Random Patterns of Medicinal Plants on a Phylogeny do not Imply Random Selections of Medicinal Plants

Existing studies report that medicinal plants are either phylogenetically clustered or that plant medicinal property is phylogenetically conserved. Each scenario is traditionally interpreted as evidence of non-random selection by humans of medicinal plants. Here, we argue that this interpretation is misleading, highlighting that both scenarios are simultaneously required for a correct interpretation of human selection of medicinal plants. We propose a framework to illustrate all possible scenarios of phylogenetic patterns and human selection options and tested the framework employing phylogenetic comparative methods on medicinal plant data collected in northern Nigeria. First, we found a mixed selection pattern in plant organs used for medicine, perhaps mirroring the within-plants heterogeneous distribution of secondary compounds as predicted in the optimal defense theory. Second, medicinal plants used to treat most diseases follow a random distribution on the phylogeny, whereas their medicinal properties are convergent on the phylogeny. This pattern is plausible only when humans select preferentially less related species for medicinal uses. That we found a random phylogenetic structure for species used to treat most diseases would, traditionally, have been interpreted as random selection of medicinal plants. We, therefore, call for caution while interpreting phylogenetic patterns in ethnobiology.