Does developmental plasticity direct the divergence among species?

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Selection pressure is determined by ecological opportunity, but the response to selection depends on individual form (1). Individual form matters because individuals in species are characterised by a distinctive set of codependent traits. Selection on one trait can therefore generate a response to selection in others. This evolutionary constraint due to trait covariation is imposed by heredity or natural selection and can limit adaptation by channelling evolution in the direction of least evolutionary resistance (2) characterised by abundant phenotypic variability (3). Allometric relationships of size-shape covariance are classic lines of least evolutionary resistance. Allometries have been categorised into within-individual, among-individual and among-species scales (reviewed by 4), making them an ideal framework to investigate how variation among individuals generates variation among species.Contradicting recent re-analyses of over 300 animal taxa (5,6), Brombacher, Ezard et al. (invited resubmission, American Naturalist) found a decoupling between among-individual and among-species allometries in 12,633 individuals of two- species of planktonic foraminifera exposed to the intensification of Northern Hemisphere glaciation during the Pliocene. Neither published study (5,6) sampled through a major climate transition. The generality of when among-individual and among-species allometries become decoupled, as well as the macroevolutionary consequences of this decoupling are testable with PISTON's data from WP2, WP4 and WP5 for the first time.Our overarching hypothesis for PhD1 is that within-individual changes catalyse the breakdown in among-individual and among-species allometries. This question is one of the oldest in evolutionary biology: von Baer's third law states that individuals in different species become increasingly dissimilar from one each other during development, implying we should see changes at the within-individual level before species diverge. In reality, variation among individuals is a dynamic force that represents the ghost of evolution past. Under supervision of Watson and Ezard, PhD1 will extend well-developed learning theory models, familiar in computer science, to formally characterise how the cumulative outcome of historical selection facilitates future adaptability (7). Watson's existing theoretical models find that developmental correlations will align with directional selection creating 'lines of least resistance' (2) that facilitate directional adaptive change (7,8). This PhD project will extend learning theory with adaptive dynamics models (9) parameterised by foraminiferal trait distributions and environmental reconstructions to address three overarching questions:1: Does within-species plasticity precede and direct divergence among species along lines of least evolutionary resistance? 2: Are certain parts of the life-course (e.g., a developmental window early in life prior to canalisation) more predictive of divergence among species than others?3: How do historical environmental change and intraspecific variation interact to determine whether the allometric relationships will break down across individual, population and species scales?References: (1) Lande & Arnold 1983 Evolution 37, 1210. (2) Schluter 1996 J. Evol. Biol. 50, 1766. (3) Hunt 2007 Evolution 61, 1560. (4) Pelabon et al. 2014 Ann. New York Acad. Sci. 1320, 58. (5) Voje et al. 2014 Evolution 68, 866. (6) Firmat et al. 2014 Phil. Trans. R. Soc. B 369, 20140057. (7) Watson & Szathmary 2016 Trends Ecol. Evol. 31, 147. (8) Watson et al. 2014 Evolution 68, 1124. (9) Doebeli & Dieckmann (2000). Amer. Nat. 156, S77.