Speciation in Fishes of the Genera Cyprinodon and Empetrichthys, Inhabiting the Death Valley Region

Miller, Robert

The limited fish fauna inhabiting the Death Valley area is rich in material for the student of evolution and is highly endemic. This account discusses the speciation of six species represented by the cyprinodontid genera Cyprinodon and Empetrichthys. Speciation in the cyprinodontid fishes inhabiting the Death Valley region is correlated with the disruption of waters since late Pleistocene times, with population size, with the number of generations produced each year, and with physical and chemical conditions of the remnant springs and creeks. There are two living genera, Cyprinodon with 4 species, one of which has 6 subspecies, and Empetrichthys with 2 species of which one has 3 subspecies. All of the species and subspecies are endemic, as is Empetrichthys which is evidently an ancient relict. Three fossil representatives of Fundulus, probably of late Pliocene or early Pleistocene age, indicate a lowland connection to the southwest. One of the species may have been near the ancestral stock from which Empetrichthys was derived. The Cyprinodon reached Death Valley from the southeast, by way of the Colorado River basin or its antecedent. Most of the subspecies of Cyprinodon nevadensis show racial differentiation, particularly C. n. mionectes of Ash Meadows, Nevada. High temperature is believed to have accelerated the time rate of evolution in warm-spring populations by increasing the mutation rate and the number of generations produced each year. Many of the local subspecies and races have very small ranges, which may be closely approximated. One unique species, Cyprinodon diabolis, is confined to a single small spring in which its total numbers fluctuate from about 50 to 400 individuals. Experimental studies have demonstrated that F1 generations of pure stocks of three subspecies of Cyprinodon nevadensis, reared under environmental conditions greatly altered from those of their natural habitat, maintained essentially the same meristic characters as are found in the natural populations. A few exceptions, however, indicate that altered environment, as well as heredity, is a factor in determining the number of segments.

Speciation in Fishes of the Genera Cyprinodon and Empetrichthys, Inhabiting the Death Valley Region

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