Niche Evolution and Adaptive Radiation: Testing the Order of Trait Divergence

Abstract

In the course of an adaptive radiation, the evolution of niche parameters is of particular interest for understanding modes of speciation and the consequences for coexistence of related species within communities. We pose a general question: In the course of an evolutionary radiation, do traits related to within-community niche differences (a niche) evolve before or after differentiation of macrohabitat affinity or climatic tolerances (b niche)? Here we introduce a new test to address this question, based on a modification of the method of independent contrasts. The divergence order test (DOT) is based on the average age of the nodes on a tree, weighted by the absolute magnitude of the contrast at each node for a particular trait. The comparison of these weighted averages reveals whether large divergences for one trait have occurred earlier or later in the course of diversification, relative to a second trait; significance is determined by bootstrapping from maximum-likelihood ancestral state reconstructions. The method is applied to the evolution of Ceanothus, a woody plant group in California, in which co-occurring species exhibit significant differences in a key leaf trait (specific leaf area) associated with contrasting physiological and life history strategies. Co-occurring species differ more for this trait than expected under a null model of community assembly. This a niche difference evolved early in the divergence of two major subclades within Ceanothus, whereas climatic distributions (b niche traits) diversified later within each of the subclades. However, rapid evolution of climate parameters makes inferences of early divergence events highly uncertain, and differentiation of the b niche might have taken place throughout the evolution of the group, without leaving a clear phylogenetic signal. Similar patterns observed in several plant and animal groups suggest that early divergence of a niche traits might be a common feature of niche evolution in many adaptive radiations.

Adaptive Radiation, Ecological Opportunity, and Evolutionary Determinism

“…For example, Diamond (1986) proposed that New Guinean birds diverge first in habitat type and subsequently in prey size and food type; Richman and Price (1992) suggested that leaf warblers (genus Phylloscopus) diverged first in body size, then in behavior and foraging morphology, and finally in habitat use; and Streelman et al (2002) suggested that parrotfish diversified sequentially in habitat, diet, and sexually selected traits (reviewed by Streelman and Danley [2003] and Ackerly et al [2006]). These hypotheses share a common form: a clade first diversifies in one way, such as in habitat use.…”

“…The problem with these hypotheses is that they usually rely on phylogenetic reconstruction of ancestral traits (an exception is Diamond 1986; see discussion in Ackerly et al 2006 andLosos 2009). Unfortunately, we are unlikely to be able to have much confidence in ancestral reconstructions for traits that evolve rapidly relative to the frequency of cladogenesis (Schluter et al 1997;Oakley and Cunningham 2000).…”

“…In light of the evolutionary lability of this trait, we can have little confidence in phylogenetic inferences of habitat use for ancestors deep in the phylogeny. Unfortunately, this precludes testing one aspect of a stages-ofradiation hypothesis, that traits inferred to have diverged in later stages of a radiation did not also diverge early in the radiation; an alternative possibility would be that body size diverged early in the radiation without much subsequent change, but that habitat use has been diverging throughout the radiation (Ackerly et al 2006;Price 2007). This ambiguity, in turn, makes it difficult to assess whether different adaptive radiations have diversified in the same sequence.…”

Conservatism and diversification of plant functional traits: Evolutionary rates versus phylogenetic signal

“…For Ceanothus, results were averaged over a set of 100 equally parsimonious trees (57), and variation among trees was negligible for the results presented here. None of the trait data were obtained from the specimens used for molecular analysis, so trait data for species were matched to corresponding species names at the tips.…”

Microsatellite variation in three calcium-tolerant species of peat moss detected specific genotypes of Sphagnum warnstorfii on magnesium-rich bedrock

“…The adaptive role of magnesium for peat mosses, together with the previously reported adaptive role of pH (Mikulášková et al 2015), represent a rather recent evolutionary mechanism, forming a beta niche in the sense of Ackerly et al (2006), which is in marked contrast to the adaptation to water level that drives the macroevolution of peat mosses ). An analogous pattern is recorded for vascular plants, although the physiological background of calcicole behaviour may be different .…”