Evolutionarystudies of Salamanders

AN INTEGRATED APPROACH TO EVOLUTIONARYSTUDIES OF SALAMANDERS DAVIDB. WAKE* CONTENTS der Caudata. Studying organisms from one Introduction ...................................................... 163 perspective can inform and direct investiga- Phylogenetics .................................................... 163 tions that have other goals, and an integra- Hierarchical Approaches .................................. 166 tive approach to organismal evolution can Ontogeny and Phylogeny ................................. 168 result. As evolutionary biologists, our ultimate Recognition of Species ...................................... 170 goal is to understand how biological diversifi- Species Diversity .............................................. 172 cation occurs, and as herpetologists our pri- A Plea for Conservation .................................... 174 mary focus is the whole organism, as exempli- Acknowledgments ............................................ 175 fied by amphibians and reptiles. The examples Literature Cited ............................................... 175 I use are mainly from my own laboratory, because I can see most clearly in these cases how Abstract: This essay is an examination of the ways integration can be achieved. Although this in which seemingly disparate studies focused on a paper is topical in nature, I attempt to provide single, rather small taxon, the Order Caudata, can connections between topics. provide a foundation for development of programs in evolutionary biology. Topics addressed include Any evolutionary study focused at the the use of individual characters as contrasted with complexly integrated systems of characters in phy- organismal level requires a framework of logenetic analysis, the nature of integration and its phylogenetics, with clearly stated hypotheses impact on specializations and adaptation, and on of relationships underlying all data analyses. the conceptualization and recognition of species. The At the same time, however, one must frankly possibility that salamanders and other amphibians acknowledge that the Caudata have been and might play a role as bio-indicators in environmental remain a controversial group; we have yet to studies is raised. achieve a robust phylogenetic hypothesis for the group as a whole, or even for the genera INTRODUCTION within the larger families. Thus, I envision phylogenetics as an on-going process with re- Y GOAL IS TO SHOW HOW STUDIES BROADLY ciprocal illumination occurring as we learn M focused on the organismal level can be more of the nature of characters, character used to develop a framework for an under- complexes, and novel data. standing of the evolutionary biology of a major taxon-in this case, the salamanders, Or- PHYLOGENETICS is * Department of Integrative Biology and Museum of There growing acceptance that the living Vertebrate Zoology, University of California, Berke- amphibians form a monophyletic group, the ley, California 94720, USA. Lissamphibia, and that the Order Caudata 163 164 DAVID B. WAKE also is a monophyletic group (Trueb and characters used by Duellman and Trueb Cloutier 1991; Duellman and Trueb 1985). I (1985), and when these recoded characters will not deal with these issues further. were placed on the trees of Duellman and Trueb (based mainly on morphology) and of Progress has been made in understanding Larson (based on DNA sequences), the fit to phylogenetic relationships at high taxonomic the former was somewhat more parsimonious levels, but one of the most vexing problems than to the latter; Larson argues that the facing a salamander biologist is the difficul- difference is not statistically significant. ties encountered in generating even a rela- Therefore,Larson’s hypothesis is equallyvalid. tively robust phylogenetic hypothesis concern- Hillis (1991) has attempted to combine some ing the relationships of salamander families. of the morphological data and sequence data The hypothesis of Duellman and Trueb (1985) available at the time he wrote his paper, but serves as a convenient point of departure, for as yet the result is not robust and has low it displays the traditional arrangement of a internal consistency. basal position for the Sirenidae, a monophyletic group including Hynobiidae and A major problem is the reconciliation of Cryptobranchidae that is also relatively basal, these different data sets. Larson (1991) has and a deeply nested position for the Pletho- made an initial effort along these lines and his dontidae. Thus, the taxa with external fertili- results demonstrate anew that any hypoth- zation (Hynobiidae and Cryptobranchidae, esis of relationships of the salamander fami- and presumably Sirenidae) are separated from lies requires extensive homoplasy for nucleo- a monophyletic group