Abstract
The effect of size-selective predation on prey communities and their traits is well documented, but the relative roles of genetic adaptation and phenotypic plasticity continue to be debated. We looked for evidence of genetic adaption in a population of the water flea Daphnia pulex that faced a novel, introduced predator, Eurasian perch (Perca fluviatilis), selectively preying upon large zooplankton. Theory predicts adaptive changes towards a faster life history. We compared growth, age and length at maturation, egg size, and fecundity of two groups of clones kept in common-garden conditions, 13 clones isolated at around the time of the perch introduction and 14 isolated 3 years after. All animals were photographed daily and observed every third hour to detect maturation and measure the clutch size. Post-introduction clones matured earlier, but this was an indirect response triggered by genetic change in growth: post-introduction clones had faster growth prior to maturation than pre-introduction ones, reaching earlier the size threshold for maturation, but the threshold itself remained unchanged. Post-introduction clones showed also higher clutch size for 2nd and 3rd clutch, and slower growth from maturation (first appearance of eggs) to the moult after the release of the first clutch. Egg size did not differ between the periods. The experiment shows how life-history responses to predation involve multiple interlinked traits and both direct and indirect genetic responses.
Highlights
The effect of predation on changes in life-history traits is well documented (Roff 1992; Stearns 1992), but we still know relatively little how predation structures the genetic composition of individual prey populations
While there was no overall difference in clutch size between the periods, the model with perioddependent allometric exponent and intercept was significantly better than the null model (2nd clutch: Χ2 = 11.3, d.f. = 2, P = 0.004; 3rd clutch: Χ2 = 11.2, d.f. = 2, P = 0.004)
Because we had maintained pre-introduction clones in the laboratory, without allowing for sexual reproduction, we could use a common-garden experiment to establish whether genetic adaptation had occurred between clones collected at the time of the introduction and 3 years later
Summary
The effect of predation on changes in life-history traits is well documented (Roff 1992; Stearns 1992), but we still know relatively little how predation structures the genetic composition of individual prey populations. Life-history theory predicts a shift in resource allocation towards the least vulnerable life-stages (Law 1979; Taylor and Gabriel 1992; Lampert 2006). If small individuals are targeted, selection might favour faster growth to escape the vulnerable size ranges, whereas if large individuals are targeted, earlier maturation at smaller size might be favoured. Selection is based on phenotypes, but if the individual phenotypic differences have genetic basis, size-selective predation will result in evolutionary change. Predation can affect phenotypes directly, through induced defences (Gilbert 1966; Tollrian and Harvell 1999), or by changing the environment, for example resource availability and thereby growth (Claessen et al 2000; Enberg et al 2012). It may prove difficult to identify evolutionary changes and disentangle them from phenotypically plastic changes
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