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Evolutionary theory predicts that selective harvest-ing will shift the frequency of genes for heritabletraits towards those that express well-adapted phe-notypes (Law 2007, this Theme Section). Applyingthis logic, commercial fisheries that select on thebasis of size could reasonably be expected to favorthe genotype for maturation at smaller sizes (and/oryounger ages), given that rapidly maturing individu-als will be more likely to reproduce prior to capture.This form of fisheries-induced evolutionary responseis a parsimonious explanation for the observed long-term trend towards earlier maturation that hasbeen observed in several commercially exploited fishstocks (Rijnsdorp 1993, Trippel 1995). Unfortunately,it is not possible to test this hypothesis directly, be-cause the genes associated with maturation have notbeen isolated.Probabilistic maturation reaction norms (PMRNs)(Heino et al. 2002) have been proposed as a statisticaltool for helping to disentangle genetic from plasticeffects on maturation (reviewed by Dieckmann &Heino 2007, this Theme Section). For a given cohort,the PMRN describes the maturation probability of indi-viduals that have survived to reach a given age andsize. A midpoint curve, illustrating the age and size atwhich the maturation probability is 50%, is often usedto summarise the PMRN. A shift in cohort-specificPMRNs over time is interpreted as being consistentwith (although not direct evidence of) a genetic changeat the population level. For example, a shift through*Email: c.t.marshall@abdn.ac.uk