杂交后代的生存之道

On May 15, 1985, trainers at Hawaii Sea Life Park were stunned when a 400- pound gray female bottlenose dolphin named Punahele gave birth to a dark-skinned calf that partly resembled the 2,000-pound male false killer whale with whom she shared a pool. The calf was a wholphin, a hybrid that was intermediate to its parents in some characteristics, like having 66 teeth compared with the bottlenose’s 44 and the 88 of the false killer whale, a much larger member of the dolphin family.

图片提供：Bernd Thissen/European Pressphoto Agency, via Corbis

一匹名为Eclyse的斑马马

撰文：Sean B Carroll

发表时间：2010年9月13日

1985年5月15日，夏威夷海洋公园（Hawaii Sea Life Park）的驯兽员们被眼前的一幕惊了个目瞪口呆：一只重达180公斤的雌性灰宽吻海豚Punahele生下了一头通体黝黑的小海豚，说是海豚，其实外表看上去和那只曾与它母亲在同一个池子里待过的雄性伪虎鲸更为接近点。准确的说，这只小家伙应该叫鲸豚（wholphin），它是个在某些特性上介于其父母中间的混种儿，比如它长有66颗牙齿，宽吻海豚有44颗牙齿，而在海豚科中数量最多的伪虎鲸则拥有88颗牙齿。

Christian Charisius/Reuters, via Corbis

图片提供：Christian Charisius/Reuters, via Corbis

A male liger (lion-tiger) named Bahier.

一只名叫Bahier的雄性狮虎（liger）

Sea Life Park Hawaii

图片提供：夏威夷海洋公园

A wholphin (dolphin-false killer whale) named Kekaimalu.

一只名叫Kekaimalu的鲸豚

In 2006, a hunter in the Canadian Arctic shot a bear that had white fur like a polar bear’s but had brown patches, long claws and a hump like a grizzly bear’s. DNA analysis confirmed the animal was a hybrid of the two species.

2006年，一位猎人在加拿大的北极地区射杀了一只奇怪的熊，它的皮毛如同北极熊一般呈现出白色，但夹杂着褐色斑点，长爪子和弓起的背部又表现出灰熊的特征来。DNA分析证实了这只怪熊是北极熊和灰熊的杂交产物。

While one might think that these oddities are examples of some kind of moral breakdown in the animal kingdom, it turns out that hybridization among distinct species is not so rare. Some biologists estimate that as many as 10 percent of animal species and up to 25 percent of plant species may occasionally breed with another species. The more important issue is not whether such liaisons occasionally produce offspring, but the vitality of the hybrid and whether two species might combine to give rise to a third, distinct species.

对于某些道德洁癖者来说，这种怪胎的出现是动物王国道德沦陷的最好例证，但事实证明，不同物种之间的杂交并不是什么罕见的奇观。一些生物学家估计，有多达10%的动物物种和25%的植物物种会偶尔和不同物种进行交配。这种暧昧关系的重要作用并不在于制造后代，而在于利用杂交的优势产生第三种和亲代完全不同的物种。

While several examples of human-bred animal hybrids are well known and can thrive in captivity including zorses (zebra-horse), beefalo (bison-beef cattle) and, of course, mules (donkey-horse), naturally occurring animal hybrids have many factors working against their longer-term success.

人为繁育杂交动物的成功案例有很多，其中圈养类就包括斑马马（zorses：斑马和马）、皮弗洛牛（beefalo：北美野牛和肉牛）以及众所周知的骡子（驴和马），但在自然状态下发生的动物杂交行为却有很多不利因素，这些因素阻碍了这种模式的长期成功。

One of the main obstacles is that, even if members of different species might mate, when the two species are too distant genetically or carry different numbers of chromosomes, the offspring are usually inviable or infertile (like zorses and mules), and are therefore evolutionary dead ends. A second problem is that any hybrid will usually be vastly outnumbered and outcompeted by one or both parent species.

其中最主要的障碍在于，即便不同物种之间的某些成员交配成功，但由于物种间的遗传差异过大或携带了不同数目的染色体，其产生的后代通常无法存活或无法生育（如斑马马和骡子），进化走入了死胡同。另一大障碍在于杂交物种在数量和竞争能力上都无法和双亲的物种相匹敌。

But because species hybrids create new combinations of genes, it is possible that some combinations might enable hybrids to adapt to conditions in which neither parent may fare as well. Several such examples are now known from nature. Furthermore, DNA analysis is now allowing biologists to better decipher the histories of species and to detect past hybridization events that have contributed new genes and capabilities to various kinds of organisms including, it now appears, ourselves.

