title: Body size and allometric shape variation in the molly Poecilia vivipara along a gradient of salinity and predation.
abstract: BACKGROUND
  Phenotypic diversity among populations may result from divergent natural selection acting directly on traits or via correlated responses to changes in other traits. One of the most frequent patterns of correlated response is the proportional change in the dimensions of anatomical traits associated with changes in growth or absolute size, known as allometry. Livebearing fishes subject to predation gradients have been shown to repeatedly evolve larger caudal peduncles and smaller cranial regions under high predation regimes. Poecilia vivipara is a livebearing fish commonly found in coastal lagoons in the north of the state of Rio de Janeiro, Brazil. Similar to what is observed in other predation gradients, lagoons inhabited by P. vivipara vary in the presence of piscivorous fishes; contrary to other poeciliid systems, populations of P. vivipara vary greatly in body size, which opens the possibility of strong allometric effects on shape variation. Here we investigated body shape diversification among six populations of P. vivipara along a predation gradient and its relationship with allometric trajectories within and among populations.
  
  
  RESULTS
  We found substantial body size variation and correlated shape changes among populations. Multivariate regression analysis showed that size variation among populations accounted for 66% of shape variation in females and 38% in males, suggesting that size is the most important dimension underlying shape variation among populations of P. vivipara in this system. Changes in the relative sizes of the caudal peduncle and cranial regions were only partly in line with predictions from divergent natural selection associated with predation regime.
  
  
  CONCLUSIONS
  Our results suggest the possibility that adaptive shape variation among populations has been partly constrained by allometry in P. vivipara. Processes governing body size changes are therefore important in the diversification of this species. We conclude that in species characterized by substantial among-population differences in body size, ignoring allometric effects when investigating divergent natural selection's role in phenotypic diversification might not be warranted.
authors.affiliation: Departamento de Ecologia, Universidade Estadual Paulista, Rio Claro, SP, Brazil. msaraujo@rc.unesp.br.
authors.forename: Márcio S
authors.initials: MS
authors.lastname: Araújo
authors.affiliation: División Antropología, Facultad de Ciencias Naturales y Museo, Universidad Nacional de La Plata, CONICET, La Plata, Argentina. ivanperezmorea@gmail.com.
authors.forename: S Ivan
authors.initials: SI
authors.lastname: Perez
authors.affiliation: Departamento de Ecologia, Universidade Estadual Paulista, Rio Claro, SP, Brazil. mjmagazoni@gmail.com.
authors.forename: Maria Julia C
authors.initials: MJ
authors.lastname: Magazoni
authors.affiliation: Núcleo em Ecologia e Desenvolvimento Socioambiental de Macaé - NUPEM, Universidade Federal do Rio de Janeiro - UFRJ, Macaé, RJ, Brazil. petryanacristina@gmail.com.
authors.forename: Ana C
authors.initials: AC
authors.lastname: Petry
mesh_terms: D000818:Animals; D005075:Biological Evolution; D049628:Body Size; D001938:Brazil; D005260:Female; D014644:Genetic Variation; D008297:Male; D010641:Phenotype; D011036:Poecilia / Q000033:anatomy & histology* / Q000502:physiology; D011235:Predatory Behavior; D054712:Salinity; D012641:Selection, Genetic