Syndromes, wherein multiple traits evolve convergently in response to a shared selective driver, form a central concept in ecology and evolution. Recent work has questioned the utility and indeed the existence of some of the classic syndromes, such as pollination and seed dispersal syndromes. Here, we discuss some of the major issues that have plagued research into syndromes in macroevolution. First, observation of co-evolving traits (sometimes called "trait syndromes'') is often used as evidence of adaptation to a particular driver, even when the link between traits and adaptation is not well-tested. Second, the study of syndromes often uses a biased sampling approach, focusing on the most extreme examples, which may obscure significant continuous variation between traits. Finally, researchers often focus on the traits that are easiest to measure even though these may not be the most directly relevant to adaptive hypotheses. We argue that these issues can be avoided by combining macroevolutionary studies of trait variation across entire clades with explicit tests of adaptive hypotheses, and that taking this approach will lead to a better understanding of syndrome-like evolution and its drivers.

Abstract— Clematis subgenus Viorna of North America currently consists of 21 species with most of the diversity found in the southeastern United States. Past taxonomic treatments of Clematis subg. Viorna have varied greatly, which has led to unclear species limits across multiple species complexes. Often recognized as a single, polymorphic species with a high degree of ecological amplitude, variation in the Clematis reticulata species complex has previously been attributed to phenotypic plasticity. A combination of herbarium specimen examination and extensive fieldwork led to the formation of seven a priori morphological groups, or hypothesized taxa, in the C. reticulata species complex based on previously used and novel morphological characters. We employ an integrative approach to species delimitation with data from morphometric studies of herbarium and common garden datasets, as well as a phylogeny inferred from ddRADseq. Of the seven hypothesized taxa, six were supported by evidence from morphology and phylogeny with corroborating biogeography and ecology. Recognition of Clematis terminalis sp. nov. renders C. subreticulata comb. nov. paraphyletic. Because of its morphological, ecological, and phenological distinctiveness, it may represent a case of peripheral speciation by isolation. Four new species are recognized: C. arenicola sp. nov., C. cumberlandensis sp. nov., C. ouachitensis sp. nov., and C. terminalis sp. nov. Viorna subreticulata is given a new combination in Clematis and the newly defined C. reticulata s.s. is re-circumscribed. An updated dichotomous key, species descriptions, ecological notes, and distribution maps are provided.