The growth rate hypothesis (GRH), a central concept of ecological stoichiometry, posits that the relative body phosphorus content of an organism is positively related to somatic growth rate as protein synthesis, which is necessary for growth, requires P-rich rRNA and has strong support at the interspecific level. Here, we explore the use of the GRH to predict microevolutionary responses in consumer body stoichiometry. For this, we subjected zooplankton populations to selection for fast population growth (PGR) in P-rich (HPF) and P-poor (LPF) food environments. With common garden transplant experiments, we demonstrate that in HP populations evolution towards increased PGR was concomitant with an increase in relative phosphorus content. In contrast, LP populations evolved higher PGR without an increase in relative phosphorus content. We conclude that the GRH has the potential to predict microevolutionary change, but that its application is contingent on the environmental context. Our results highlight the potential of cryptic evolution in determining the performance response of populations to elemental limitation of their food resources.

This study consists of 4 seperate experiments

1. Evolution experiment: Fourteen replicate populations with identical genetic composition, were exposed to culturing conditions that selected for fast clonal population growth, seven of the populations were allocated to a P-rich (HPF) and the other seven populations to a P-poor (LPF) diet. Every 24 hours we transferred 60 haphazardly selected individuals and all resting eggs from each population to a new culturing flask with a fresh food suspension. After the daily transfer, we counted the remaining individuals to calucate population growth rates.

2. Common garden one: Using the populations from the evolution experiment we performed fully reciprocal common garden experiment to test for genetic adaptation to selection for fast growth in the two food quality treatments. Every 24 hours we transferred 10 haphazardly chosen individuals from each experimental unit into a fresh algal suspension. We counted the remaining animals to estimate PGR. We preserved the remaining individuals to estimate demographic composition.

3. Common garden two: Using the populations from the evolution experiment we performed large scacle fully reciprocal common garden experiment to evaluate the effect of selection history on organismal carbon (C), nitrogen (N), and phosphorus (P) content. We determined rotifer C and N contents using a FLASH 2000 organic element analyzer (Interscience B.V., Breda, Netherlands), and P content with a QuAAtro segmented flow autoanalyzer (Beun de Ronde, Abcoude, Netherlands). For each of these analyses we used a sample of 100 individuals with a single parthenogenetic egg.

4. Life History Experiment in LP Food: Using the populations from the evolution experiment conducted a life history experiment in low-P food with the populations. During the life table 15-18 individuals from each population were monitored every two hours from birth until the production of the first juvenile or until confirmed as carrying a sexual egg.

All raw data is provided please see associated analytical code for data processing.