Morphological stasis and genetic divergence without reproductive isolation in the Rhinichthys cataractae species complex : insights from a zone of secondary contact in the lower Fraser Valley, British Columbia

The Nooksack dace (Rhinichthys cataractae putative subspecies; NSD) and longnose dace (R. cataractae; LND) form a zone of post-glacial secondary contact in three streams in British Columbia's lower Fraser River valley, providing a valuable opportunity to study contact between populations separated during the Pleistocene glaciations. They are morphologically cryptic, despite an estimated 2-3 million years of separation. The NSD is currently listed as Endangered under Canada's Species at Risk Act (SARA) and my study clarifies its taxonomic and conservation status. NSD and LND have highly divergent mitochondrial DNA types, and dace carrying each mtDNA type have been found in roughly equal numbers in the zone of secondary contact. However, it was unknown whether this represented ongoing hybridization or reproductive isolation in sympatry. I conducted a morphological analysis using 11 morphometric measurements and two meristic characters (N = 582, 23 sampling locations) to uncover any subtle variation between the two dace, as well as to test for morphological intermediacy in the zone of sympatry. I then employed a 10-locus microsatellite DNA assay (N = 374, 12 sampling locations) to test for introgression between LND and NSD in the zone of secondary contact. I found that the two dace could not be reliably distinguished: there was overlap in all morphological characters measured, and both morphological and microsatellite analyses showed a greater effect of location than mtDNA clade, even when restricted to allopatric populations. There was no evidence of population structure within the sympatric populations, indicating complete admixture. The LND and NSD provide an example of "ephemeral speciation" - two lineages which, despite long separation, have developed no apparent barriers to reproduction and have collapsed into a single interbreeding population where they come into secondary contact. The zone of secondary contact should ideally be conserved for its evolutionary significance, and is a good illustration of the ...