EMBRYONIC TEMPERATURE ADAPTATIONS AND GENETIC COMPATIBILITY IN TWO ALLOPATRIC POPULATIONS OF THE SPADEFOOT TOAD, SCAPHIOPUS HAMMONDI

Evolutionary studies of disjunct, allopatric populations should provide valuable information with respect to the processes of geographic speciation. However, as Blair (1964) has stated, the majority of such studies among anuran amphibians have been unrewarding as there has been either too little or too much differentiation. Of particular interest has been the study of potential isolating mechanisms which might serve to separate the diverging populations as distinct breeding units under conditions of sympatry. One of these isolating mechanisms, hybrid inviability, is usually considered to originate as a by-product of genetic differentiation while the diverging populations are geographically separated (Mayr, 1963). Furthermore, the polygenic character of isolation mechanisms suggests that the development of incipient sexual isolation might be expected to develop more often in those allopatric populations which demonstrate significantly different adaptations. As Patterson and Stone (1952) and Volpe (1955) have suggested, the genes, or factors, which are the basis of the isolation mechanisms may be fixed incidentally while the diverging populations adapt to their own environment, rather than as a result of selection pressure during sympatry. Among anuran amphibians the relationship between hybrid inviability and adaptive differentiation in allopatric populations is best known in the leopard frog, Rana pipiens (Moore, 1949; Ruibal, 1955, 1962; Volpe, 1954, 1957a). These studies showed that Rana pipiens consists of a number of geographic races which differ in their embryonic temperature adaptations. Furthermore, crossbreeding tests between R. pipiens populations differing in temperature adaptations demonstrated a high degree of hybrid inviability. Moore (1957) suggested that the presumed genetic basis for the embryonic temperature adaptations was also the basis for an isolation mechanism, hybrid inviability. Volpe (1955) discussed differences in embryonic rate of development as a possible factor responsible for hybrid inviability between two species of the genus Bufo, and he suggested that genetic compatibility might be greatly influenced by conflicting modes of action of temperature-related processes in developing hybrid embryos. The work reported here is a study of embryonic temperature adaptations and genetic compatibility in disjunct, allopatric populations of the spadefoot toad, Scaphiopus hammondi. The populations studied are from southeastern Arizona and southern California. Scaphiopus hammondi does not occur in the extremely arid region of eastern California and western Arizona. In California hammondi breeds during the winter and spring while in Arizona hammondi breeds during the summer. The wide area of disjunction between these two populations and the fact that they breed at different times of the year and under different climatic conditions were the primary reasons for studying the extent of genetic divergence and embryonic 1 Part of a dissertation in partial fulfillment of the requirements for the degree Doctor of Philosophy in the Department of Life Sciences, University of California, Riverside. 2 Present address: Department of Biology, Western Washington State College, Bellingham.