Srivastava, Alka
Hipp, Andrew
Weber, Jaime A.

Who am I this time? The affinities and misbehaviors of Hill's oak (Quercus ellipsoidalis)

Date created
Oaks, it has long been recognized, readily hybridize. They do not obey the limit to interspecific hybridization that is the hallmark of the biological species concept [1], and they have in fact been described by two of the leaders in the field of speciation as a "worst case scenario for the biological species concept" [2]. Pioneering work by James Hardin [3] showed hybridization among 14 of the 16 species recognized within the white oak subgenus members of eastern North America, with hybridization occurring almost anywhere that white oaks grow in sympatry. In the molecular era, hybridization is often suggested by chloroplast sharing among species [4-6]. But in spite of this fact, nuclear markers in sufficient numbers can distinguish oak species [7-9], and the genetic groupings that result generally accord closely with our understanding of oak species limits based on morphology, geography, and ecology. The congruence among these data sources strongly suggests that oak species are genetically coherent across wide ranges. In the North American Great Lakes region, the taxonomy of Hill's oak (Quercus ellipsoidalis E.J.Hill) and scarlet oak (Q. coccinea Münchh.) has long been recognized as problematic [8, 10-16]. The two species are largely allopatric but overlap in characters of the end buds, leaves, and acorns. In their purest expressions, there is no confusing them, but the overlap raises the question of whether the two are better treated as separate species [8, 17-19] or endpoints on a morphological continuum [11, 12, 14, 15, 20, 21]. A second issue is the degree to which these species hybridize with black oak (Quercus velutina Lam.), which is widespread throughout the region occupied by both species and morphologically entangled with them [19]. Previous molecular genetics work on these species demonstrated that scarlet oak is distinct from both Hill's oak and black oak, that black oak and Hill's oak are genetically similar to one another and may hybridize, and that and genetic intermediacy between black oak and Hill's oak correlates poorly with morphological intermediacy [8]. In demonstrating that Hill's oak and black oak grade into each other morphologically and genetically in the Great Lakes region, while Hill's oak and scarlet oak grade into each other morphologically but apparently not genetically, the study raised the intertwined issues of the evolutionary origins of Hill's oak and relatives and their ongoing gene flow. Is Hill's oak derived from within black oak, its similarity to scarlet oak an expression of shared ancestral characters that have been lost in black oak? Alternatively, is Hill's oak more closely related to scarlet oak, and genetically similar to black oak due to interspecific gene flow? The alternative effects of gene flow and phylogeny in explaining genetic similarity can be difficult to tease apart [22, 23], and in this study we utilize a combination of population-genetic, phylogenetic, and biogeographic approaches to try to do so.
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Volume, Page Number
21, 27-36