Auk’s Top Cited, #1: Lumping versus Splitting

It’s here at last! For months now, we’ve been counting down our top ten most cited papers from 2014 and 2015, and this week we can finally reveal that The Auk‘s single most cited paper from that time period is…

Species taxonomy of birds: Which null hypothesis? by Frank Gill, a Perspective paper from the April 2014 issue of The Auk.

The question of where exactly to draw the lines between species has long vexed ornithologists and birders. Do we lump potentially interbreeding birds together into as few species as possible? Or do we analyze minute differences between populations and split them up as separate species?

The most commonly accepted definition of a species says that populations are considered to be the same species if they have the potential to interbreed, even if they’d never encounter each other in the real world, putting the burden of proof on the “splitters.” However, Gill argues that new advances in our understanding of the nature of reproductive isolation, the genetics of speciation, the limited role of gene flow, the power of directional selection, and the dynamics of hybridization all suggest that it’s time to rethink this approach. Narrow hybridization zones with limited gene flow may be a common part of the process of speciation, and Gill suggests that many so-called “polytypic species” may actual be groups of multiple closely related but distinct species.

Read the full paper at