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).