Environment, area and diversification in the Iridaceae plant family

How fast a lineage divides may explain why some areas contain more species than others. The Cape of South Africa is one of the most floristically diverse regions on Earth and many species are found nowhere else. There are two broad explanations for high species richness in the Cape: either the Cape represents an old, relatively undisturbed area that has accumulated species richness gradually over time or the recent onset of its Mediterranean-type climate triggered rapid diversification.

In a study published in the September issue of The American Naturalist, T. Jonathan Davies (Imperial College London) and colleagues use a phylogenetic tree of the iris family to show that lineages within the Cape have speciated at a faster rate than those found elsewhere, even in comparison to regions of similar Mediterranean climates. Irises represent a family of herbaceous, seasonal geophytes, and it is perhaps these features and associated biological traits that have provided the key for lineages to prosper in the Cape region, thereby exploiting the potential for rapid diversification conveyed by the Cape environment. An appreciation of the interaction between biological traits and environment will likely prove critical to an understanding and interpretation of the distribution of species richness across the globe and among the branches of the tree of life.