Meeting in the middle: biogeography of Pecos drainage Cyprinodon

Authors: Christopher Hoagstrom

Year: 2019

Abstract

The Pecos River drainage hosts three endemic species within Cyprinodon. Although their biogeography is fundamentally understood [Echelle & Echelle 1978, Copeia 1978(4):569-582; Echelle et al. 2005, Copeia 2005(2):320-339], this study provides an updated synthesis of essential details. The three species ultimately descend from the same widespread coastal ancestor, but directly descend from two separate geographic lineages of Pliocene age: (1) C. elegans from a lower Rio Grande branch, (2) C. pecosensis-bovinus from a Red River branch. The Rio Grande taxa evolved in a karst-dominated watershed and C. elegans eventually colonized springs, similar to relatives elsewhere. In the ancestral Pecos River drainage, these occurred in Cretaceous limestones cropping out across the slope northeast of the Davis and Glass mountains. In contrast, the Red River taxa evolved in sediment-laden plains rivers, within a region with widespread salt dissolution and high rates of evaporation. There, saline habitats (backwaters, wetlands, springs) were widespread and are still occupied by members of the Red River clade. The most recent common ancestor of C. pecosensis and C. bovinus (sister species) colonized the incipient Pecos River via stream capture. Although secondary contact between pupfishes of Rio Grande and Red River descent resulted in mtDNA introgression, the lineages remained distinct [Echelle et al. 2005, op. cit.]. Based on recent secondary contact of C. elegans with C. variegatus (an invasive species), habitat segregation was likely an important isolating mechanism, restricting hybridization to ecotones [Stevenson & Buchanan 1973, Copeia 1973(4):682-692; Echelle & Echelle 1994, Copeia 1994(3):590-597]. Captive breeding experiments indicate mate selectivity and hybrid sterility/inviability could have also been involved [Tech 2006, J. Evol. Biol. 19:1830-1837]. In contrast, representatives of the Red River clade lack reproductive isolation from C. variegatus [Wilde & Echelle 1992, Trans. Amer. Fish. Soc. 121(3):277-286; Echelle & Echelle 1997, Conserv. Biol. 11(1):153-161], consistent with a heritage of similar habitat use. Both endemic species require strict protection from introgression with the nonnative invader. Speciation of C. bovinus versus C. pecosensis reflects isolation of Leon Creek on a horst where C. bovinus is endemic. The horst became elevated above an otherwise subsiding, surrounding landscape. Ephemeral stream reaches between the horst and the Pecos River, possible dispersal barriers (i.e. vertical drops) at the horst boundary, and spring adaptation presumably explain interspecific reproductive isolation. Today, only residual habitats remain available to each of the three endemic species. All require conservation management, are severely imperiled, and—history suggests—could quickly disappear.