Evolutionary capacity of upper thermal limits: beyond single trait assessments

Abstract

Thermal tolerance is an important factor influencing the distribution of ectotherms, but we still have limited understanding of the ability of species to evolve different thermal limits. Recent studies suggest that species may have limited capacity to evolve higher thermal limits in response to slower, more ecologically relevant rates of warming. However, these conclusions are based on univariate estimates of adaptive capacity. To test these findings within an explicitly multivariate context, we used a paternal half-sibling breeding design to estimate the multivariate evolutionary potential for upper thermal limits in Drosophila melanogaster. We assessed heat tolerance using static (basal and hardened) and ramping assays. Additive genetic variances were significantly different from zero only for the static measures of heat tolerance. Our G matrix analysis revealed that any response to selection for increased heat tolerance will largely be driven by static basal and hardened heat tolerance, with minimal contribution from ramping heat tolerance. These results suggest that the capacity to evolve upper thermal limits in nature may depend on the type of thermal stress experienced.

Evidence for lower plasticity in CTMAX at warmer developmental temperatures

“…Furthermore, recent work suggests laboratory maintenance does not have large effects on physiological patterns in Drosophila (Maclean et al ., ). In addition, previous studies (van Heerwaarden & Sgro, ; Blackburn et al ., ) show that populations maintained under these conditions and for similar timescales harbour significant levels of adaptive genetic diversity.…”

Evolutionary potential of multiple measures of upper thermal tolerance in Drosophila melanogaster

“…Using a multivariate approach, Blackburn et al . () further showed that ramping and static knockdown time were not significantly genetically correlated with each other, suggesting that these traits are largely genetically independent.…”

“…The response to selection for increased heat tolerance is, however, not only dependent on genetic variation but also on the covariation in the traits under selection (Lynch & Walsh ; Brooks ; Blackburn et al . ). Measuring one component of heat tolerance might therefore over‐ or underestimate the adaptive potential to increase heat tolerance if different components of heat tolerance have a different genetic basis.…”

“…), and heritability estimates for ramping and hardened heat knockdown time seem to be much smaller than for static heat knockdown time (Mitchell & Hoffmann ; Blackburn et al . ). Using a multivariate approach, Blackburn et al .…”

“…Whether these different traits share a genetic basis has, however, rarely been tested (but see Van Heerwaarden & Sgrò ; Blackburn et al . ), and studies have revealed contrasting results on the extent that upper thermal limits can evolve (Hoffmann, Chown & Clusella‐Trullas ).…”