Letters Response Dynamic affairs–could be if we let it! Charlie K. Cornwallis and Tobias Uller Edward Grey Institute, Department of Zoology, University of Oxford, South Parks Road, Oxford, UK, OX1 3PS The aim of our opinion paper was to highlight the potential importance of fluctuating selection and phenotypic plasticity in the evolution of sexual traits, topics that have been fundamental for understanding how non-sexual traits evolve [1]. Safran et al. [2] state that they disagree with aspects of our contribution, advocating that business should continue as usual with respect to empirical research, but what is ‘urgently required’ is ‘the develop- ment of new theoretical models’. In our paper we tried to highlight that we think new theory is important by stating ‘we need more explicit theory’ in the first point of the discussion and concluding with a call for the ‘joint pro- gression of theoretical and empirical studies’ [1]; it would be a shame to have new models without new data to test their predictions. It therefore seems that Safran et al. [2] agree more with our perspective than they disagree. How- ever, some of Safran et al.’s [2] other remarks indicate to us that their perspective might perhaps obscure the devel- opment of such a theoretical framework, and so making a few additional remarks on their response might be worth- while. First, we proposed that a better understanding is needed of how the evolution of sexual traits is influenced by the interaction between different selection pressures (sexual, viability and kin) over space and time [1]. Safran et al. [2] state that ‘no one doubts’ this and reiterate the classic view that viability selection curbs the exaggeration of sexual traits. However, the point is not if this is theor- etically possible or not (in fact, it is possible), but rather that there is relatively little research on how different selection pressures interact (beyond viability selection curbing sexual selection) and change over space and time in populations with different genetic structures and demo- graphics [3–5]. We are by no means the first to point this out. Recent reviews have also noted the lack of integration between research on sexual selection and other disciplines such as kin selection [6]. We raised this issue again within the context of fluctuating selection on sexual traits with the hope of generating more integrative research in the future. Second, Safran et al. [2] take exception to our point that sexual traits are often studied in isolation and results are extrapolated over temporal and spatial scales. They state ‘we note that the use of multiple sexual signals (unimodal and multimodal) and the relationship between signal evol- ution and environmental context have received extensive support’. We believe this point is best answered by a review [5] of this topic in the May 2010 issue of TREE, which states ‘research [on multiple signalling]... has taken a largely static view of the world’ concluding that ‘Future challenges will include identifying the circumstances under which environmental fluctuations lead to multiple signalling’. Third, Safran et al. [2] seem happy to see the role of phenotypic plasticity being dealt with only in terms of condition-dependence. As briefly outlined in our opinion and reviewed in more depth elsewhere [7], we believe condition-dependence does not capture the role of plasticity in the evolution of sexual traits. To illustrate this using a basic example from within the standard research frame- work of sexual selection, imagine a situation where all individuals are in exactly the same condition, but are placed in different social contexts, say one male with two females vs. two males with one female. Males are predicted to plastically adjust the amount of sperm they allocate to females [8], but this does not have anything to do with their condition; it could, but it doesn’t have to. More generally, we believe that only examining plasticity from the perspective of condition dependence ignores many important and interesting aspects of plasticity, including its role in the origin of novel secondary sexual traits and its impact on the direction of evolutionary trajectories (in- cluding the evolution of condition dependent expression of sexual traits [9]) [7,10,11]. We believe examining plasticity in sexual traits more generally than just condition depen- dence is a more encompassing approach that will draw direct links with other areas of evolutionary ecology [10– 12]. In fact, it has been noted that a proper understanding of the role of plasticity in evolution, using empirical and theoretical approaches, is considered one of the greatest challenges in evolutionary research [10–12]. Studies of sexual traits in this context might hold important insights and are well worth pursuing [7,9]. Finally, Safran et al. [2] disagree with our opinion by stating ‘plasticity is not a cause of natural or sexual selection’. We, and others, believe it is [7,10,11]. Simply put, if organisms change their phenotype in response to environmental conditions (plasticity), this can change their interactions with other individuals. It is well recog- nised that biotic interactions are an important com- ponent of selection. Consequently, plasticity can cause changes in selection acting on surrounding individuals and in this sense plasticity is a cause of selection [7].A similar effect could also be achieved if organisims change their abiotic environment [13]. In fact, Safran et al. [2] again probably agree more than they disagree since they state that ‘plasticity facilitates or constrains evolution- ary change by altering the form and direction of selec- tion’. If plasticity alters the form and direction of selection, it seems reasonable to us to call it a cause of variation in selection. To summarize, we reiterate our original point [1], which seems to agree in part with Safran et al.’s main message, Update Corresponding author: Cornwallis, C.K. (charlie.cornwallis@zoo.ox.ac.uk). TREE-1253; No. of Pages 2 1