Species delimitation under the general lineage concept: an empirical example using wild North American hops...

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 among concepts depending on the perceived importance of various attributes of evolving populations. We have examined five different species criteria to ask whether the three taxonomic varieties of Humulus lupulus (hops) native to North America are distinct lineages. Three criteria (monophyly, absence of genetic intermediates, and diagnosability) focus on evolutionary patterns and two (intrinsic reproductive isolation and niche specialization) consider evolutionary processes. Phylogenetic analysis of amplified fragment length polymorphism (AFLP) data under a relaxed molecular clock, a stochastic Dollo substitution model, and parsimony identified all varieties as monophyletic, thus they satisfy the monophyly criterion for species delimitation. Principal coordinate analysis and a Bayesian assignment procedure revealed deep genetic subdivisions and little admixture between varieties, indicating an absence of genetic intermediates and compliance with the genotypic cluster species criterion. Diagnostic morphological and AFLP characters were found for all varieties, thus they meet the diagnosability criterion. Natural history information suggests that reproductive isolating barriers may have evolved in var. pubescens, potentially qualifying it as a species under a criterion of intrinsic reproductive isolation. Environmental niche modeling showed that the preferred habitat of var. neomexicanus is climatically unique, suggesting niche specialization and thus compliance with an ecological species criterion. Isolation by distance coupled with imperfect sampling can lead to erroneous lineage identification using some species criteria. Compliance with complementary pattern- and process-oriented criteria provides powerful corroboration for a species hypothesis and mitigates the necessity for comprehensive sampling of the entire species range, a practical impossibility in many systems. 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. Thus, we support recognition of vars. neomexicanus and pubescens as species, but delay making a recommendation for var. lupuloides until sampling of genetic variation is complete or a stable biological process can be identified to explain its observed genetic divergence.