Understanding the determinants of the species range, the area within which members of a given species occur for at least part of their lives, is a fundamental goal of modern ecology and the basis for management of rare and invasive species (Mayr 1963; Hoffman & Blows 1994; Kirkpatrick & Barton 1997; Holt & Keitt 2005). Interspecific variation in range size is large, spanning 12 orders of magnitude (Brown, Stevens & Kaufman 1996). The factors contributing to this variation are complex and include historical influences such as species age and biogeographic limits to expansion, as well as contemporary interactions between the environment and species’ ecological tolerances. Ecological tolerances, which govern the position and breadth of occurrence of species along environmental gradients, are the products of the physiological, morphological and developmental attributes of its members (Good 1974; Holt 2003) and can be heritable. Therefore, by extension, range position and breadth are potentially shaped by evolution and the limits to adaptation (Antonovics 1976; Kirkpatrick & Barton 1997).
To the extent that ecological tolerance has a genetic basis, species range size and position will be the product of two contrasting evolutionary forces: phylogenetic history, which results in resemblance between related species, and selection or drift in contrasting environments, which causes divergence. There is little consensus about whether phylogeny influences a species’ range. Some authors have suggested that the geographic range shows a phylogenetic signal because of the inheritance of ecological tolerances (Peat & Fitter 1994; Kelly & Woodward 1996; Waldron 2007), whereas others maintain that small differences in biology will lead to large differences in range attributes, and therefore, the effect of phylogenetic history will be negligible (Brown, Stevens & Kaufman 1996; Webb & Gaston 2003). Research on birds indicates that most variation in range area occurs at the species level (Gaston 1998), suggesting little phylogenetic signal. However, a study of fossil mollusks found significant correlations in range size between sister species (Jablonski 1987).
In plants, sources of variation in range area have been examined in only two studies, both of which focused on native or naturalized floras of islands, Crete and Great Britain (Peat & Fitter 1994; Kelly & Woodward 1996). As with birds, most of the variation occurred among species. However, the small geographic scale of these studies may have artificially constrained range size variation and limited the ability to detect a phylogenetic signal (Jones, Sechrest & Gittleman 2005). Furthermore, the ecological attributes of species ranges (e.g. breadth and position with respect to climate) have rarely been studied in this context, although they may also be conserved or constrained by the inheritance of ecological tolerances. To our knowledge, only one study has examined variation in ecological dimensions of plant ranges (Prinzing et al. 2001). In their study, Prinzing et al. (2001) found that for half of the climatic variables (temperature, light and soil conditions) they measured, more than 50% of the variance was explained above the species level. Thus, the results regarding phylogenetic influences on species range size and position are heterogeneous and conflicting. In plants, this can be addressed by studying both geographical and ecological attributes of species ranges on a larger spatial scale.
Among extant species, polyploidy is considered an important genetic cause of changes in the geographical and ecological range of species (Levin 2002). Genetic divergence among species of the same ploidy (homoploid divergence) via mutation, recombination, gene duplication and rearrangement, will influence ecological tolerance, but these changes are generally considered to have smaller effects than the transgressive influences of genome multiplication (Levin 2002). Two types of polyploids are recognized: autopolyploids, which form by duplicating the genome of a single species, and allopolyploids, formed by duplicating the genome of interspecific hybrids. The difference between the effects of these two types of polyploidy are not well explored, however, both types of polyploidization can potentially affect species ranges in two major ways (Muntzing 1936; Stebbins 1950). First, polyploidy can cause an immediate shift in phenotype and physiological tolerances directly as a result of a larger genome and cell size. Secondly, polyploidy may affect the evolution of a species’ range by increasing the capacity for genetic variation, the probability of beneficial mutations and the potential for novel adaptive responses to selection (Stebbins 1985; Levin 2000; Otto & Whitton 2000; Soltis & Soltis 2002). Indeed, the potential effects of polyploidization have led biologists to expect that both allo- and autopolyploidy will cause changes to the position of a species’ range along an ecological gradient, specifically towards more extreme environments and more northern latitudes, and to the breadth of environments occupied compared to diploids (Stebbins 1950; reviewed by Levin 2000). The distinction between these two effects is not well examined and the empirical evidence for greater breadth is conflicting. Early work by Muntzing (1936) and Stebbins (1950) corroborated these expectations, and more recently, a study of Clarkia found that polyploids had significantly larger ranges than diploids within the genus (Lowry & Lester 2006). In contrast, broader taxonomic surveys have found no evidence for this pattern (Stebbins & Dawe 1987; Petit & Thompson 1999). These broader studies, however, have not been conducted within a phylogenetic framework. This could obscure the effect of genome duplication on range attributes if the phylogenetic history of a species also has a significant effect on range size (Levin 2000).
In this study, we examined the evolutionary basis of species ranges in flowering plant species of North America. We estimated both geographical (area, overlap) and ecological attributes (precipitation and temperature) of species ranges. These attributes were used to characterize range breadth (geographic area, range of temperatures and precipitation) and position (latitude, mean precipitation and temperature) along ecological gradients. To account for phylogenetic history, we collected data for two diploid and one polyploid species from each of 144 genera. This sampling design allowed us to compare the effects of genome duplication (i.e. divergence between species with different ploidy – heteroploid species) to differences between homoploid species (species with the same ploidy). Because diploids are considered the ancestral state (Stebbins 1971), we used the differences between two related diploids as the evolutionary baseline against which the effects of polyploidy were evaluated. Specifically, we explored the following questions: (i) What proportion of the variation in each of the geographic and climatic range attributes is explained by taxonomic membership above the species level (i.e. order, family and genus)? (ii) Are range attributes correlated among relatives, and are heteroploid relatives less correlated than diploid relatives? (iii) Are polyploids different from their diploid relatives with respect to range size, overlap, climate position and climate breadth, and are these differences greater than those between diploid relatives?