Pupfish

Abstract: Adaptive radiations are known for rapid morphological and species diversification in response to ecological opportunity, but it remains unclear if distinct mechanisms drive this pattern. Here, we show that rapid rates of morphological diversification are linked to the evolution of novel ecological niches in two independent Cyprinodon radiations nested within a wide-ranging group repeatedly isolated in extreme environments. We constructed a molecular phylogeny for the Cyprinodontidae, measured 16 functional traits across this group, and compared the likelihoods of single or multiple rates of morphological diversification. We found that rates of morphological diversification within two sympatric Cyprinodon clades containing unique trophic specialists are not part of an adaptive continuum with other clades, but are instead extreme outliers with rates up to 131 times faster than other Cyprinodontidae. High rates were not explained by clade age, but were instead linked to unique trophic niches within Cyprinodon, including scale-eating, zooplanktivory, and piscivory. Furthermore, although both radiations occur in similar environments and have similar sister species, they each evolved unique trophic specialists and high rates of morphological diversification in different sets of traits. We propose that the invasion of novel ecological niches may be a key mechanism driving many classic examples of adaptive radiation.

Abstract: The relict Cyprinodon (pupfish) species of Death Valley and adjacent hydrologic basins of western North America have often been cited as examples of extremely rapid post-Pleistocene evolution. Three of these species, C. nevadensis, C. salinus, and C. diabolis, inhabit remnants of an integrated system of rivers and lakes that filled Death Valley during late Pleistocene times (Miller, 1948, 1950). Two other species, C. radiosus and C. macularius, inhabit river systems that were connected with Death Valley some time in the more distant past (Table 1). The taxonomy, ecology, and geochronology of these species have been discussed in detail by Miller (1948, 1950, 1961) and Liu (1969). The morphological divergence among the species has been relatively extensive. Major morphological and behavioral differences are listed in Table 2. The morphological differences alone are striking, and include some characters (e.g., dentition) likely to be under polygenic control with epistatic interactions, and which thus have a high probability of reflecting major evolutionary changes (Kluge and Farris, 1969). In aggregate, these differences seem at least as great as those which separate the majority of congeneric teleost species. The marked ecological differences among their habitats (Table 1), especially in temperature regime and salinity, have been regarded as the major selective factors that mediated the evolution of the species (e.g., Miller, 1950). For example, the virtually constant 34 C temperature of Devil's Hole is surely among the highest at which fishes permanently live, while the other habitats are generally lower in average temperature. Similarly, Salt Creek is often %, or more as saline as sea water, while most of the other

Abstract: Apparently between 1980 and 1984, Cyprinodon variegatus was introduced into an area of the Pecos River in Texas, where it hybridized with an endemic species, C. pecosensis. Protein electrophoresis indicated that, by 1985, panmictic admixtures of these two pupfishes occupied approximately 430 river-kilometers of the Pecos River, roughly one-half of the historic range of the endemic species. The average frequency of introduced alleles at four diagnostic loci ranged from 0.18 to 0.84 at the 15 sites sampled from the Pecos River in Texas. Clinal patterns in allele frequencies suggest that C. variegatus was introduced into a mid-reach of the river and that this was followed by both upstream and downstream dispersal of the introduced alleles. All pairwise combinations of loci showed significant linkage disequilibrium. The level of disequilibrium indicates chromosomal linkage for one gene-pair, Gpi-A and Est-1. The change in pupfish allele frequencies in the Pecos River represents an extreme example of rapid natural selection in a seminatural situation.

Abstract: Pupfish (Cyprinodon nevadensis amargosae), which inhabit shallow, thermally instabile desert waters were acclimated to a series of constant temperatures (15, 25 and 35 ° C) and to a symmetrical fluctuating temperature regime (15–35 ° C). A range or scope of thermal tolerance of 39 ° C between critical thermal maxima and minima exists for this species (Fig. 1). Upon acclimation to cycling temperatures, this tolerance range increased significantly to almost 41 ° C (Fig. 2). Hence, these fish can apparently compensate physiologically for both warm and cold temperatures simultaneously.