Divergent natural selection has the potential to drive the evolution of reproductive isolation. The euryhaline killifish Lucania parva has stable populations in both fresh water and salt water. Lucania parva and its sister species, the freshwater L. goodei, are isolated by both prezygotic and postzygotic barriers. To further test whether adaptation to salinity has led to the evolution of these isolating barriers, we tested for incipient reproductive isolation within L. parva by crossing freshwater and saltwater populations. We found no evidence for prezygotic isolation, but reduced hybrid survival indicated that postzygotic isolation existed between L. parva populations. Therefore, postzygotic isolation evolved before prezygotic isolation in these ecologically divergent populations. Previous work on these species raised eggs with methylene blue, which acts as a fungicide. We found this fungicide distorts the pattern of postzygotic isolation by increasing fresh water survival in L. parva, masking species/population differences, and underestimating hybrid inviability. 1. Introduction There is substantial evidence that adaptation to different environments can lead to the evolution of reproductive isolation between populations, a process referred to as ecological speciation [1–6]. Ecological speciation predicts the evolution of both prezygotic and environmentally dependent postzygotic isolation. Prezygotic isolation can evolve as mating signals and preferences adapt to different environments [7–12]. Extrinsic (environmentally dependent) postzygotic isolation may also evolve since hybrids have intermediate phenotypes and are poorly adapted to parental habitats [13–17]. Currently, there is less evidence that genetic incompatibilities between populations (intrinsic postzygotic isolation) can evolve simply as a consequence of adaptation to different habitats [18–20]. Most identified intrinsic isolating barriers have no clear relationship to adaptation and may have arisen subsequent to ecological divergence [21–23]. However, theoretical and empirical work suggests intrinsic isolation can arise through ecological divergence if there are epistatic interactions between alleles conferring environment-specific adaptations [24–26]. When prezygotic, extrinsic, and intrinsic postzygotic reproductive isolating barriers evolve as byproducts of adaptation, the probability that they will lead to speciation depends on their cumulative strength and ability to persist in the face of gene flow when incipient species come into contact [27]. If the cumulative strength of
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