内容来源：https://www.quantamagazine.org/how-ecotypes-harbor-the-genetic-memory-of-a-species-past-20260521/

内容总结：

研究发现：同一物种内部存在“生态型”，堪称物种的“遗传记忆”

一项最新基因组学研究揭示，自然界中同一物种的种群会根据不同局部环境演化出截然不同的特征，这种现象被称为“生态型”。科学家指出，这些隐藏在同一物种基因组中的多样特性，犹如物种对过往生存环境的“遗传记忆”，能在环境剧变时迅速被激活，推动种群在短短数代内完成适应性进化。

半世纪前的困惑：同一物种，何以形态迥异？

早在上世纪20年代，瑞典植物学家约特·图雷松就注意到，不同海岸线上的盐生灌木存在开花时间、茎秆长度等显著差异，且这些差异具有遗传基础。他于1922年提出“生态型”概念，用以描述同一物种为适应局部环境而产生的亚种群。但当时遗传学尚处萌芽阶段，这一观点饱受质疑。

上世纪70年代，进化生物学家克斯廷·约翰内松在瑞典群岛考察时，也观察到类似现象：生活在潮间带的滨螺壳更厚，以抵御螃蟹捕食；而栖息在浪击岩礁上的滨螺壳则更薄、体型更小。两者之间还存在形态过渡的中间类型。尽管看上去像不同物种，但她怀疑它们只是同一物种的不同生态型。

基因组学揭开谜底：幸存基因的秘密库

直到21世纪初，全基因组测序技术普及，图雷松的假说才得以验证。科学家发现，许多物种的基因组中“存有”适应不同环境的备用基因——这种现象被称为“持久存在的遗传变异”。即使这些变异基因在种群中出现频率不高，但一旦环境改变，自然选择就能“调取”这些历史记忆，快速催生适应新环境的生态型。

一个经典案例是1964年阿拉斯加大地震后，原本生活在海洋中的三刺鱼被困于新形成的淡水湖。按常识它们可能灭绝，但在短短几十年内，这些原本长有骨板防御海洋捕食者的鱼类，却演化出淡水生态型的特征——骨板减少。2015年一项长达50年的基因组研究揭示，海洋三刺鱼种群中本身就零散携带着淡水生存所需的基因变体。实验证明，将海洋三刺鱼移入淡水，它们可在20至30年内完成向淡水生态型的转换，远比等待新突变出现要快得多。

染色体“组团”：锁定适应性基因的关键机制

适应性特征往往涉及数百个基因的协同表达。那么，自然选择如何同时作用于所有这些基因？研究发现，关键在于染色体的结构变异，特别是“倒位”现象。

倒位是指染色体的一段DNA断裂后，旋转180度再重新接回，导致这段基因序列在两条同源染色体上方向相反。这一过程有效阻止了该区域在减数分裂时的基因重组，使一组相关基因像“超级基因”一样被锁定在一起，代代相传。例如，滨螺的厚壳特性与躲避螃蟹的逃逸行为相关基因就被打包锁定，确保了后代同时继承这些协同适应特征。

类似现象在不同物种中广泛存在：加州竹节虫通过倒位维持有纹和无纹两个生态型的伪装特征；三刺鱼则依靠倒位保持海洋与淡水生态型各自基因组的完整性，同时允许种群间仍能交配。约翰内松团队在滨螺基因组中已发现近20个与生态型相关的染色体倒位，其中一些甚至已有数百万年历史，存在于共同祖先中。

生态型：通往新物种的“灰色地带”还是终点站？

进化生物学家长期争论：生态型是否是物种分化的第一步？三刺鱼的海洋与淡水生态型形态差异显著，似乎暗示它们终将分道扬镳。但数百万年过去，它们仍未形成独立物种。相反，当海水变为淡水时，自然选择只是在两个生态型间“切换”，而不丢弃另一套备用基因。

伦敦大学学院进化遗传学家肖恩·斯坦科夫斯基指出，物种定义长久以来存在争议，而生态型概念与达尔文对物种的认知高度契合。他甚至认为，达尔文雀和维多利亚湖丽鱼这些经典的辐射进化例子，按严格标准可能并非不同物种，而是同一物种的不同生态型。

斯坦科夫斯基说：“基因组学让我们看清了表象下的运行机制。生态型代表了一种遗传记忆，反映了一个物种在不同栖息地的生存历史。”这一发现不仅改写了对物种形成过程的理解，也为应对环境变化和生物多样性保护提供了全新视角。

中文翻译：

生态型如何承载物种的遗传记忆
引言
20世纪70年代，还是研究生的进化生物学家克斯汀·约翰内松经常在瑞典群岛的海岸上漫步，仔细搜寻地面上移动的小石子：那是海螺。她的导师是一位分类学家，交给她的任务是记录这些物种的特征，从而描述它们。她注意到，壳更厚的海螺留在岸上，而壳更薄的则更喜欢栖息在波浪拍打的岩石上，介于这两种栖息地之间的海螺，壳的厚度也处于中间状态。虽然它们看起来像是不同的物种，但约翰内松不禁怀疑，这些海螺是否只是同一种生物的不同类型。

