One of biology’s most important concepts, no?
A lizard lineage which has evolved over the last 19 million years has helped scientists to re-think one of the most important concepts of modern biology.
‘Adaptive radiation’ is recognised as a pillar of evolutionary science. It describes the development of new biodiversity, and is triggered when a species encounters a new environment with plenty of available resources –this is called ‘ecological opportunity’. This single species then makes the most of these resources and multiplies rapidly into several new forms. When all these resources have been used up by new species, the process of biodiversity proliferation slows down dramatically.
‘Early-bursts’ of new species diversification have previously been seen as a central part of this process – scientists have for decades referred to this trend as a key component of adaptive radiation. However, new research published in the academic journal BMC Evolutionary Biology suggests that the term should not be defined by these early rapid surges.
Here’s the abstract:
Background: Adaptive radiation theory posits that ecological opportunity promotes rapid proliferation of phylogenetic and ecological diversity. Given that adaptive radiation proceeds via occupation of available niche space in newly accessed ecological zones, theory predicts that: (i) evolutionary diversification follows an ‘early-burst’ process, i.e., it accelerates early in the history of a clade (when available niche space facilitates speciation), and subsequently slows down as niche space becomes saturated by new species; and (ii) phylogenetic branching is accompanied by diversification of ecologically relevant phenotypic traits among newly evolving species. Here, we employ macroevolutionary phylogenetic model-selection analyses to address these two predictions about evolutionary diversification using one of the most exceptionally species-rich and ecologically diverse lineages of living vertebrates, the South American lizard genus Liolaemus.
Results: Our phylogenetic analyses lend support to a density-dependent lineage diversification model. However, the lineage through-time diversification curve does not provide strong support for an early burst. In contrast, the evolution of phenotypic (body size) relative disparity is high, significantly different from a Brownian model during approximately the last 5 million years of Liolaemus evolution. Model-fitting analyses also reject the ‘early-burst’ model of phenotypic evolution, and instead favour stabilizing selection (Ornstein-Uhlenbeck, with three peaks identified) as the best model for body size diversification. Finally, diversification rates tend to increase with smaller body size.
Conclusions: Liolaemus have diversified under a density-dependent process with slightly pronounced apparent episodic pulses of lineage accumulation, which are compatible with the expected episodic ecological opportunity created by gradual uplifts of the Andes over the last ~25My. We argue that ecological opportunity can be strong and a crucial driver of adaptive radiations in continents, but may emerge less frequently (compared to islands) when major events (e.g., climatic, geographic) significantly modify environments. In contrast, body size diversification conforms to an Ornstein-Uhlenbeck model with multiple trait optima. Despite this asymmetric diversification between both lineages and phenotype, links are expected to exist between the two processes, as shown by our trait-dependent analyses of diversification. We finally suggest that the definition of adaptive radiation should not be conditioned by the existence of early-bursts of diversification, and should instead be generalized to lineages in which species and ecological diversity have evolved from a single ancestor. Open access – Daniel Pincheira-Donoso, Lilly P. Harvey, and Marcello Ruta
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Here’s the little swine that challenges orthodoxy: