巴拿馬地峽
Despite their sympatry, the closest relative of "E. rathbunae" is not "E. analoga"; rather, "E. rathbunae" is part of a clade that also includes species from the western Atlantic Ocean, including "E. portoricensis" and "E. benedictii". The last common ancestor of the genus is thought to have lived in the Pacific Ocean, and to have colonised the Gulf of Mexico when the Isthmus of Panama was submerged, and "E. rathbunae" is thought to have similarly recolonised the Pacific from ancestors that lived on the Atlantic side of the Isthmus of Panama.