Mallet James, Beltrán Margarita, Neukirchen Walter, Linares Mauricio
Galton Laboratory, University College London, Wolfson House, 4 Stephenson Way, London NW1 2HE, UK.
BMC Evol Biol. 2007 Feb 23;7:28. doi: 10.1186/1471-2148-7-28.
To understand speciation and the maintenance of taxa as separate entities, we need information about natural hybridization and gene flow among species.
Interspecific hybrids occur regularly in Heliconius and Eueides (Lepidoptera: Nymphalidae) in the wild: 26-29% of the species of Heliconiina are involved, depending on species concept employed. Hybridization is, however, rare on a per-individual basis. For one well-studied case of species hybridizing in parapatric contact (Heliconius erato and H. himera), phenotypically detectable hybrids form around 10% of the population, but for species in sympatry hybrids usually form less than 0.05% of individuals. There is a roughly exponential decline with genetic distance in the numbers of natural hybrids in collections, both between and within species, suggesting a simple "exponential failure law" of compatibility as found in some prokaryotes.
Hybridization between species of Heliconius appears to be a natural phenomenon; there is no evidence that it has been enhanced by recent human habitat disturbance. In some well-studied cases, backcrossing occurs in the field and fertile backcrosses have been verified in insectaries, which indicates that introgression is likely, and recent molecular work shows that alleles at some but not all loci are exchanged between pairs of sympatric, hybridizing species. Molecular clock dating suggests that gene exchange may continue for more than 3 million years after speciation. In addition, one species, H. heurippa, appears to have formed as a result of hybrid speciation. Introgression may often contribute to adaptive evolution as well as sometimes to speciation itself, via hybrid speciation. Geographic races and species that coexist in sympatry therefore form part of a continuum in terms of hybridization rates or probability of gene flow. This finding concurs with the view that processes leading to speciation are continuous, rather than sudden, and that they are the same as those operating within species, rather than requiring special punctuated effects or complete allopatry. Although not qualitatively distinct from geographic races, nor "real" in terms of phylogenetic species concepts or the biological species concept, hybridizing species of Heliconius are stably distinct in sympatry, and remain useful groups for predicting morphological, ecological, behavioural and genetic characteristics.
为了理解物种形成以及作为独立实体的分类群的维持，我们需要有关物种间自然杂交和基因流动的信息。
在野生的红蛱蝶属和纹蛱蝶属（鳞翅目：蛱蝶科）中，种间杂交经常发生：根据所采用的物种概念，红蛱蝶亚族中26%-29%的物种涉及其中。然而，就个体而言，杂交很少见。对于一个经过充分研究的在邻域接触中杂交的物种案例（红带袖蝶和希美红带袖蝶），表型上可检测到的杂交个体约占种群的10%，但对于同域分布的物种，杂交个体通常占个体总数不到0.05%。无论是种间还是种内，馆藏标本中自然杂交个体的数量随着遗传距离大致呈指数下降，这表明存在一种类似于在某些原核生物中发现的简单“指数失败定律”的兼容性规律。
红蛱蝶属物种间的杂交似乎是一种自然现象；没有证据表明它因近期人类对栖息地的干扰而增加。在一些经过充分研究的案例中，回交在野外发生，并且在昆虫饲养室中已证实有可育回交个体，这表明基因渐渗很可能发生，并且最近的分子研究表明，在一些但并非所有位点上，等位基因在同域分布的杂交物种对之间进行交换。分子钟定年表明，物种形成后基因交换可能持续超过300万年。此外，一个物种，赫氏红蛱蝶，似乎是杂交物种形成的结果。基因渐渗可能经常促进适应性进化，有时也通过杂交物种形成对物种形成本身有贡献。因此，同域共存的地理种群和物种在杂交率或基因流动概率方面构成一个连续统一体。这一发现与以下观点一致：导致物种形成的过程是连续的，而非突然的，并且这些过程与在物种内部起作用的过程相同，而非需要特殊的间断效应或完全的异域分布。尽管在定性上与地理种群没有区别，从系统发育物种概念或生物学物种概念的角度来看也不是“真实的”，但红蛱蝶属的杂交物种在同域分布中稳定地保持不同，并且仍然是预测形态、生态、行为和遗传特征的有用类群。
