Abstract
Understanding the ecological pressures that generate variation in body shape is important because body shape profoundly affects physiology and overall fitness. Using Fundulus, a genus of fish that exhibits considerable morphological and physiological variation with evidence of repeated transitions between freshwater and saltwater habitats, we tested whether habitat salinity has influenced the macroevolution of body shape at different stages in development. After accounting for phylogenetic inertia, we find that body shape deviates from the optimal streamlined shape in a manner consistent with different osmoregulatory pressures exerted by different salinity niches at every stage of ontogeny that we examined. We attribute variation in body shape to differential selection for osmoregulatory efficiency because: (1) saline intolerant species developed body shapes with relatively low surface areas more conducive to managing osmoregulatory demands and (2) inland species that exhibit high salinity tolerances have body shapes similar to saline tolerant species in marine environments.
Highlights
Natural selection is a major driver of the evolution of morphological diversity among species (Arnold, 1983; Schluter and Smith, 1986; Berner et al, 2008)
Salinity niche, which we describe as all aspects of a species’ environment that change with salinity, appears to have driven morphological variation in fish, especially as it relates to osmoregulation, and the gills and kidneys
Regardless of which aspect of body shape was compared among species (i.e., Procrustes distance, landmark pairs, or fineness ratio (FR)), and whether an adaptive or correlative macroevolutionary model was used, a clear pattern between historical salinity niche and body shape emerged
Summary
Natural selection is a major driver of the evolution of morphological diversity among species (Arnold, 1983; Schluter and Smith, 1986; Berner et al, 2008). Other factors, such as genetic drift (Ackermann and Cheverud, 2004) and evolutionary constraints, which prevent traits from reaching their optima (e.g., phylogenetic inertia; Hansen and Orzack, 2005; Pienaar et al, 2013), have the potential to influence the direction and magnitude of morphological evolution among species. Few studies have examined ecological pressures that might have driven selection for morphological variation at several stages in development
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