但由于物种杂交实现了基因的新组合，有可能某些新组合让杂交后代产生了对环境的适应能力，而在这样的环境中它的双亲却无法存活。在自然界中这样的例子已有不少为人类所知。此外，DNA分析技术让生物学家们能够对物种的进化历史作更为精确的解读，他们甚至可以对发生在过去的杂交事件进行检测，而正是这样的杂交行为贡献了新基因，并产生了今日这般多种生物体（包括人类）共存的可能性。

The familiar sunflower has provided great examples of adaptation by hybrids. Loren H. Rieseberg of the University of British Columbia and colleagues have found that two widespread species, the common sunflower and prairie sunflower, have combined at least three times to give rise to three hybrid species: the sand sunflower, the desert sunflower, and the puzzle sunflower.

普通的向日葵为我们提供一个通过杂交产生适应性的绝佳例证。不列颠哥伦比亚大学（University of British Columbia ）的洛伦·里瑟博格（Loren H. Rieseberg）和他的同事发现了两种广泛存在的向日葵品种：普通向日葵和具柄向日葵（prairie sunflower），这两种向日葵至少杂交了三次，并产生了三种杂交后代：沙地向日葵、沙漠向日葵和奇异向日葵（puzzle sunflower）。

The parental species thrive on moist soils in the central and Western states, but the hybrids are restricted to more extreme habitats. The sand sunflower, for instance, is limited to sand dunes in Utah and northern Arizona and the puzzle sunflower to brackish salt marshes in West Texas and New Mexico.

亲本向日葵在美国中部和西部的大多数湿性土壤中都可以茁壮成长，但杂交种对产地却有严格限制，以沙地向日葵为例，它只能生长在犹他州和亚利桑那州北部的沙丘之中，而奇异向日葵则偏爱以德克萨斯州西部和新墨西哥州的盐沼平原为家。

The species distributions suggest that the hybrids thrive where the parents cannot. Indeed, recent field tests that examined the relative ability of the parental species to thrive in the hybrids’ habitat, and vice versa, found that the sand sunflower was better able than its parents to germinate, grow and survive in its dune habitat but fared relatively poorly in parental habitats. Similarly, the puzzle sunflower was much better at growing in salty conditions than its parents.

这种物种分布结果表明，杂交种可以在在亲本无法生长的环境中茁壮成长。最近的野外试验也证实了亲本向日葵在杂交向日葵适宜的栖息地中生长较差，而且反过来也是一样，沙地向日葵虽然可以在沙丘地上生根发芽，但一被移到湿性土壤中就表现糟糕。同样，在盐碱条件下，奇异向日葵比亲本要长得更旺盛。

One lesson from the sunflowers appears to be that hybrids may succeed if they can exploit a different niche from their parents. The same phenomenon has been discovered in animal hybrids.

从向日葵身上我们似乎学到了一课：如果杂交后代可以在一个完全不同的小生态位中扎根，它就有可能会成功。在动物杂交物种上，我们也发现了同样的现象。

In the past 250 years, various forms of honeysuckle have been introduced to the Northeastern states. In the late 1990s, researchers led by Bruce McPheron of Pennsylvania State University discovered that this invasive honeysuckle was infested by a particular fruit fly species they called the Lonicera fly. When they analyzed DNA to determine its relationship to others, they were stunned to find that it was a hybrid of two closely related flies, the blueberry maggot and the snowberry maggot.

在过去的250年中，陆续有各种各样的忍冬属植物被移植到美国东北部。上世纪九十年代后期，宾夕法尼亚州立大学的布鲁斯·麦克费隆（Bruce McPheron）所领导的研究小组发现，这些侵入的忍冬属植物被一种名为忍冬蝇（Lonicera fly）的特别果蝇所感染。研究人员对忍冬蝇的DNA进行分析，以确定它们和其他果蝇之间的亲缘关系，结果相当令人惊讶，忍冬蝇竟然是蓝莓蝇和雪莓蝇这两种关联果蝇物种的杂交后代。

In laboratory experiments, the researchers found that the Lonicera hybrid preferred its honeysuckle host plant over its parent species’ host plants and that each parent species preferred its own host plant over the other’s. However, both parents also accepted honeysuckle. The researchers suggest that since the two parental species were thus more likely to encounter each other on honeysuckle in the wild, the newly invasive weed served as a catalyst for matings between the species and the formation of the hybrid species that now prefers honeysuckle.

