Evol Ecol Res 14: 147-167 (2012)     Full PDF if your library subscribes.

Phenotypic divergence of exotic fish populations is shaped by spatial proximity and habitat differences across an invaded landscape

Peter A.H. Westley,* Corinne M. Conway and Ian A. Fleming

Ocean Sciences Centre, Memorial University of Newfoundland, St. John’s, Newfoundland, Canada

Correspondence: P.A.H. Westley, School of Aquatic and Fishery Sciences, University of Washington, Box 355020, Seattle, WA 98195, USA.
e-mail: resolute@uw.edu


Background: Brown trout (Salmo trutta) were introduced into, and subsequently colonized, a number of disparate watersheds on the island of Newfoundland, Canada (110,638 km2), starting in 1883.

Questions: Do environmental features of recently invaded habitats shape population-level phenotypic variability? Are patterns of phenotypic variability suggestive of parallel adaptive divergence? And does the extent of phenotypic divergence increase as a function of distance between populations?

Hypotheses: Populations that display similar phenotypes will inhabit similar environments. Patterns in morphology, coloration, and growth in an invasive stream-dwelling fish should be consistent with adaptation, and populations closer to each other should be more similar than should populations that are farther apart.

Organism and study system: Sixteen brown trout populations of probable common descent, inhabiting a gradient of environments. These populations include the most ancestral (∼130 years old) and most recently established (∼20 years old).

Analytical methods: We used multivariate statistical techniques to quantify morphological (e.g. body shape via geometric morphometrics and linear measurements of traits), meristic (e.g. counts of pigmentation spots), and growth traits from 1677 individuals. To account for ontogenetic and allometric effects on morphology, we conducted separate analyses on three distinct size/age classes. We used the BIO-ENV routine and Mantel tests to measure the correlation between phenotypic and habitat features.

Results: Phenotypic similarity was significantly correlated with environmental similarity, especially in the larger size classes of fish. The extent to which these associations between phenotype and habitat result from parallel evolution, adaptive phenotypic plasticity, or historical founder effects is not known. Observed patterns of body shape and fin sizes were generally consistent with predictions of adaptive trait patterns, but other traits showed less consistent patterns with habitat features. Phenotypic differences increased as a function of straight-line distance (km) between watersheds and to a lesser extent fish dispersal distances, which suggests habitat has played a more significant role in shaping population phenotypes compared with founder effects.

Conclusion: Recently established brown trout populations exhibit phenotype-by-environment correlations consistent with adaptation to newly encountered environments, a characteristic that may aid their spread to additional systems.

Keywords: allometry, biological invasions, coloration patterns, geometric morphometrics, microevolution, phenotypic divergence with distance, phenotypic plasticity, salmonid fishes.

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