Chiba Satoshi
Institute of Geosciences, Shizuoka University, Oya, Shizuoka, 422, Japan.
Evolution. 1993 Oct;47(5):1539-1556. doi: 10.1111/j.1558-5646.1993.tb02174.x.
Geographical and temporal variation in gene exchange between two endemic land snail species, Mandarina aureola and Mandarina ponderosa, was studied on Hahajima Island of the Bonin Islands. Allozyme variation in modern samples, and variation in the color and shell morphology of modern and fossil samples, suggest a complex geographical and historical pattern of hybridization. These two species occur in sympatry, and their shell morphologies and protein genotypes are markedly divergent. However, many specimens of M. aureola, collected from the middle region of the island, exhibit intermediate shell morphologies and possess marker alleles of M. ponderosa. Fossil samples of the two species strongly suggest that these intermediates were hybrids with M. ponderosa that were produced since the end of the Pleistocene. Each of these species, in addition, is subdivided into two genetically and morphologically divergent parapatric races. Interspecific hybridization appears to have produced genetical and morphological admixture among these four distinctive groups of populations. The past distribution and geographic variation of M. ponderosa can be traced in the distribution of M. ponderosa-derived genotypes in current populations of M. aureola. Temporal changes of the color pattern in the fossil populations of hybrids suggest that the traits introduced from M. ponderosa to M. aureola have been affected by natural selection and could replace traits of living species when advantageous. Moreover, these introgressed genes appeared to provide novel properties that enabled M. aureola to advance into a new environment. Relatively independent change in shell color and morphology further suggests mosaic evolution following the hybridization events. Connectively, these data reveal how hybridization events may be an important source of evolutionary novelties and make it clear that the phenomenon of reticulate evolution cannot be ignored.
在小笠原群岛的母岛对两种地方性陆地蜗牛物种——金色曼陀罗蜗牛（Mandarina aureola）和粗壮曼陀罗蜗牛（Mandarina ponderosa）之间的基因交流进行了地理和时间变化的研究。现代样本中的等位酶变异以及现代和化石样本的颜色与壳形态变异表明存在复杂的杂交地理和历史模式。这两个物种同域分布，它们的壳形态和蛋白质基因型明显不同。然而，从该岛中部采集的许多金色曼陀罗蜗牛标本呈现出中间壳形态，并拥有粗壮曼陀罗蜗牛的标记等位基因。这两个物种的化石样本有力地表明，这些中间类型是自更新世末期以来与粗壮曼陀罗蜗牛杂交产生的杂种。此外，这两个物种各自又细分为两个在遗传和形态上不同的邻域种族。种间杂交似乎在这四个独特的种群组之间产生了遗传和形态混合。粗壮曼陀罗蜗牛过去的分布和地理变异可以通过金色曼陀罗蜗牛当前种群中源自粗壮曼陀罗蜗牛的基因型分布来追溯。杂交化石种群颜色模式的时间变化表明，从粗壮曼陀罗蜗牛引入到金色曼陀罗蜗牛的性状受到了自然选择的影响，并且在有利时可能取代现存物种的性状。此外，这些渐渗基因似乎提供了新特性，使金色曼陀罗蜗牛能够进入新环境。壳颜色和形态的相对独立变化进一步表明杂交事件后发生了镶嵌进化。连贯起来，这些数据揭示了杂交事件如何可能是进化新奇性的重要来源，并清楚地表明网状进化现象不容忽视。