通过室内实验，研究者们发现忍冬蝇偏爱以忍冬作为寄生植物，同样，其亲本果蝇也有各自偏爱的寄生植物，但同时它们也不排斥忍冬属植物。研究人员认为，亲本果蝇很可能是在野外的忍冬植物上邂逅的，这种新近侵入的野草成为了两种果蝇结合的催化剂，并导致了偏爱寄生性杂交后代的形成。

The sunflower and Lonicera fly examples raise the question of whether hybridization between species has been more frequent than biologists once assumed. The most provocative report of possible hybridization came from the recent analysis of more than 60 percent of the Neanderthal genome sequence, which raised the specter of our ancestors commingling their genes with a long-diverged cousin.

向日葵和忍冬蝇的例子提出了这样一个问题：物种之间的杂交频度是不是要比生物学家们先前预测的要更为频繁？一项最新分析报告表明，经过对尼安德特人（Neanderthal）60%以上的基因组序列进行分析后发现，我们的祖先可能与这位一直以来与现代人类分道扬镳的表亲发生过混血。

Analyses of the overall genetic distance between Neanderthals and modern humans reveal that our DNA is 99.84 percent identical to that of Neanderthals. This small divergence indicates that the two lines split off from each other about 270,000 to 440,000 years ago. The fossil evidence shows that Neanderthals were restricted to Europe and Asia, whereas Homo sapiens originated in Africa. Various kinds of evidence indicate that modern humans migrated out of Africa and reached the Middle East more than 100,000 years ago and Europe by about 45,000 years ago, and would have or could have encountered Neanderthals for some time in each locale. The crucial question for paleontology, archaeology, and paleogenetics has been what transpired between the two species. To put it a little more crudely, did we date them or kill them, or perhaps both?

尼安德特人和现代人类的全面遗传距离分析结果揭示，我们有99.84%的DNA与他们是完全相同的，微小的差异则暗示这两个谱系早在27万至44万年前就彼此分离了。化石证据表明，尼安德特人只存在于欧洲和亚洲，而人类则起源于非洲。人类走出非洲以后，在10万年前抵达中东，4.5万年前到达欧洲，我们有可能在某些时段和某个地区遭遇过尼安德特人。古人类学家、考古学家和古遗传学家最感兴趣的关键问题在于：这两种物种之间究竟发生了什么？通俗点说就是，人类是和他们发展出一段不伦之恋呢？还是将他们直接屠戮？或者两者兼而有之。

If the former, then there could be a bit of Neanderthal in some or all of us. The first comparisons of small sections of Neanderthal DNA did not indicate any hybridization, and the lack of interbreeding became a widely accepted conclusion. That remained the case until this year, when a much greater portion of the Neanderthal genome was obtained by Svante Paabo and colleagues at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany. It now appears that 1 percent to 4 percent of the DNA sequence of Europeans and Asians, but not Africans, was contributed by Neanderthals mixing with Homo sapiens, perhaps in the Middle East 50,000 to 80,000 years ago. It is possible that some Neanderthal versions of genes enabled modern humans to adapt to new climates and habitats.

如果是前者，那么在所有或部分人类身上就存在尼安德特人的血统。与少量尼安德特人DNA的第一次比对并没有发现任何杂交的迹象，人类和他们不存在异种繁殖也成为了一种被广泛接受的结论。但直到今年，研究又发生了新变化，来自德国莱比锡马普进化人类学研究院（Max Planck Institute for Evolutionary Anthropology）的斯万特·帕珀（Svante Paabo）和同事获取了更多的尼安德特人DNA，在分析以后发现，欧洲人和亚洲人（不包括非洲人）的DNA序列中有1%至4%是由与古人类混血的尼安德特人贡献的，这种杂交过程可能发生在5万至8万年之前的中东地区。有可能是某些尼安德特人的基因让现代人类适应了新气候和新栖息地。

The discovery of hybrid species and the detection of past hybridizations are forcing biologists to reshape their picture of species as independent units. The barriers between species are not necessarily vast, unbridgeable chasms; sometimes they get crossed with marvelous results.

杂交物种的发现和对过去杂交过程的检测迫使生物学家们改变将物种视为独立单元的固有看法。物种之间的阻隔也许并不如想象中的那般巨大，它并非是一道无法逾越的天堑；有时候，某些不经意的交叉会产生奇迹般的效果。

Sean B. Carroll is a molecular biologist and geneticist at the University of Wisconsin.

肖恩·卡罗尔（Sean B. Carroll）是威斯康星大学（University of Wisconsin）的分子生物学家和遗传学家