大约50年前，植物学家约特·图雷松在类似的环境中也有过类似的发现。他在瑞典海岸考察时注意到，来自不同海岸线的盐角草植物具有独特的特征——开花时间或早或晚，茎秆或长或短——而在不同栖息地之间，这些特征则介于中间。他在自家花园中培育了这些植物，发现这些独特的特征有遗传基础，尽管它们来自同一个物种。1922年，他发表了研究成果，并创造了“生态型”一词，用来描述适应某个超局部栖息地的物种亚群。

当时，物种的定义比现在还要模糊。基因仍停留在理论层面，而DNA的结构还要等30年后才被发现。如今担任哥德堡大学谢尔讷海洋实验室主任的约翰内松说，图雷松“很难获得认可”。一个物种如何能包含多种不同的表型（或特征集合），却不分化为两个物种？她说：“他费了很大力气，才试图让同事们相信物种内部存在遗传差异和局部适应。”

直到21世纪初，全基因组测序技术让进化生物学家能够使用，图雷松关于生态型的想法才得以检验。通过比较从海螺到三刺鱼、竹节虫等生命树中各生态型的DNA序列，科学家们可以在分子水平上研究适应和物种形成（新物种形成的过程）。

伦敦大学学院的进化遗传学家肖恩·斯坦科夫斯基说：“在我看来，这绝对是有史以来作为一名生物学家最激动人心的时代——也许除了回到达尔文那个时代。即使我们了解到生物体是由遗传密码编程的，我们也从未真正能解读它。而现在，我们正逐一查看基因组中的每一个[核苷酸分子]——A、T、G、C（腺嘌呤、胸腺嘧啶、鸟嘌呤和胞嘧啶）。”

约翰内松、斯坦科夫斯基及其他研究人员对基因组生态型的分析，解释了某些物种如何能维持多种适应的DNA序列，从而让进化过程在环境条件变化时（有时只需几代）有效地在生态型之间进行选择。数据还表明，一些经典的多样物种群，包括达尔文雀，可能根本不是独立的物种，而是同一种物种的不同生态型。

斯坦科夫斯基说，生态型代表了一种“遗传记忆”，反映了物种在不同栖息地中的生存历史。他说：“基因组学让我们看清了表象之下的一切。”当科学家们深入探究时，他们发现了适应的引擎。

遗传记忆
1964年3月，北美有记录以来最大的地震撼动了阿拉斯加湾。在四分钟内，海湾中的一些岛屿被抬升了38英尺，河流与海洋隔绝，形成了淡水湖。十年后，科学家惊讶地发现三刺鱼在那里繁衍生息。

这种鱼通常生活在咸水海洋中。生物学家原本以为它们会在淡水湖中灭绝。然而，更有趣的事情发生了。原本武装着骨板以抵御海洋捕食者的海洋三刺鱼，开始看起来和行为方式都像它们的淡水近亲——骨板更少。这种变化在几十年内就完成了，速度快到来不及形成新物种。

是什么驱动了这些快速的表型变化？2015年发表的一项长达50年的基因组数据研究揭示，在海洋三刺鱼的整个种群基因组中，潜藏着在淡水环境中生存所需的基因。研究这种鱼类的伯尔尼大学进化遗传学家凯瑟琳·派歇尔说，这些备用基因零星分布在基因组的约100个区域中。

这种遗传多样性的存在——即同一基因拥有携带不同特征的多重形式——被称为“驻留变异”。即使数量很少，这些备用基因也有机会被表达，就好像自然选择可以回溯过去，在需要时重新部署这些基因。另一个实验室的研究表明，被移植的海洋三刺鱼只需20到30年就能转变为淡水生态型。而新特征的产生和选择可能需要远长于此的时间——前提是鱼类在初次适应完全不同栖息地的冲击中幸存下来。

斯坦科夫斯基说：“这几乎就像种群对不同环境经历的遗传记忆。”

回到瑞典，约翰内松的海螺似乎也携带着这种遗传记忆。1988年，在她完成博士学位的几年后，一次罕见的水华用石灰绿色的黏液覆盖了斯堪的纳维亚的海岸线，几乎杀死了所有海洋生物。“我所有的海螺都死了，”她回忆道。但她把这场悲剧变成了机遇。许多孤立的岩石岛屿变得空荡荡，于是她进行了一次自然实验，看看能否促使她的海螺转变生态型。

壳大而厚的海螺有盔甲保护，可抵御岸上的捕食性螃蟹；壳较小的海螺则更容易紧贴在波浪拍打的岩石上。在当年年仅3岁的女儿的帮助下，约翰内松收集了数百只大海螺，并将它们放在暴露于海水的空岩石上。随着岁月流逝，一代代海螺生息更替，整个种群变得体型更小，壳更薄。在不到30年的时间里，受波浪冲击的生态型在整个种群中得到了选择。

