The tree below is a faithful rendering of Figure 1 of the important 2002 article by Ned K. Johnson and Carla Cicero in the journal Molecular Ecology, Volume 11, pages 2065-2081. It is entitled "The role of ecologic diversification in sibling speciation of Empidonax flycatchers (Tyrannidae): Multigene evidence from mtDNA. The 16 twigs across the top of this tree represent the 15 currently recognized species of Empidonax, plus an offshore population of the Pacific-slope Flycatcher ("Island Flycatcher") that is genetically distinct from the mainland population. Four Middle American species that are not mentioned in North American field guides are indicated with brown text. Although they are not covered in the species pages of this guide, the evolutionary story would not be complete without them, so they are fully integrated in this section (and in the Maps Section).

A tree of this sort tells us how (genetically) similar each species is to each other. From genetic similarity we infer genealogical similarity. For example, if you go down the tree from the twig for the Buff-breasted Flycatcher to the first branching point (called a "node"), then go back up, you find the Black-capped Flycatcher. This means they are each other's closest relatives, or "sisters." By the same token, Alder and Willow Flycatchers are sister species, although the White-throated Flycatcher is almost as closely related to both. Sister species share a common ancestor, which is represented by the node joining their branches. It is the branching pattern, the topology of the tree, that contains the genealogical information. The lengths of the branches are not necessarily proportional to the time since two branches split.

The genus Empidonax is divided into four distinct lineages, represented by the four branches you see on the tree. These four groups of species have distinctive coloration, habitat, nests, eggs, and vocalizations. The following pages show how different these four branches are. Click here to continue the story of Empidonax evolution.