Nyman Tommi, Farrell Brian D, Zinovjev Alexey G, Vikberg Veli
Department of Biology, University of Oulu, P.O. Box 3000, FI-90014 Oulu, Finland.
Evolution. 2006 Aug;60(8):1622-37.
Adaptive radiations consist of two intertwined processes, diversification of species and diversification of their ecological niches, but it is unclear whether there is a causal link between the processes. In phytophagous insects, ecological diversification mainly involves shifts in host-plant associations and in larval feeding habits (internal or external) on different plant parts, and several observations indicate that speciation is facilitated by host shifts. Data on host use in individual species suggest that internal feeders are less likely to colonize new hosts than external-feeding taxa and, consequently, increases in collective host ranges and species numbers should be slowed down in endophagous lineages. We tested these related hypotheses by using phylogenetic information to reconstruct the evolutionary history of larval resource use in the sawfly subfamily Nematinae, a group of 1000 plus species with a broad range of niches: the subfamily's combined host range includes over 20 plant families, and larvae may feed externally on leaves or needles, or internally, for example, in buds, fruits, leaves, or galls. The results show that: (1) Most internally feeding groups have evolved independently from external-feeding ancestors, but several distinct internal habits have appeared convergently multiple times; (2) Shifts among host taxa are clearly more common than changes in larval habits; (3) The majority of host switches have occurred among phylogenetically close plant groups, but many shifts are manifest among distantly related, ecologically proximate hosts; (4) Although external feeding characteristic of the common ancestor of Nematinae is associated with relatively high rates of host-shifting, internal feeders are very conservative in their host use; (5) In contrast, the effect of endophagy on speciation probabilities is more variable: net speciation rates are lowered in most internal-feeding groups, but a striking exception is found in species that induce galls on Salicaceae. The loose connection between collective host ranges and species diversity provides empirical support for theoretical models suggesting that speciation rates are a function of a complex interplay between "intrinsic" niche width and resource heterogeneity.
适应性辐射由两个相互交织的过程组成，即物种多样化及其生态位多样化，但尚不清楚这两个过程之间是否存在因果联系。在植食性昆虫中，生态多样化主要涉及寄主植物关联的转变以及在不同植物部位的幼虫取食习性（内部或外部）的转变，并且一些观察结果表明寄主转移促进了物种形成。关于单个物种寄主利用的数据表明，与外部取食类群相比，内部取食者定殖新寄主的可能性较小，因此，内食性谱系中寄主范围和物种数量的总体增加应该会减缓。我们通过利用系统发育信息来重建叶蜂亚科线虫叶蜂幼虫资源利用的进化历史，来检验这些相关假设，该亚科有1000多种物种，具有广泛的生态位：该亚科的寄主范围组合包括20多个植物科，幼虫可能在叶片或针叶外部取食，也可能在内部取食，例如在芽、果实、叶片或虫瘿中取食。结果表明：（1）大多数内部取食类群是从外部取食祖先独立进化而来的，但几种不同的内部习性多次趋同出现；（2）寄主类群之间的转移明显比幼虫习性的变化更常见；（3）大多数寄主转换发生在系统发育关系密切的植物类群之间，但许多转换也出现在亲缘关系较远、生态相近的寄主之间；（4）虽然线虫叶蜂共同祖先的外部取食特征与相对较高的寄主转移率相关，但内部取食者在寄主利用方面非常保守；（5）相比之下，内食性对物种形成概率的影响更具变异性：大多数内部取食类群的净物种形成率降低，但在杨柳科上诱导虫瘿的物种中发现了一个显著例外。寄主范围总体与物种多样性之间的松散联系为理论模型提供了实证支持，这些模型表明物种形成率是“内在”生态位宽度与资源异质性之间复杂相互作用的函数。
