Speciation and demographic history of the Cortez bonefish, Albula sp. A (Albuliformes: Albulidae), in the Gulf of California inferred from mitochondrial DNA

Nucleotide sequence data from a segment of the mitochondrial cytochrome b (Cyt b) gene were used to infer demographic history and examine conditions that may have led to speciation in the Cortez bonefish (Albula sp. A) in the Gulf of California, Mexico, a currently undescribed species of bonefish in the Albula vulpes complex. Analysis of molecular variance in 39 individuals collected from three localities along the eastern gulf coast, over c. 850 km, revealed a lack of population structure among localities (overall ΦST=−0·015), with 100% of the genetic variation distributed within populations. Analysis of combined sequences from these individuals using neutrality tests and the mismatch distribution provided evidence of a population expansion dating to the Pleistocene. The population expansion was supported by maximum likelihood estimates of changes in long‐term female effective population size (Nef). A molecular clock estimate of divergence, provisionally calibrated for the Cyt b gene in Albula, indicates that Albula sp. A and its sister species in the eastern Pacific, Albula esuncula, diverged from a common ancestor c. 5·0–8·8 million years ago. This date is about the time the Baja California peninsula separated from mainland Mexico during the formation of the Gulf of California. Oceanographic and ecological changes associated with the opening of the gulf likely provided conditions favourable for adaptive radiation and reproductive isolation, ultimately resulting in the allopatric formation of two separate lineages. The co‐occurrence of Albula sp. A and A. esuncula found today in the coastal Pacific waters of northern Mexico is most probably the result of secondary contact after speciation.