Evolutionary divergence of leaf width and its correlates

Premise of the study: The question why leaf dimensions vary so much between species has long puzzled ecologists. Presumably, variation arises from selective forces acting on leaf function but which selective forces and which leaf functions? This investigation assesses the consistency of divergence in plant traits and habitat variables in association with leaf width divergence in the flora of NSW, Australia. 

Methods: More than 80 traits and habitat variables were measured for 25 independent evolutionary divergence events (PICs). Each PIC was represented by two related plant species that had diverged substantially in leaf width. Outgroup species provided indications of the direction of divergence. Most PICs were within genus, so divergences represent relatively recent evolutionary events. 

Key results: No plant traits or habitat variables were 100% consistently associated with a divergence in leaf width, and surprisingly few diverged in a consistent direction significantly more than what might be expected by chance. This surprising lack of consistent divergence with leaf width contrasted with the result that many of these traits and habitat variables were correlated with leaf width across all species in our data set and in line with correlations reported from other studies. Subcategorizing PICs according to the probable direction of leaf width divergence did not improve consistency. 

Conclusions: These results indicate that evolutionarily recent leaf width divergence events are not tightly tied to divergences in other leaf traits or in environmental situations, despite the broad correlations that have been observed across many species. Rather, cross species correlations are underpinned by earlier divergence events in the phylogeny.