Patterns of Diversification in a Neotropical Radiation of Birds (Aves: Furnariidae)

Abstract

Ecology and the role of natural selection in lineage diversification has been a central topic in evolutionary biology since Darwin. At the macroevolutionary scale, this idea is embodied in the ecological theory of adaptive radiation, which posits that rapid diversification is driven by ecological adaptive radiation in which speciation is coupled with niche divergence. Within species, the theory of ecological speciation proposes that local adaptation drives speciation by reducing gene flow among populations occupying different environments either by directly reducing migration or by reducing the fitness of migrants. Much progress has been made testing these predictions in a multitude of organisms, but there remains a lack of studies addressing the role of ecology in diversification at multiple evolutionary scales within the same lineage. Herein, I use the Neotropical bird radiation of ovenbirds (Passeriformes: Furnariidae) as a model system to examine the role of ecology in speciation and lineage diversification. I show that, across furnariid subclades, rates of lineage diversification are best predicted by the rate of climatic-niche evolution rather than ecomorphological evolution, although both are clearly important. This result is consistent with a role for environmental gradients in driving speciation through the process of isolation-by-adaptation (IBA). I then compared the relative support for IBA against the null model of isolation-by-distance (IBD) in a species of furnariid, Cranioleuca antisiensis, that shows signs of incipient speciation and is distributed across a broad environmental gradient. Using genetic, phenotypic, and environmental data from across its distribution, I found evidence of local adaptation in body size. However, I found that IBD was the best explanation for genetic differentiation along the cline, suggesting a limited role for the environmental gradient in reducing gene flow among populations of C. antisiensis. Finally, I explore the properties of the speciation mechanism ‘speciation-by-extinction’. Speciation-by-extinction (SBE) is an alternative to the standard model of allopatric speciation where speciation results from divergence accrued following the isolation of two undifferentiated populations. SBE, in contrast, proposes that speciation can result from the partitioning of standing phenotypic or genetic variation through the local extinction of intermediate populations.