EVOLUTION IN A PAEDOMORPHIC LINEAGE. I. AN ELECTROPHORETIC ANALYSIS OF THE MEXICAN AMBYSTOMATID SALAMANDERS.

Paedomorphosis, or the retention of ancestral juvenile features in descendant adults (Gould, 1977), is one of a general class of heterochronic events which has recently been invoked as a possible mode of evolutionary change. Gould (1977) and Alberch et al. (1979) among others have reiterated the arguments of Goldschmidt (1940) that it is simpler to change morphology through shifts in the timing of established developmental events than by incorporating mutations that may disrupt the developmental status quo. Although a much more thorough understanding of the genetics of such developmental shifts is necessary before arguments on their evolutionary flexibility can be evaluated, paedomorphosis does seem to be an important mode of morphological evolution (see Gould, 1977, for many zoological examples, and Guerrant, 1982, for a botanical case study). The importance of paedomorphosis in salamander evolution has long been recognized (Noble, 1931; Tihen, 1958; Wake, 1966), and represents one of the major ways in which new morphological characters are acquired in these amphibians. Paedomorphosis has been invoked to explain the evolution of single elements (e.g., the nasal bone in Rhyacotriton olympicus, Wake, 1980a), as well as the simultaneous change of many aspects of salamander morphology (e.g., Wake, 1966; Alberch and Alberch, 1981). The most striking example of the complete reorganization of the adult morphology attributable to paedomorphosis occurs when salamanders fail to metamorphose to the terrestrial condition and mature while retaining virtually the entire larval morphology throughout life. I term this phenomenon larval reproduction. (Pierce and Smith, 1979; Sullivan, 1980; and Wake, 1980b discuss this terminological point.) Operationally, reproductive larvae retain external gills and an enlarged tail fin, lack movable eyelids, and are obligatorily aquatic as sexually mature adults. Although larval reproduction is found, at least occasionally, in most families of salamanders, it reaches its greatest variability of expression in the Ambystoma tigrinum-mexicanum-dumerilii-Rhyacosiredon complex of the family Ambystomatidae (Tihen, 1958). This complex includes about 17 currently recognized species, distributed across North America from southern Canada to the southern rim of the Mexican plateau (Gehlbach, 1967). The group reaches its greatest taxonomic and metamorphic diversity at its southern extreme, in the Mesa Central of central Mexico. Fifteen of the 17 species are endemic to this region (Smith and Taylor, 1948), and individuals exhibiting larval reproduction are known from well over half of these taxa. Of these, the axolotl, A. mexicanum, has been widely cited as a prime example of paedomorphosis in vertebrate evolution (deBeer, 1958; Gould, 1977). Before the importance of metamorphic failure in ambystomatid evolution can be addressed, a phylogeny of the group must be established as a framework for examining the evolution of metamorphosis. Here, I use starch-gel electrophoresis to evaluate the phylogenetic history of the Mexican ambystomatid salaman-