Abstract
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.