The Evolution of the Thermal Niche Across Locally Adapted Populations of the Copepod Tigriopus californicus

Thermal performance is a key component of fitness particularly for ectotherms living in thermally variable environments. Local adaptation can occur within populations of a species that inhabit regions with divergent thermal conditions, but this adaptation may result in trade-offs in other measures of fitness. If these trade-offs affect other aspects of thermal performance, several different patterns are possible (Huey and Kingsolver 1993). One potential pattern from a trade-off is a shift in the thermal niche, meaning that an organism that can handle a new range of higher temperatures can no longer handle colder temperatures as well. A second type of pattern is a generalist/specialist trade-off whereby populations may have broader thermal niches but lower fitness at optimal temperatures [i.e. “a jack-of-all-trades is a master of none” (Huey and Hertz 1984)]. Another possibility is that increased investment associated with local thermal adaptation (i.e. high temperature tolerance) may result in trade-offs in non-thermally dependent traits (Angilletta et al. 2003). The nature and structure of these trade-offs could determine the degree to which organisms will be able to respond to a changing climate. The copepod Tigriopus californicus (Baker, 1912) has become an important system in which to study the evolution of local adaptation to the thermal environment. Geographically distinct populations of this copepod occur in upper intertidal pools along the Pacific coast from central Baja Mexico to Alaska. These populations often show high degrees of genetic divergence from one another indicating that levels of gene flow between populations can be very limited over long periods of time (Burton 1997; Edmands 2001; Willett and Ladner 2009). There is also a clear latitudinal gradient in high temperature survival that is suggestive of local thermal adaptation for this species (Willett 2010; Kelly et al. 2012; Leong et al. 2018). This latitudinal gradient for high temperature tolerance has been seen for nauplii and copepodids as well as adults (Tangwancharoen and Burton 2014). Local thermal adaptation in T. californicus is also suggested by studies of fitness components and competitive fitness under non-extreme temperatures. Hong and Shurin (2015) examined 15 populations of T. californicus from Vancouver Island, BC, Canada, to southern California (CA) for a set of life history traits that contribute to fitness under four different temperature conditions (from 15°C to 30°C). They estimated the net fitness effect of these traits by calculating an intrinsic population growth rate (r) and found a consistent shift in the thermal niche from south to north and also higher r in the northern populations. Willett (2010) also found that for comparisons across a set of moderate temperatures there was a flip in competitive fitness between pairs of southern and central CA T. californicus populations. Central CA populations outcompeted southern populations at 16°C while the opposite pattern was observed in a fluctuating environment with an average temperature of 24°C (a 20°C to 28°C daily cycle). Combined these results suggest that