What can multiple phylogenies say about the latitudinal diversity gradient? A new look at the tropical conservatism, out-of-the-tropics and diversification rate hypotheses
ancestral area reconstruction, evolutionary time, latitudinal zone transitions, sister group analysis, tropical origination
We reviewed published phylogenies and selected 111 ones representing mammals, birds, insects and flowering plants. We then mapped the latitudinal range of all taxa to test the relative importance of the tropical conservatism, out-of-the-tropics and diversification rate hypotheses in generating latitudinal diversity gradients. Most clades originated in the tropics, with diversity peaking in the zone of origin. Transitions of lineages between latitudinal zones occurred at 16–22% of the tree nodes. The most common type of transition was range expansions of tropical lineages to encompass also temperate latitudes. Thus, adaptation to new climatic conditions may not represent a major obstacle for many clades. These results contradict predictions of the tropical conservatism hypothesis (i.e., few clades colonizing extratropical latitudes), but support the out-of-the-tropics model (i.e., tropical originations and subsequent latitudinal range expansions). Our results suggest no difference in diversification between tropical and temperate sister lineages; thus, diversity of tropical clades was not explained by higher diversification rates in this zone. Moreover, lineages with latitudinal stasis diversified more compared to sister lineages entering a new latitudinal zone. This preserved pre-existing diversity differences between latitudinal zones and can be considered a new mechanism for why diversity tends to peak in the zone of origin.