Variation in the strength of selection but no trait divergence between elevational extremes in a tropical rainforestDrosophila

Abstract

Evolutionary responses to environmental change require heritable variation in traits under selection. Both heritability and selection vary with the environment, and may also covary, meaning that environmental variation can be an important source of evolutionary constraint. However, estimates of heritability and selection along environmental gradients in the field are rare. We estimated environmental variation in selection on three traits (cold tolerance, heat tolerance and wing size) of the rainforest fly Drosophila birchii by transplanting flies in cages along two elevational gradients in north-east Queensland, Australia, and calculating the genetic covariance of trait values with cage productivity at each elevation. We estimated heritability of each trait from laboratory crosses, and environmental variation in heritability of wing size from the correlation of mothers and daughters in cages at each elevation. We then used estimates of selection and heritability to predict selection responses along the elevation gradients. Laboratory assays revealed low-moderate genetic variation in all traits and low covariation among traits, suggesting the potential for a strong response to selection. Estimated selection responses predicted divergence of cold tolerance with elevation at one gradient. However, this was not observed at either gradient, with no difference between high and low elevation populations for this trait. Despite substantial variation in heritability (and predicted selection response) of wing size, this appeared random with respect to elevation, preventing overall divergence and suggesting that local environmental variation constrains evolutionary responses along natural ecological gradients. Such an effect, if widespread, may significantly slow evolutionary responses to environmental change.

Environmental variation and biotic interactions limit adaptation at ecological margins: lessons from rainforestDrosophilaand European butterflies

“…The standardized selection differentials ranged from 0.01-0.48 for cold tolerance, heat tolerance and wing size [53]. This is similar to the range of values observed in a large review of phenotypic selection in the wild (most estimates between 0 and 0.3, median selection gradient = 0.16) [55].…”

“…per generation at the lowest elevation and a decrease of 0.44 s.d. per generation at the highest elevation [53]. This predicted adaptive response is interesting given clines in cold shock recovery time were observed in 2004 (see §3a(i) above).…”

“…These data, therefore, do not detect trade-offs that would make parts of these elevational gradients locally steeper, as we might predict where density declines suddenly (but see §3a(xi) below). Laboratory assays by O'Brien et al [53] also found no significant genetic correlations between heat and cold tolerance, or between either of these traits and wing size. Furthermore, these (non-significant) estimates of genetic correlation were always positive between these traits in the laboratory, suggesting that trade-offs between heat and cold tolerance do not limit evolutionary responses along these gradients.…”

“…This predicted adaptive response is interesting given clines in cold shock recovery time were observed in 2004 (see §3a(i) above). However, the predicted response to selection from these 2012 estimates is a reduction in productivity following cold shock at high elevations, suggesting either trade-offs in the field with other traits that are not obvious in the laboratory (see §3a(ix) below), or that delaying reproduction during cold periods is an adaptive strategy at high elevations [53].…”

“…We used relatives of the offspring reared from each family that were used in the transplant cages to estimate variation in cold tolerance, heat tolerance and body size [53]. This meant that we could connect trait means for each family measured in the laboratory with the fitness of their relatives transplanted to each cage in the field.…”