ARS | Publication request: Species delimitation under the general lineage concept: An empirical example using wild North American hops (cannabaceae: Humulus lupulus)

Submitted to: Systematic Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 4, 2010
Publication Date: January 1, 2011
Citation: Reeves, P.A., Richards, C.M. 2011. Species delimitation under the general lineage concept: An empirical example using wild North American hops (cannabaceae: Humulus lupulus). Systematic Biology. 60:45-59.

Interpretive Summary: Within the field of genetics resources conservation, species concepts are not simply academic but are critical in determining effective sampling strategies within and among genetic lineages. This is particularly evident in plant species that are subject to frequent hybridization and introgression. We develop a comprehensive test of species concepts for use in understanding the historical relationship between varieties within North American hops (Humulus lupulus). We use a set of complimentary methods including molecular, phenotypic and GIS/niche modeling to infer the process of speciation that may have lead to the observed ecotypic differentiation. Increasingly combinations of methods such as we use here will be important to characterize variation for sampling and utilization.

Technical Abstract: There is an emerging consensus that the intent of most species concepts is to identify evolutionarily-distinct lineages. However, the criteria used to identify lineages differ between concepts depending on the perceived importance of various attributes of evolving populations. We have applied tests integral to five common species concepts to ask whether the three taxonomic varieties of Humulus lupulus (hops) native to North America are distinct lineages. Using parsimony analysis of AFLP data we show that varieties are monophyletic and therefore satisfy the requirement for delimitation as species under one version of the phylogenetic species concept. Principal coordinate analysis and a Bayesian assignment procedure revealed deep genetic subdivisions and little admixture between varieties, indicating compliance with the genotypic cluster species concept. Diagnostic morphological and AFLP characters were found for vars. pubescens and neomexicanus, which therefore form species under another version of the phylogenetic species concept. Natural history information suggests that partial intrinsic reproductive isolation may have evolved in var. pubescens, potentially qualifying it as a biological species. Ecological niche modeling showed that var. neomexicanus habitat is climatically unique, thus it complies with the ecological species concept. Accordingly, vars. pubescens and neomexicanus are recognizably-distinct lineages and merit elevation to species rank. We hypothesize that var. pubescens maintains its genetic identity, despite substantial niche overlap with var. lupuloides, via the evolution of partial reproductive isolating mechanisms. Variety neomexicanus, conversely, will likely persist as a distinct lineage, regardless of limited gene flow with vars. lupuloides and pubescens, because of ecological isolation—adaptation to the unique conditions of the Rocky Mountain cordillera. Variety lupuloides may be a nascent species that has emerged since the last glacial maximum, one that has not yet attained many of the characteristics associated with mature lineages.