Incipient species formation in salamanders of the Ensatina  complex

Abstract

The Ensatina eschscholtzii complex of plethodontid salamanders, a well-known ''ring species,'' is thought to illustrate stages in the speciation process. Early research, based on morphology and coloration, has been extended by the incorporation of studies of protein variation and mitochondrial DNA sequences. The new data show that the complex includes a number of geographically and genetically distinct components that are at or near the species level. The complex is old and apparently has undergone instances of range contraction, isolation, differentiation, and then expansion and secondary contact. While the hypothesis that speciation is retarded by gene f low around the ring is not supported by molecular data, the general biogeographical hypothesis is supported. There is evidence of a north to south range expansion along two axes, with secondary contact and completion of the ring in southern California. Current research targets regions once thought to show primary intergradation, but which molecular markers reveal to be zones of secondary contact. Here emphasis is on the subspecies E. e. xanthoptica, which is involved in four distinct secondary contacts in central California. There is evidence of renewed genetic interactions upon recontact, with greater genetic differentiation within xanthoptica than between it and some of the interacting populations. The complex presents a full array of intermediate conditions between well-marked species and geographically variable populations. Geographically differentiated segments represent a diversity of depths of time of isolation and admixture, ref lecting the complicated geomorphological history of California. Ensatina illustrates the continuing difficulty in making taxonomic assignments in complexes studied during species formation.

Mitochondrial DNA evolution in the Anaxyrus boreas species group

“…Second, refugia from Pleistocene glaciations resulted in shared phylogeographic distributions of species. The Klamath-Siskiyou Mountains, near the border of Oregon and California, remained unglaciated throughout the Pleistocene and still contain high biological diversity and endemism (e.g., Wake, 1997;Wilke and Duncan, 2004 (Slug); Mead et al, 2005;Steele and Storfer, 2006). This region could have served as a refugium for boreal toads in the NW group, and allowed the divergence of the NW-coastal minor group from the more northern NW-northern assemblage (Fig.…”

Speciation durations and Pleistocene effects on vertebrate phylogeography

“…Species radiations within several other ¢sh groups in closed lacustrine environments imply speciation durations often much less than 0.3 Ma, and speciation durations in some island birds (and arthropods) are reportedly about 0.6^1.0 Ma (McCune 1997). Near the other end of the scale, huge genetic distances betweeǹ conspeci¢c' phylogroups in the salamander Ensatina eschscholtzii indicate periods of population isolation far longer than most vertebrate speciation durations (Moritz et al 1992;Wake 1997). Other such outlier examples can be seen in ¢gures 2^4.…”

Wherefore and Whither the Ring Species?

“…Today these islands are incorporated into the California Coast Ranges. Phylogenetic tests of competing biogeographic models supported the ring species scenario of Stebbins, as modified by Wake (1997).…”

“…A particularly large amount of phylogeographic diversity among populations was found in the San Francisco Bay area of central California, including three subspecies and ten mtDNA clades (Wake, 1997;Kuchta et al, 2009b;Pereira and Wake, 2009). In contrast with points of ring closure, the highest levels of interbreeding are found between secondary contacts within the distributional arms, such as between northern and southern platensis, or between xanthoptica and oregonensis (Wake, 1997;Pereira and Wake, 2009;Pereira et al, 2011). However, contacts within the distributional arms are not as thoroughly studied as the points of ring closure, and more detailed investigations of contacts within the distributional arms would be a fruitful avenue of research.…”

“…This creates obvious problems for Stebbins' original biogeographic scenario. Wake (1997) postulated that connections between islands and the mainland were repeatedly forged and broken, providing an opportunity for dispersal and differentiation of the unblotched lineages of Ensatina. Today these islands are incorporated into the California Coast Ranges.…”