A Test of an Allopatric Speciation Model for Congeneric Amphipods in an Isolated Aquatic Ecosystem

Abstract

We quantified amphipod population distributions in Montezuma Well, characterized two habitats, and tested alternative hypotheses for speciation using behavioral observations and genetic analyses. Amphipod distributions within Montezuma Well, Arizona, indicate nearly complete separation of habitat ranges between two congeneric populations. Hyalella montezuma inhabits the pelagic zone and a Potamogeton-dominated portion of the littoral zone, while H. azteca is restricted to a narrow band of shoreline Berula and Eleocharis roots. A population of H. azteca from nearby Stoneman Lake was included in the study for comparison with populations in Montezuma Well. We estimated genetic distance measures by random amplification of polymorphic DNA (RAPD) using the polymerase chain reaction with ten-base primers of arbitrary sequences. Based on 96 amplified fragments, the genetic distance between H. montezuma from Montezuma Well and H. azteca from Stoneman Lake was 0.19, whereas the distance between H. montezuma and H. azteca from Montezuma Well was 0.77. Observations of swimming behavior were consistent with genetic data: H. montezuma and H. azteca from Stoneman Lake spent significantly more time swimming than did H. azteca from Montezuma Well. These results suggest speciation resulted from genetic differentiation before introduction of ancestral populations to Montezuma Well.