Adaptive, maladaptive, neutral, or absent plasticity: Hidden caveats of reaction norms

Abstract

Adaptive phenotypic plasticity may improve the response of individuals when faced with new environmental conditions. Typically, empirical evidence for plasticity is based on phenotypic reaction norms obtained in reciprocal transplant experiments. In such experiments, individuals from their native environment are transplanted into a different environment, and a number of trait values, potentially implicated in individuals' response to the new environment, are measured. However, the interpretations of reaction norms may differ depending on the nature of the assessed traits, which may not be known beforehand. For example, for traits that contribute to local adaptation, adaptive plasticity implies non-zero slopes of reaction norms. By contrast, for traits that are correlated to fitness, high tolerance to different environments (possibly due to adaptive plasticity in traits that contribute to adaptation) may, instead, result in flat reaction norms. Here we investigate reaction norms for adaptive versus fitness-correlated traits, and how they may affect the conclusions regarding the contribution of plasticity. To this end, we first simulate range expansion along an environmental gradient where plasticity evolves to different values locally and then perform reciprocal transplant experiments in silico. We show that reaction norms alone cannot inform us whether the assessed trait exhibits locally adaptive, maladaptive, neutral or no plasticity, without any additional knowledge of the traits assessed and species' biology. We use the insights from the model to analyse and interpret empirical data from reciprocal transplant experiments involving the marine isopod Idotea balthica sampled from two geographical locations with different salinities, concluding that the low-salinity population likely has reduced adaptive plasticity relative to the high-salinity population. Overall, we conclude that, when interpreting results from reciprocal transplant experiments, it is necessary to consider whether traits assessed are locally adaptive with respect to the environmental variable accounted for in the experiments, or correlated to fitness.

Adaptive, maladaptive, neutral, or absent plasticity: Hidden caveats of reaction norms

“…Another possibility is that plasticity with respect to salinity has been reduced in the Baltic Sea (i.e., in our low-salinity population) because the salinity gradient is shallower there than in the Öresund (Johannesson et al 2020) and/or because the low-salinity population experiences weaker fluctuations in salinity than the high-salinity population (because changes in wind and outflow from the Öresund result in larger fluctuations in salinity levels in the high-salinity environment than in the low-salinity environment; Bendtsen et al 2009; Maar et al 2011; see also ICES data https://www.ices.dk/data/dataset-collections/Pages/default.aspx)). A shallower local environmental gradient and/or weaker temporal fluctuations in the environmental conditions (as is the case here for the low-salinity I. balthica population compared to the high-salinity population) have been theoretically shown to result in lower local plasticity (Eriksson & Rafajlović 2022).…”

A decade of progress in marine evolutionary biology

“…The majority of marine studies on this topic have been limited in duration, number of drivers, and in the ability to extrapolate to fitness consequences over time. One crucial factor highlighted in this Special Issue is that the relationship between measured traits and fitness in common‐garden or transplant experiments needs to be well characterized to draw adequate conclusions about plastic or adaptive capabilities ( Eriksson et al, 2022 ) . The fitness value of a particular trait is often hard to determine and even more in an understudied ecological context.…”

“…Although common‐garden and reciprocal transplant experiments are powerful tools to test local adaptation and stress tolerance, the interpretation of results may be challenging. Plastic responses can be invaluable to mitigate species' vulnerability to rapid global change; however, Eriksson et al ( 2022 ) argue that traditional approaches comparing reaction norms of organisms exposed to different environments may not account for the adaptive value of plastic responses. The authors supported this argument by comparing modeling simulations of adaptive vs fitness‐correlated traits.…”

“…One major topic studied within the Marine Evolutionary Biology community is the interplay between genetic and plastic mechanisms in the processes of local adaptation, ecotype formation, and speciation, and how these processes may be impacted by, e.g., sexual selection, individuals' dispersal patterns, or species‐specific genomic properties, such as the size and distribution of chromosomal rearrangements (e.g., Faria et al, 2021 ; Ravinet et al, 2017 ). Herein, these processes are addressed in diverse empirical model systems ranging from diatoms to mollusks and fish (Green et al, 2022 ; Le Moan et al, 2022 ; Sefbom et al, 2022 ; Zhang et al, 2022 ), as well as in silico in two theoretical modeling studies (Eriksson et al, 2022 ; Marshall & Connallon, 2022 ). The link between genotype and phenotype is also the focus of several empirical studies here (Gefaell et al, 2022 ; Stenger et al, 2022 ; Walker et al, 2022 ), as is the seascape genomics of a wide range of taxa (Delaval, Bendall, et al, 2022 ; Delaval, Frost, et al, 2022 ; Fitz et al, 2022 ; Lapègue et al, 2022 ; Matias et al, 2022 ; Palumbi et al, 2022 ; Pampoulie et al, 2022 ), which has important implications for management of marine resources.…”

“…One major topic studied within the Marine Evolutionary Biology community is the interplay between genetic and plastic mechanisms in the processes of local adaptation, ecotype formation, and speciation, and how these processes may be impacted by, e.g., sexual selection, individuals' dispersal patterns, or species-specific genomic properties, such as the size and distribution of chromosomal rearrangements (e.g., Faria et al, 2021;Ravinet et al, 2017). Herein, these processes are addressed in diverse empirical model systems ranging from diatoms to mollusks and fish (Green et al, 2022;Le Moan et al, 2022;Sefbom et al, 2022;Zhang et al, 2022), as well as in silico in two theoretical modeling studies (Eriksson et al, 2022;Marshall & Connallon, 2022).…”