Layers of contingency shroud pervasive ecological divergence in a local radiation of land snails

Abstract

Received 8 March 2015; revised 5 April 2015; accepted for publication 8 April 2015The predictability of evolution depends on the roles that selection and historical contingency play in determiningits outcomes, but the relative importance of these evolutionary mechanisms has attracted considerable debate.One view is that historical events have such a profound impact on the genetic structure of populations thatpatterns of phenotypic evolution are essentially unpredictable. The opposing view is that selection is so powerfulthat evolutionary change is primarily deterministic, and thus highly predictable. By controlling for the effects ofphylogeny, geographic location and habitat, this study examined the relative roles of contingency anddeterminism in a local radiation of land snails, genus Rhagada, in a continental archipelago. Informed byprevious studies on a single island, which revealed a strong association between low-spired shells and rockyhabitats, 28 population pairs were sampled in directly adjoining rocky and spinifex plain habitats. Whenconsidered in their respective pairs, the effect of habitat was remarkably consistent, with lower-spired shellsobserved in the rocky habitat in 24 of the comparisons. However, when analyzed outside the context of thosepairs, the association was obscured by broad variation in shell shape within habitat types and among lineages.These results reveal the complex nature of a morphological radiation; while the pattern of ecological divergence ishighly predictable at the scale that selection acts, deterministic evolution is largely obscured by phylogenetic andpopulation history. © 2015 The Linnean Society of London, Biological Journal of the Linnean Society, 2015, 00,000–000.