Species delimitation in systematics: inferring diagnostic differences between species

Species are fundamental units in studies of systematics, biodiversity and e cology, but their delimitation has been relatively neglected methodological ly. Species are typically circumscribed based on the presence of fixed (int raspecifically invariant or non-overlapping) diagnostic morphological chara cters which distinguish them from other species. In this paper, we argue th at determining whether diagnostic characters are truly fixed with certainty is generally impossible with finite sample sizes and we show that sample s izes of hundreds or thousands of individuals may be necessary to have a rea sonable probability of detecting polymorphisms in diagnostic characters at frequencies approaching zero. Instead, we suggest that using a non-zero fre quency cut-off may be a more realistic and practical criterion for characte r-based species delimitation (for example, allowing polymorphisms in the di agnostic characters at frequencies of 5% or less). Given this argument, we then present a simple statistical method to evaluate whether at least one o f a set of apparently diagnostic characters is below the frequency cut-off. This method allows testing of the strength of the evidence for species dis tinctness and is readily applicable to empirical studies.