Pincheira-Donoso Daniel, Harvey Lilly P, Ruta Marcello
Laboratory of Evolutionary Ecology of Adaptations, School of Life Sciences, University of Lincoln, Brayford Campus, Lincoln, LN6 7DL, UK.
Laboratory of Evolutionary Palaeobiology, School of Life Sciences, University of Lincoln, Brayford Campus, Lincoln, LN6 7DL, UK.
BMC Evol Biol. 2015 Aug 7;15:153. doi: 10.1186/s12862-015-0435-9.
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.
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.
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.
适应性辐射理论认为，生态机遇促进了系统发育和生态多样性的快速增殖。鉴于适应性辐射是通过占据新进入的生态区域中可用的生态位空间来进行的，该理论预测：（i）进化多样化遵循“早期爆发”过程，即它在一个进化枝的历史早期加速（当可用生态位空间促进物种形成时），随后随着新物种使生态位空间饱和而减缓；以及（ii）系统发育分支伴随着新进化物种之间与生态相关的表型性状的多样化。在此，我们采用宏观进化系统发育模型选择分析，利用现存脊椎动物中物种最为丰富且生态最为多样的谱系之一——南美蜥蜴属鬃狮蜥，来检验关于进化多样化的这两个预测。
我们的系统发育分析支持了一种密度依赖的谱系多样化模型。然而，谱系随时间的多样化曲线并未为早期爆发提供有力支持。相反，表型（体型）相对差异的进化程度较高，在鬃狮蜥进化的大约最后500万年中显著不同于布朗模型。模型拟合分析也拒绝了表型进化的“早期爆发”模型，而是支持稳定选择（奥恩斯坦 - 乌伦贝克模型，识别出三个峰值）作为体型多样化的最佳模型。最后，多样化速率倾向于随着体型变小而增加。
鬃狮蜥在一个密度依赖的过程中实现了多样化，谱系积累有略微明显的间歇性脉冲，这与安第斯山脉在过去约2500万年中逐渐隆升所创造的预期间歇性生态机遇相符。我们认为，生态机遇可能很强大，并且是大陆上适应性辐射的关键驱动因素，但当重大事件（如气候、地理事件）显著改变环境时，它可能比岛屿上出现得更不频繁。相比之下，体型多样化符合具有多个性状最优值的奥恩斯坦 - 乌伦贝克模型。尽管谱系和表型之间存在这种不对称的多样化，但正如我们对多样化的性状依赖分析所示，预计这两个过程之间存在联系。我们最后建议，适应性辐射的定义不应以多样化的早期爆发的存在为条件，而应推广到物种和生态多样性从单一祖先进化而来的谱系。
