Abstract
Intraspecific variation in vertebral number is taxonomically widespread. Much scientific attention has been directed towards understanding patterns of variation in vertebral number among individuals and between populations, particularly across large spatial scales and in structured environments. However, the relative role of genes, plasticity, selection, and drift as drivers of individual variation and population differentiation remains unknown for most systems. Here, we report on patterns, causes and consequences of variation in vertebral number among and within sympatric subpopulations of pike (Esox lucius). Vertebral number differed among subpopulations, and common garden experiments indicated that this reflected genetic differences. A QST-FST comparison suggested that population differences represented local adaptations driven by divergent selection. Associations with fitness traits further indicated that vertebral counts were influenced both by stabilizing and directional selection within populations. Overall, our study enhances the understanding of adaptive variation, which is critical for the maintenance of intraspecific diversity and species conservation.
Highlights
The causes and consequences of discontinuous phenotypic variation have fascinated evolutionary biologists for more than a century[1]
That intraspecific variation of VN has an additive and heritable genetic basis has been demonstrated by common garden experiments as well as by heritability estimates derived from quantitative genetics approaches[4,8,15,28]
Our main findings were: i) number of vertebrae varied among subpopulations in the natural environment, ii) variation among subpopulations was, at least in part, genetically based and driven by divergent stage-specific selection as evidenced by a common garden experiment and a QST-FST comparison, iii) resemblance between captive-reared and wild-caught individuals suggested that the pattern of variation documented in natural subpopulations reflected genetic differences rather than plasticity, iv) cross-sectional comparisons indicated directional viability selection on VN, v) associations with juvenile body size, growth trajectories and reproductive effort suggested that individuals with an intermediate number of vertebrae had higher fitness, indicative of stabilizing selection within subpopulations
Summary
The causes and consequences of discontinuous phenotypic variation have fascinated evolutionary biologists for more than a century[1]. Patterns of variation among species suggest that vertebral number ( VN) is associated with ecology, life-style, body form (e.g., elongation), and body size[9,12]. Vertebral number varies among populations within various species of fish, salamanders and reptiles[2,7,9,15,16]. The evolution of an elongated snake- (and fish-) like body form with a de-regionalized pre-cloacal axial skeleton is best explained by retention of standard vertebrate Hox gene domains with alteration of downstream expression that suppresses development of distinct regions and increases somite numbers[25,26,27]. Subpopulations are genetically differentiated (as evidenced by analysis of microsatellite marker data)[40] and display local adaptations of larval traits[42] and in growth trajectories and adult body size to their specific breeding habitats[39]
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