Adaptation to climate change: contrasting patterns of thermal-reaction-norm evolution in Pacific versus Atlantic silversides

Abstract

How organisms may adapt to rising global temperatures is uncertain, but concepts can emerge from studying adaptive physiological trait variations across existing spatial climate gradients. Many ectotherms, particularly fish, have evolved increasing genetic growth capacities with latitude (i.e. countergradient variation (CnGV) in growth), which are thought to be an adaptation primarily to strong gradients in seasonality. In contrast, evolutionary responses to gradients in mean temperature are often assumed to involve an alternative mode, 'thermal adaptation'. We measured thermal growth reaction norms in Pacific silverside populations (Atherinops affinis) occurring across a weak latitudinal temperature gradient with invariant seasonality along the North American Pacific coast. Instead of thermal adaptation, we found novel evidence for CnGV in growth, suggesting that CnGV is a ubiquitous mode of reaction-norm evolution in ectotherms even in response to weak spatial and, by inference, temporal climate gradients. A novel, largescale comparison between ecologically equivalent Pacific versus Atlantic silversides (Menidia menidia) revealed how closely growth CnGV patterns reflect their respective climate gradients. While steep growth reaction norms and increasing growth plasticity with latitude in M. menidia mimicked the strong, highly seasonal Atlantic coastal gradient, shallow reaction norms and much smaller, latitude-independent growth plasticity in A. affinis resembled the weak Pacific latitudinal temperature gradient.

Absence of countergradient and cogradient variation in an oceanic silverside, the California grunion Leuresthes tenuis

“…This was followed by post hoc Tukey comparisons. The power to detect growth differences among populations was analyzed assuming a maximum difference (δ) of 0.07 mm d −1 , since this represented a realistic growth rate difference that may be observed between the most widespread grunion populations based on previous findings for Atherinops affinis (maximum difference = 0.24 mm d −1 spanning 15° latitude, Baumann & Conover 2011). Power (P) was estimated for α = 0.05 and for both n = 2 and n = 3 replicates.…”

“…However, power analyses indicated sufficient ability to detect moderate population growth differences (≤0.07 mm d −1 ) in laboratory-reared L. tenuis (with the exception of the first experimental period in 2009). Furthermore, significant population differences in estuarine silversides have been found over a latitudinal range of 4 to 5°or less (Conover & Present 1990, Billerbeck et al 1997, Yamahira & Conover 2002, Yama hira et al 2006, Baumann & Conover 2011, Baumann et al 2012.…”

“…Growth capacity at the temperature of maximal growth increases from south to north by 4.6% per degree of latitude in the Atlantic silverside Menidia menidia and by 2.7% per degree of latitude in the Pacific topsmelt Atherinops affinis, thereby countering the environmental effect of decreasing temperatures, which would otherwise result in slower growth in northern wild populations (Conover & Present 1990, Baumann & Conover 2011Fig. 4).…”

“…This enables them to compensate for reduced growing seasons and colder average temperatures that occur at high versus low latitudes (Conover & Present 1990, Yamahira & Conover 2002, Baumann & Conover 2011. In addition, vertebral number increases with latitude in Atlantic and Pacific silversides (Jordan's Rule; Jordan 1891), due to reinforcing environmental and genetic influences, representing a CoGV pattern (Billerbeck et al 1997, Yamahira et al 2006, Baumann et al 2012).…”

“…Examination of the co-variation between genetic and environmental influences within species may reveal spatial patterns of adaptation to natural environmental variation. Additionally, this knowledge may provide clues to potential evolutionary responses to temporal environmental change , Baumann & Conover 2011, which is becoming increasingly important to understand as global climate change may alter species distributions and life histories.…”