences to those of more distantly related populations
or even different species.