On Adaptive Accuracy and Precision in Natural Populations

Abstract

Adaptation is usually conceived as the fit of a population mean to a fitness optimum. Natural selection, however, does not act only to optimize the population mean. Rather, selection normally acts on the fitness of individual organisms in the population. Furthermore, individual genotypes do not produce invariant phenotypes, and their fitness depends on how precisely they are able to realize their target phenotypes. For these reasons we suggest that it is better to conceptualize adaptation as accuracy rather than as optimality. The adaptive inaccuracy of a genotype can be measured as a function of the expected distance of its associated phenotype from a fitness optimum. The less the distance, the more accurate is the adaptation. Adaptive accuracy has two components: the deviance of the genotypically set target phenotype from the optimum and the precision with which this target phenotype can be realized. The second component, the adaptive precision, has rarely been quantified as such. We survey the literature to quantify how much of the phenotypic variation in wild populations is due to imprecise development. We find that this component is often substantial and highly variable across traits. We suggest that selection for improved precision may be important for many traits.

Whither adaptation?

“…The various definitions of adaptation and maladaptation can be wearisome in their diversity, contradiction, and controversy (Williams 1966;Gould and Lewontin 1979;Rose and Lauder 1996;Crespi 2000;Nesse 2005;Hansen et al 2006). I therefore simply outline my own view, which experience suggests is shared by at least some other evolutionary ecologists.…”

“…I will variously invoke both response variables in the following sections. Of additional interest, although not considered here, is the spread of individual phenotypes around an optimum (Hansen et al 2006), and the extent to which that variation might or might not be adaptive.…”

Floral specialization and angiosperm diversity: phenotypic divergence, fitness trade-offs and realized pollination accuracy

“…At the level of the population, there are at least three components, which are additive (Armbruster et al 2004, 2009 a ; Hansen et al 2006; Pélabon et al 2012): (i) optimality of the mean, which is how far the mean of events departs from the optimum (=‘maladaptive bias’), (ii) the variance, which is how much individuals vary from the mean (=‘adaptive imprecision’), and (iii) the variance in the optimum. By extrapolation from measurement theory (Armbruster et al 2004; Pélabon et al 2012), these three components sum to the adaptive inaccuracy as \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} }{}$$\eqalign{{\rm adaptive}\,{\rm inaccuracy} = ({{\rm trait}\,{\rm mean}{\rm \ - }{\rm \ optimum}\,{\rm value}} )^{\rm 2} \cr {\rm + trait}\ {\rm variance + optimum}\,{\rm variance}}$$\end{document} …”

Heritabilities of Directional Asymmetry in the Fore- and Hindlimbs of Rabbit Fetuses

“…On average, albeit only small for each bone, the degree of DA was higher for forelimbs (≈0.15% of trait size) compared to hindlimbs (≈0.05% of trait size) (Table 3). Coefficients of phenotypic variation were high, ranging from ∼300 to ∼1000%, reflecting small average DA and a very imprecise expression (Hansen et al 2006 [29]). Pairwise comparisons of forelimb vs. hindlimb elements, corrected for multiple testing, showed that DA was higher in forelimbs ( P<0.01 for all comparisons).…”