Fig. 4.

a When different species (B–I) evolve from a shared ancestor (A), they do not “split up at once”, which would result in the same degree of relatedness, as shown in this sketch. b Rather, speciation events starting from a shared ancestor occur subsequently, while the time span between speciation events also differ. The result is a “phylogenetic tree” shown in this sketch, with different degrees of relatedness between the species (B–I). When traits of a set of related species (such as B–I) are compared to find evidence for selective forces shaping adaptations, tests for simple correlations between trait data and environmental and/or life style data are, therefore, not adequate. Special methods are necessary to account statistically for the complex data structure represented by a phylogenetic tree