Many species of plants, animals, and microorganisms exchange genes well after the point of evolutionary divergence at which taxonomists recognize them as species. Genomes contain signatures of past gene exchange and, in some cases, they reveal a legacy of lineages that no longer exist. But genomic data are not available for many organisms, and particularly problematic for reconstructing and interpreting evolutionary history are communities that have been depleted by extinctions. For these, morphology may substitute for genes, as exemplified by the history of Darwin’s finches on the Galápagos islands of Floreana and San Cristóbal. Darwin and companions collected seven specimens of a uniquely large form of Geospiza magnirostris in 1835. The populations became extinct in the next few decades, partly due to destruction of Opuntia cactus by introduced goats, whereas Geospiza fortis has persisted to the present. We used measurements of large samples of G. fortis collected for museums in the period 1891 to 1906 to test for unusually large variances and skewed distributions of beak and body size resulting from introgression. We found strong evidence of hybridization on Floreana but not on San Cristóbal. The skew is in the direction of the absent G. magnirostris. We estimate introgression influenced 6% of the frequency distribution that was eroded by selection after G. magnirostris became extinct on these islands. The genetic residuum of an extinct species in an extant one has implications for its future evolution, as well as for a conservation program of reintroductions in extinction-depleted communities.

许多植物、动物和微生物物种在分类学家将它们识别为物种的进化分歧点之后就开始交换基因。基因组包含过去基因交换的特征，在某些情况下，它们揭示了不再存在的谱系遗产。但是许多生物体没有基因组数据，重建和解释进化历史尤其成问题的是因灭绝而枯竭的群落。对于这些，形态可以代替基因，正如达尔文雀在加拉帕戈斯群岛的弗洛雷亚纳和圣克里斯托瓦尔的历史所证明的那样。达尔文和他的同伴们收集了七个独特的大地螺标本在 1835 年。在接下来的几十年里，种群灭绝，部分原因是引进的山羊破坏了仙人掌，而Geospiza fortis一直持续到现在。我们对 1891 年至 1906 年期间为博物馆收集的大量G. fortis样本进行了测量，以测试由基因渗入导致的异常大的差异以及喙和体大小的偏态分布。我们在 Floreana 上发现了强有力的杂交证据，但在 San Cristóbal 上却没有。偏斜是在不存在的G. magnirostris的方向上。我们估计基因渗入影响了在G. magnirostris之后被选择侵蚀的频率分布的 6%在这些岛屿上灭绝了。现存物种中已灭绝物种的遗传残留物对其未来的进化以及在灭绝耗尽的群落中重新引入的保护计划具有影响。