经济学人下载:性选择和物种形成(2)

Genetic sequencing shows just how similar these species are. Only 12 of their genes differ—less than 0.1% of their genomes. Intriguingly, one of the 12 is part of one of the sex chromosomes, hinting at a role for sexual selection. Another three of them play a role in the colouration of male plumage, which is also suggestive. For, although the species are of similar sizes and shapes, adult male tawny-bellied seedeaters, as their name implies, have brownish-orange chests, while male Ibera seedeaters have black throats and sandy-coloured bodies. Their songs are distinct, too. They use the same range of frequencies, but different syllables. 

基因测序显示了这些物种的极度相似性。他们只有12个基因不同——不到基因组的0.1%。有趣的是，这12条染色体中有一条是其中一条性染色体的一部分，这暗示了性选择的作用。另外三条对雄性羽毛的颜色也有影响，这也很有启发性。因为，尽管这两个物种的大小和形状相似，但成年雄性茶腹食籽雀(如它们的名字所示)的腹部是茶色的，胸部是橙色的，而雄性伊比利亚食籽雀的喉咙是黑色的，身体是沙色的。此外它们的鸣啭也截然不同，使用的频率范围是相同的，但音节不同。

First, Ms Turbeck and her colleagues established that females of both species did, indeed, regularly select the right mates. Observation of pairs suggested so, and genetic testing of young (needed because more than half the females under study mated with extra males in addition to their long-term partners) did not find any exceptions, either. That done, they looked at males' abilities to distinguish conspecific rivals from heterospecific neutrals. During the 2019 breeding season they mounted decoy males in the territories of 40 male tawny-bellied seedeaters and 36 male Ibera seedeaters. They then monitored the residents' responses when a female was around. 

首先，图贝克女士和她的同事证实了一点，即这两个物种的雌性确实会定期选择合适的配偶，对配偶的观察也证实了这一点，而且对年轻配偶进行的基因检测(需要做是因为在研究中有超过一半的雌性除了长期伴侣外还与其他雄性交配)也没有发现任何例外。研究完成后他们观察了雄性区分同种对手和异种中性对手的能力。在2019年的繁殖季节，他们在40只雄性茶腹食籽雀和36只雄性伊比利亚食籽雀的领地上安置诱饵雄性。然后，当有雌性在附近时，他们监测“居民们”的反应。

They used four decoys. One had the plumage and song of a male Ibera seedeater. One similarly resembled a male tawny-bellied seedeater. The other two had, in one or other combination, the appearance of one species and the song of the other. 

他们用了四种诱饵。一种有着雄性伊比利亚食籽雀的羽毛和鸣啭，一种和雄性茶腹食籽雀相似。另外两种在一种或另一种组合下有着一种物种的外表和另一种物种的鸣啭。

If a resident male viewed a decoy as competition he would act aggressively, by flying to and pecking at it. And males could certainly tell the difference. They were most aggressive towards decoys that resembled their own species, moderately so towards decoys that combined features of both, and least towards those that resembled the other species. Since these were not real birds, it must have been the colouration and the song that were the cues. 

如果一只雄性居民把诱饵视为竞争对手，那么他会飞过去啄它，表现得很有攻击性。雄性当然能分辨出区别。它们对与自己种类相似的诱饵最具攻击性，对结合了两者特征的诱饵的攻击性较弱，而对那些与其他种类相似的诱饵的攻击性最低。既然这些不是真正的鸟，那么颜色和鸣啭一定就是线索。

The differences between the species' songs may be cultural, and have come about after the split. But the differences in plumage are clearly genetic. What is more, they are so small that they could have happened on a single occasion as a result of an accidental hybridisation, for similar genetic differences are found in varying combinations throughout southern capuchino seedeaters. Also, what happened once could presumably have happened often. It therefore seems plausible that Ms Turbeck has uncovered the nub of the mechanism by which this group has become so diverse. If so, then she has helped write another page in that as-yet-unpublished volume, "The Real Origin of Species". 

这两种物种间鸣啭上的差异可能是由文化造成的，而且是在物种分裂之后产生的，但羽毛上的差异显然是遗传的。更重要的是，差异如此之小，所以可能是偶然杂交的结果，因为在南部卡布奇诺食籽雀的不同组合中发现了类似的基因差异。而且，曾经发生过的事情可能会经常发生。因此，图贝克女士揭示了这一群体变得如此多样化的核心机制似乎是合理的。如果是这样的话，那么她又能在尚未出版的《物种的真正起源》一书中再添上一页了。