Phylogeny can make the mid-domain effect an inappropriate null model

The mid-domain effect (MDE), a bias in species richness towards the midpoint of a given geographical dimension, has been used as a null model in macro-ecological studies. Departures from a MDE are often thought of as interesting. The MDE is a product of the interaction between geometric boundaries and range locations, with species being forced to occupy more central positions in proportion to their range size. We criticize this mechanism for assuming species' locations to be wholly independent from their evolutionary past. We present a simple simulation model that shows how range locations arising as part of a phylogenetic process depart from a MDE. The amount of departure is positively correlated with phylogenetic imbalance (tree shape), but a deviation from an equal-rates Markov speciation model is not necessary to negate a MDE. We suggest that the MDE is an appropriate ecological null model only when phylogenetic influence on range location is demonstrably low or non-existent.