Variation in thermal tolerance
Higher genetically based variance in a trait results in higher rates of phenotypic evolution (Lande 1976). While no studies (to our knowledge) have measured within-population variation in mosquito thermal tolerance, several studies have investigated variation between populations (Mogi 1992, Reisen 1995, Dodson et al. 2012, Vorhees et al. 2013, Ruybal et al. 2016, Chu et al. 2019). Overall, these studies find genetically based, but often trait-specific variation that did not always clearly support local thermal adaptation (i.e., a correlation between trait values and local climatic conditions; Appendix C: Table S1). For example, some studies have found thermal tolerance varying predictably with the population’s thermal environment of origin—upper thermal limits of mosquito respiration and survival after heat shock were positively correlated with the temperature of origin for Cx. tarsalis and An. gambiae, respectively (Rocca et al. 2009, Vorhees et al. 2013). However, several other studies have found the opposite pattern (Ruybal et al. 2016), found minimal or no variation in thermal responses between populations (Dodson et al. 2012, Mogi et al. 1992), or found that certain populations had uniformly higher or lower trait performance at all experimental temperatures independent of their climate of origin (Ruybal et al. 2016, Reisen et al. 1995, Chu et al. 2019, Dodson et al. 2012). Taken together, mosquito populations do sometimes vary in their thermal performance, but there is no clear evidence for existing local thermal adaptation across temperature gradients of similar magnitude to those predicted by climate change over the next several decades. This suggests either barriers to thermal adaptation or relatively weak selection on thermal performance (see ‘Strength of selection’ section). However, the lack of within-population sampling and/or idiosyncratic, trait-specific temperature relationships may have obscured true patterns of local adaptation (Bradshaw et al. 2000).