Zeh David W, Zeh Jeanne A
Smithsonian Tropical Research Institute, Unit 0948, APO AA 34002-0948, Apartado, 2072, Balboa, República de Panamá.
Evolution. 1994 Aug;48(4):1168-1182. doi: 10.1111/j.1558-5646.1994.tb05303.x.
Differences in secondary sexual characteristics of males often provide the most conspicuous means of distinguishing between closely related species. Does this therefore imply that the absence of differentiation in exaggerated male traits between allopatric populations provides evidence of a single, genetically cohesive species? We addressed this question with a comprehensive investigation of two populations (French Guiana and Panama) of the harlequin beetle-riding pseudoscorpion, Cordylochernes scorpioides. This highly sexually dimorphic pseudoscorpion is currently described as a single species, ranging throughout the Neotropics. Our morphometric analyses detected minimal differentiation between the two populations in all nine external morphological characters measured, including sexually dimorphic traits in males. Only in traits of the spermatophore was there any appreciable level of differentiation. Behavior differentiation and prezygotic reproductive isolation were also limited: 78.3% of males successfully transferred sperm to "foreign" females, and in 63.9% of these cases, females' eggs were successfully fertilized. By contrast, extensive divergence existed in two of nine electrophoretic loci, including an essentially fixed-allele difference at the Ldh locus. Most significantly, postzygotic reproductive isolation was complete, with heteropopulation zygotes invariably aborting early in development. These results strongly suggest that the two populations are, in fact, sibling species, a conclusion supported by our recently published findings on their marked divergence in minisatellite DNA. How can such interpopulation homogeneity in male sexually dimorphic traits exist in the face of strong genetic divergence? We propose that sexual selection, oscillating between favoring small and then large males, maintains such high levels of male variability within each population that it has obscured a speciation event in which genetic divergence and postzygotic incompatibility have clearly outpaced the evolution of prezygotic reproductive isolation.
雄性第二性征的差异通常是区分近缘物种最显著的方式。那么，这是否意味着异域种群间夸张的雄性特征缺乏分化就证明它们是一个单一的、基因上具有凝聚力的物种呢？我们通过对骑在丑角甲虫上的伪蝎（Cordylochernes scorpioides）的两个种群（法属圭亚那和巴拿马）进行全面调查来解决这个问题。这种具有高度两性异形的伪蝎目前被描述为一个单一物种，分布于整个新热带地区。我们的形态测量分析在测量的所有九个外部形态特征中，检测到两个种群之间的分化极小，包括雄性的两性异形特征。只有在精包的特征上存在任何明显程度的分化。行为分化和合子前生殖隔离也很有限：78.3%的雄性成功地将精子转移到“外来”雌性体内，在这些情况中，63.9%的雌性卵子成功受精。相比之下，在九个电泳位点中的两个位点存在广泛的差异，包括在Ldh位点上基本固定的等位基因差异。最显著的是，合子后生殖隔离是完全的，异种群合子在发育早期总是会夭折。这些结果强烈表明，这两个种群实际上是姐妹种，这一结论得到了我们最近发表的关于它们在微卫星DNA上存在显著差异的研究结果的支持。面对强烈的遗传差异，雄性两性异形特征在种群间为何会存在如此高的同质性呢？我们提出，性选择在青睐小体型雄性和大体型雄性之间摆动，维持了每个种群内如此高水平的雄性变异性，以至于它掩盖了一个物种形成事件，在这个事件中，遗传差异和合子后不相容性显然已经超过了合子前生殖隔离的进化。
