On Evolution

11. Cladists in Wonderland

Cladists in Wonderland 11 At least in the last half of the twentieth century, nothing quite paralleled the “cladistic revolution” in terms of mixing (inappropriately ) ideology with science. Following the publication in  of Phylogenetic Systematics, an English translation of Willi Hennig’s  treatise originally in German, at least two academic generations of systematists were often caught up in a religious-like fervor, trumpeting the virtues of cladism and crusading to convert nonbelievers and agnostics to their scientific faith. Hennig’s phylogenetic principles were indeed important if not revolutionary in biologists’ attempts to understand the various sources of evolutionary similarities and differences among organisms, but what seems even more amazing is how this one particular evolutionary topic generated so much ideological fervor. Although many important scientific contributions have emerged from cladistic reasoning, cladists often were led too far by their dogmas. Perhaps nowhere was this more true that with regard to species concepts. The following is Avise’s review of one edited book published under the cladistic banner. As you will see, he could find no better words to describe some of the cladist’s views than those issued by fictitious characters in Lewis Carroll’s Through the Looking Glass and Alice’s Adventures in Wonderland. “A hill can’t be a valley, you know. That would be nonsense,” said Alice. The Red Queen shook her head. “You may call it ‘nonsense’ if you like, but I’ve heard nonsense, compared with which that would be as sensible as a dictionary.” In the year  I attended a conservation genetics symposium at which one of the speakers claimed to have caught a speciation event in the act. Earlier in this century, tiger beetles (Cicindela dorsalis) were distributed more or less continuously along the eastern coast of the United States, but shoreline development extirpated populations in the mid-Atlantic states, causing a range disjunction between still-extant populations in New England and the southern states. By analyzing DNA sequences in living and museum-preserved specimens, small but detectable nucleotide differences were uncovered between these two extant populations, a distinction not formerly possible because mid-Atlantic populations had been polymorphic for the sequences in question. Under one version of the phylogenetic species concept (PSC), a new species had arisen precisely when the polymorphism became a fixed difference (in this case, via the extinction of intermediate demes). If this speaker’s PSC-based conclusion about species formation is to be taken seriously, and generalized, conservation biologists might naively rejoice. In the coming decades, as natural populations of many species are extirpated or reduced to small inbred units, intraspecific polymorphisms increasingly will be converted to fixed allele differences between allopatric demes. Under PSC logic, by definition, this will result in a great proliferation of new species. Thus, we may look forward to a twenty-first century in which the rate of species origin (via fixation of genetic variants) may far out-pace the rate at which currently recognized taxonomic species are driven to extinction . What most biologists had feared as a deepening valley in species numbers may instead soon become a numerical peak in taxonomic species richness! “It was much pleasanter at home,” thought poor Alice. “I almost wish I hadn’t gone down that rabbit-hole—and yet—it’s rather curious, you know.” To scientists raised under the traditional Biological Species Concept (BSC) of Theodosius Dobzhansky and Ernst Mayr, where the evolution of intrinsic (genetically based) reproductive barriers is the underlying basis of cladogenesis, the speciational world of various cladistic camps can seem as curious as that encountered by Alice in her sojourns down a rabbit’s hole or through a looking glass. To us outsiders, it can be a world of sense and nonsense often turned on its  On Evolution head, of erudite jabberwocky, of impeccably logical illogic, of surreal reality. Now, for the first time, this speciational wonderland is fully explored in a single volume. In Species Concepts and Phylogenetic Theory , Wheeler and Meier have assembled a collection of invited articles eloquently portraying a conceptual world that is simultaneously as coherent and incoherent as anything conjured up by Lewis Carroll. I heartily recommend this entertaining treatise to anyone interested in some recent developments (not advances) in phylogenetic reasoning as applied to species issues. An opening chapter by Joel Cracraft wisely enjoins readers (p. ) “to grab a favorite fetish and conjure up a...