that practices internal tide, morphological, and other kinds of fertilization. characters. We need a fresh look at the nature of mor- New studies of morphological characters phological and other characters and their and of DNA sequences challenge this orthodoxy in several important respects. Cloutier (in interrelationship, and new characters. We prep.), usingmorphologicaltraits, not only fails must give attention to the ways in which char- to find a monophyletic Hynobiidae + Crypto- acters are generated in development and the branchidae, but also believes that the family degree to which characters are developmen- Hynobiidae is paraphyletic. He does agree with tally and functionally linked. There is so much earlier authors in recognizing the Pletho- homoplasy in the morphological characters of salamanders that cladistic hypotheses that use dontidae as being deeply nested. In contrast, Larson (1991) has studied the nucleotide states that cannot be directly observedin taxa are suspect. I especially distrust characters sequences of ribosomal RNA and concludes that the Hynobiidae + Cryptobranchidae is that, on the basis of a parsimony analysis monophyletic but not basal, and that the alone, are considered to be lost and thus used Plethodontidae, Amphiumidae, and Rhyaco- as a synapomorphy low in a tree-yet reap- tritonidae (a family recognized on morpho- pear (attributed to evolutionary reversal) logical grounds by Good and Wake [in press]) higher in the tree. In general, I mistrust char- are relatively basal. According to Larson, the acters whose state must be inferred rather remaining families form a monophyletic group than directly observed. and the Dicamptodontidae and Ambystomat- We lack critical information for various taxa. idae are deeply nested sister taxa; this For example, it is amazing that we still do not challenges the findings of Edwards (1976). know whether sirenids have external or inter- Larson has re-evaluated the morphological nal fertilization. On the one hand, the absence EVOLUTIONARY STUDIES OF SALAMANDERS 165 of cloacal glands suggests that no spermato- treated as four primary taxa. (Presch [19891 phore is produced and that sperm storage does has questioned this procedure, which I defend not occur. On the other, the fact that eggs are on the grounds ofthe demonstrable homoplasy laid individually suggests that internal fer- in the group; I reject Presch?s reanalysis ofour tilization might occur. data, which involves recoding of data and use of groups of genera as primary taxa.) By ac- My focus has been the Family Pletho- cepting these four primary taxa, we are ac- dontidae, which includes over 60%of the ap- cepting that projectile tongues evolved inde- proximately 390 living species (Frost [19851 pendently at least three times in the family, and subsequent descriptions). Homoplasy is and because we think that attached projectile extensive in the family (Wake 1966, and in tongues are functionally and phylogenetically press; Wake and Larson 1987) and has led to independent of free projectile tongues, projec- problems in developing a robust phylogenetic tile tongues of some sort must have evolved at hypothesis. Since the publication of my com- least five times, assuming that our four pri- parativeosteological study ofthe family(Wake mary taxa are monophyletic. We examined 1966), however, there has been relative taxo- the 15 dichotomous trees that could be gener- nomic stability. The monophyletic nature of ated for four groups, and found that two are the family is unquestioned, as is the mono- more parsimonious than the alternatives. Our phyletic status of the two major sister taxa, choice requires that the free projectile tongue the subfamilies Desmognathinae (3 genera evolved three times and the attached projec- and 14 species) and Plethodontinae (15 gen- tile tongue evolved three times in the family. era and about 235 species). The species of the It will be difficult with any rearrangement of Plethodontinae (still about 60% of living taxa to reduce this more than by one; Lombard species!) were placed in three tribes: Pletho- and Wake (1986) treated Batrachoseps and dontini, Hemidactyliini (Dubois [ 19841 has the supergenus Bolitoglossa as sister taxa, argued that the correct name should be but if the sister taxon of Bolitoglossa is Hydro- Mycetoglossini), and Bolitoglossini. Of these, mantes the number of evolutions of free the Bolitoglossini is an unquestioned mono- tongues could be reduced by one. phyletic group and the Plethodontini is very likely a monophyletic group, but evidence for At present we have too little information to the monophyly of the third tribe is weaker. choose among the various alternatives, but data have been building that support certain Due to the extensive

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