The problem of reticulate trees: comparative phylogenetic analyses of interspecific hybrids of Cnemidophorus lizards.

Alistair J. Cullum

Department of Ecology and Evolutionary Biology
321 Steinhaus
University of California, Irvine
Irvine, CA 92697-2525

Presented at:
77th Annual Meeting of the American Society of Ichthyologists and Herpetologists

Symposium title:
"Comparisons and current uses of phylogenetic approaches to ichthyology and herpetology."
Mark Pyron and Robert Espinoza, co-chairs
Sunday, June 29th, 1997

Abstract. — The genus Cnemidophorus consists of both sexual and asexual species, with the latter species having originated via hybridization between various of the sexual species. The resulting phylogeny is reticulate (i.e. the branches recombine), and this reticulation prevents the use of most common phylogenetically-based comparative analyses. Studies involving such phylogenies must therefore use alternative techniques. I used two such techniques to compare physiological performance in sexual and asexual Cnemidophorus, testing the hypothesis that dramatically higher levels of heterozygosity in the asexuals resulted in superior performance in these species. Five whole-organism physiological traits (burst speed, endurance, maximal exertion, standard metabolic rate and evaporative water loss rate) were measured, then sexuals and asexuals were compared using both Cheverud et al.’s (1985) phylogenetic autocorrelation method and a second approach I developed. The latter method has the advantage of generating predicted values for each hybrid species based on its parental species, allowing comparison of these predicted values to observed values in a paired design. Both forms of analysis failed to support the "heterozygous superiority" hypothesis. Asexuals exhibited a general trend towards inferior performance, with endurance in particular being markedly reduced relative to sexual species.