While copious research has been conducted on the evolution of morphological characters, the macroevolution of life-history traits along environmental gradients is a relatively unknown field of study. Life-History Theory posits that the timing and duration of important events in an organism’s life, such as age at sexual maturity and first reproductive event, are shaped by natural selection. As such, life history strategies respond to biotic and abiotic factors and organisms will segregate into the habitats that they are best suited for. The environmental gradient selected for this study was pond permanence driven by hydroperiod. At one end, permanent ponds are large, stable water bodies that hold water continuously for centuries to millennia. At the other end of the spectrum are vernal ponds, which are small, ephemeral water bodies that reliably dry up every year. Temporary ponds fall between the two extremes, as they can either hold water continuously or dry up depending on pond structure and drought conditions. Pond drying imposes severe time constraints on the obligate aquatic life-history stages of some species, with vernal pond species expected to have the most rapid growth and development rates. These researchers sought to illuminate the evolution of life histories to pond permanence in three species of Lestes damselfly: L. dryas, a vernal pond specialist, L. forcipatus, a phylogenetically close relative of L. dryas and temporary-pond specialist, and L. congener, a basal temporary-pond specialist.
The researchers conducted a 3x2x2 factorial experiment, subjecting the three Lestes species to two photoperiod treatments (Early vs. Late) and two transient starvation periods during the final instar (Fed vs. Starved). Starvation treatments were performed in order to evaluate whether either fixed growth or plastic responses to photoperiod hinder larvae’s ability to deal with stressors, and photoperiod treatments approximated the Light:Dark hour ratios that nymphs would experience if they’d hatched either early or late in the season. Body mass and growth rates were calculated for each nymph. Innate immune function was quantified by scoring phenoloxidase activity after emergence in an effort to determine whether the responses to treatments impeded larval ability to deal with pathogens.
As expected, the researchers found that the vernal-pond specialist L. dryas reached each developmental stage before the other two species, emerging on average seventeen days earlier. L. dryas also had the lowest mass at emergence of the three species, corresponding with its shorter development time. L. dryas also experienced the highest growth rate of the three species. These results are consistent with the hypothesis that temporary ponds are the ancestral habitat of Lestes damselflies, that L. dryas adapted after invading the vernal pond habitat, and that regular pond drying was the definitive selective pressure driving these life history traits. All three species in the late photoperiod treatment accelerated growth and development by approximately three days, indicating that they were growing at less than their maximum physiological capacity in the early photoperiod, and starvation delayed emergence in all species and treatments. There was no clear evidence than an accelerated life history had detrimental effects on immune function at emergence, although the class agreed that this portion of the study could have used more attention and perhaps its own study in the future.
While a well-thought-out and elegant experimental design, the class spent the majority of the class discussion debating the merits and drawbacks of lab studies compared with field studies. Lab work may benefit from the removal of potentially confounding variables, but those same variables are often important pieces of the puzzle. Research performed in the lab allows the observation of a select few variables and facilitates otherwise impossible experiments; however one must be careful not to impose lab findings onto the natural environment simply because the organisms studied are the same. To be sure, each form of research has its place and the two are often complementary to one another.
Citation: De Block, M., M.A. McPeek, and R. Stoks. 2007. Life-history evolution when Lestes damselflies invaded vernal ponds. Evolution 62:485-493.
Photo Credit: Carl P. Lutz, 2009