Evolution of alternative life histories in the tiger salamander, Ambystoma tigrinum nebulosum

Howard H Whiteman, Purdue University

Abstract

In some salamander species, larvae may follow one of two alternative life histories: an individual may retain its larval morphology and aquatic existence to become a sexually mature paedomorph, or transform into a terrestrial metamorphic adult. This dimorphism is termed facultative paedomorphosis, because the alternative expressed depends on a genotype-environment interaction. The environmental factors influencing this polymorphism are known for some species, but their importance in other species is unclear. In addition, the evolutionary maintenance of the dimorphism is poorly understood. I proposed three alternative hypotheses for the evolutionary maintenance of facultative paedomorphosis, and tested the predictions of these hypotheses using a literature review and an empirical life history study. The literature review suggested that at least two of the three hypotheses maintain paedomorphosis in different species, that the polymorphism might even be maintained for different reasons in different populations of the same species, and that geographic variation in selection pressures may have produced these results. My field observations consisted of a four-year mark and recapture study of a montane population of tiger salamanders, Ambystoma tigrinum nebulosum, at the Mexican Cut Nature Preserve in the Elk Mountains of Colorado. I documented larval growth patterns and life history decisions for two different larval cohorts in four permanent ponds. I also explored several fitness components for each morph, and used these data to estimate morph-specific lifetime reproductive success. Larval growth and most fitness measures were consistent with the Best of a Bad Lot hypothesis, which suggests that paedomorphs have decreased fitness relative to metamorphs. Under this hypothesis, paedomorphosis is maintained because small individuals which mature as paedomorphs obtain higher fitness than those postponing maturity for increased growth and eventual metamorphosis. My results contrast with those obtained from several other salamander populations, which suggest that paedomorphs have a fitness advantage over metamorphs, and metamorphosis is maintained as a tactic to escape competition with larger larvae and paedomorphs. Thus, it appears that facultative paedomorphosis can be maintained through different selection mechanisms in different populations or species. More generally, my results are consistent with the view that phenotypic plasticity and variation per se may exist for different evolutionary reasons in different populations.

Degree

Ph.D.

Advisors

Howard, Purdue University.

Subject Area

Ecology|Zoology

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