Abstract
Freshwater colonization by threespine stickleback has led to divergence in morphology between ancestral marine and derived freshwater populations, making them ideal for studying natural selection on phenotypes. In an open brackish–freshwater system, we previously discovered two genetically distinct stickleback populations that also differ in geometric shape: one mainly found in the brackish water lagoon and one throughout the freshwater system. As shape and size are not perfectly correlated, the aim of this study was to identify the morphological trait(s) that separated the populations in geometric shape. We measured 23 phenotypes likely to be important for foraging, swimming capacity, and defense against predation. The lateral plate morphs in freshwater displayed few significant changes in trait sizes, but the low plated expressed feeding traits more associated with benthic habitats. When comparing the completely plated genetically assigned populations, the freshwater, the hybrids, the migrants and the lagoon fish, many of the linear traits had different slopes and intercepts in trait‐size regressions, precluding our ability to directly compare all traits simultaneously, which most likely results from low variation in body length for the lagoon and migrant population. We found the lagoon stickleback population to be more specialized toward the littoral zone, displaying benthic traits such as large, deep bodies with smaller eyes compared to the freshwater completely plated morph. Further, the lagoon and migrant fish had an overall higher body coverage of lateral plates compared to freshwater fish, and the dorsal and pelvic spines were longer. Evolutionary constraints due to allometric scaling relationships could explain the observed, overall restricted, differences in morphology between the sticklebacks in this study, as most traits have diversified in common allometric trajectories. The observed differences in foraging and antipredation traits between the fish with a lagoon and freshwater genetic signature are likely a result of genetic or plastic adaptations toward brackish and freshwater environments.
Highlights
When dispersing into new environments, novel resources, competitors, and predation regimes are agents of natural selection that may lead to new phenotypic optima (Schluter, 2000; Schluter & Conte, 2009)
All present freshwater populations are believed to have descended from marine ancestors (Bell, 1981), and the numerous independent postglacial invasions of freshwater habitats have resulted in parallel evolution of a set of predictable phenotypes
There were no major significant differences between morphs when testing the morphological trait groups separately with principal component analysis (PCA) on the residual data, but the low-plated individuals were significantly different in PC3 (F2, 91 = 3.093, p = .05) when comparing predator defense traits, having on average a shorter second dorsal spine (Figure 3c)
Summary
When dispersing into new environments, novel resources, competitors, and predation regimes are agents of natural selection that may lead to new phenotypic optima (Schluter, 2000; Schluter & Conte, 2009). Experimental studies indicate, for instance, that salinity alone can account for a large amount of the observed morphological shape differences between marine and freshwater stickleback (Mazzarella et al, 2015). Freshwater populations are typically of the low-plated morph, but completely plated stickleback do occur in fresh water (Hagen & Moodie, 1982; Kitano et al, 2008; Leinonen et al, 2012), either as resident individuals, or as anadromous fish breeding there (Harvey et al, 1997; McKinnon & Rundle, 2002; Taugbøl et al, 2014). There are many documented examples of morphological evolution in freshwater populations after colonization by a marine ancestor, where the similar freshwater phenotypes are interpreted as a result of natural selection causing the populations to adapt to the new environmental conditions. As size and shape are only imperfectly correlated, and their correlation is determined by the allometric relationships
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