Evol Ecol Res 14: 973-992 (2012)     Full PDF if your library subscribes.

Intraspecific diversity in Arctic charr, Salvelinus alpinus, in Iceland: I. Detection using mixture models

Pamela J. Woods1,2,3, Skúli Skúlason1, Sigurður S. Snorrason2,

Bjarni K. Kristjánsson1, Hilmar J. Malmquist4 and Thomas P. Quinn3

1Hólar University College, Sauðárkrókur, Iceland,  2University of Iceland, Reykjavík, Iceland, 3University of Washington, Seattle, Washington, USA and  4Natural History Museum of Kópavogur, Kópavogur, Iceland

Correspondence: P.J. Woods, Hólar University College, Háeyri 1, 551 Sauðárkrókur, Iceland. E-mail: pamelajwoods@gmail.com


Background: Finite-mixture models allow sub-populations to be described by separate parameter sets, but do not require the members of sub-populations to be identified. Although the accessibility of these methods has greatly increased in recent years, they are rarely used in studies of ecology and evolution, despite their ability to address basic and common questions regarding the detection of biological diversity.

Questions: Do mixture models uncover previously unknown polymorphism of Arctic charr (Salvelinus alpinus) within populations across Iceland? How many groups can be defined based on morphology and growth curves? How differentiated are these groups, and do these groups correspond with other differences, e.g. in life history and habitat use?

Data incorporated: Length-at-age, maturity, diet, and morphology data on Arctic charr from 50 lakes sampled in the Ecological Survey of Icelandic Lakes.

Analysis methods: We used mixture models to detect polymorphism as the presence of (1) multiple von Bertalanffy growth curves and (2) multimodality in multivariate morphology. We also analysed whether timing of maturity was related to growth and whether sexual dimorphism confounded the results. Dietary patterns were used to confirm a relationship between polymorphism and resource use. Metrics were defined to indicate the relative degree of differentiation.

Conclusions: Polymorphism occurs in more lakes than previously documented. In many cases, polymorphism in growth curves or morphology was mirrored by differences in diet. Mixture models are useful tools for identifying multimodal diversity below the species level (e.g. resource polymorphism). These results yield an important first step in understanding how the functional role of a species can vary within and among localities and how this may relate to processes of divergence.

Keywords: bimodal distribution, divergent selection, ecological speciation, individual heterogeneity, life history, maturity, mixture model, resource polymorphism, salmonid, Von Bertalanffy.

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