几年后，她揭示了使海螺快速适应成为可能的基因组特征。

染色体重排
一个给定的生态型可能需要数百个基因的表达。那么自然选择如何同时作用于所有这些基因呢？基因组研究在染色体结构中找到了解释。

在卵子和精子的形成过程中，基因可以通过一种称为“重组”的过程在染色体（高度紧凑的DNA包装体）之间进行洗牌。部分DNA可以被删除或插入。染色体可能断裂成两段，或融合成一条。整个DNA片段可以翻转，这被称为“倒位”。

倒位发生时，染色体上的一段DNA会断裂下来，旋转180度，再以相反的方向重新插入染色体。倒位之后，一组基因在一条染色体上以某种方向排列，在另一条染色体上则以相反方向排列。这有效地阻止了该区域再次发生重组，并将这组基因锁定在一起形成一个区块。如果这些基因以某种方式相关，就可能形成一个“超级基因”，即多个基因作为一个单元发挥作用。例如，海螺的厚壳和躲避螃蟹的回避行为特征就会因此关联起来，从而在后续世代中一起遗传。

“这就像你有一副扑克牌，”研究竹节虫物种形成和生态型的法国国家科学研究中心进化遗传学家帕特里克·诺西尔说，“在正常遗传中，你会完全洗牌——全部52张牌。而有了这些染色体倒位，牌组中有一部分拒绝洗牌，所以你永远无法改变里面牌的顺序。这部分正是控制使生态型不同的特征的区域。”

派歇尔认为这正是三刺鱼身上发生的情况。她说，尽管不同生态型的鱼仍然会交配（海洋三刺鱼会回到淡水入口繁殖），但过去的倒位确保了淡水基因保持在一起，海洋基因也同样如此。这使两种生态型区分开来，同时维持了物种范围的基因流动。

派歇尔的实验室正越来越接近证实这一假设。利用基因编辑技术CRISPR，她的实验室可以将一个倒位翻转回其原始方向。随着这些三刺鱼繁殖，该区域的基因将能再次洗牌，可能会破坏构成生态型的特征集合。派歇尔说：“这将是首批证据之一，证明倒位确实将区分生态型的适应性[基因]聚集并保持在一起。”

倒位并不总是有益的。它们伴随着风险，包括繁殖失败。例如，人类基因组中已识别出数千种倒位，其中一些可能导致妊娠失败。但当它们成功地将特征群保持在一起时，回报可能很高。事实上，约翰内松和斯坦科夫斯基的综述在生命树中发现了与生态型相关的倒位，包括植物、鸟类、鱼类、哺乳动物、海洋无脊椎动物和昆虫。

斯坦科夫斯基说：“没有人预料到[倒位]在生态型中会如此普遍。”“在进化遗传学中，这可能是过去二十年来最大的认识之一。”

而且，一些生态型包含许多染色体倒位。到2010年代中期，约翰内松和她的同事们已经在海螺基因组中识别出近20种与生态型相关的倒位。有趣的是，这些相同的特征群在不同种群中都能找到——在西班牙、英国以及瑞典——尽管这些种群彼此隔离且不杂交。进一步分析表明，其中一些倒位已有数百万年的历史，很可能发生在某个共同祖先身上。

约翰内松说：“这些倒位，作为DNA的大块片段，直到我们有了参考基因组和遗传图谱才真正显现出来。”“当这个[结果]出现时，我们仿佛看到了故事的结局。”

需要明确的是，并非所有染色体倒位都会形成生态型，生态型也未必由倒位形成。事实上，另一种保留基因区块的染色体重组过程可以在加利福尼亚干燥、灌木丛生的查帕拉尔山区找到，那里的克里斯蒂娜蒂马竹节虫（Timema cristinae）天生擅长伪装。

诺西尔说，它的一个生态型背部有条纹，以窄叶植物为食；另一个生态型是亮绿色且无条纹，以阔叶植物为食。每个生态型都能与它偏好的食物颜色融为一体，从而帮助其躲避捕食者。在这些竹节虫身上，与三刺鱼一样，染色体倒位使每个生态型的特征群在基因组上保持在一起。但诺西尔的研究发现，染色体的变化并不止于此。

易位是染色体DNA的另一种结构变化，可能在重组过程中偶然发生。基因从染色体的一端被重新定位到另一端——这一事件可能变得“非常混乱”，诺西尔说。基因组的某些部分可能完全消失，而其他基因则可能被插入到新的位置。

对于结构重排为何会形成生态型，还有另一种解释。染色体断裂很少是整齐的，有时可能产生有益的新特征。“于是那个特征就会被自然选择所青睐，”诺西尔说。他认为，结构变化不仅能阻止重组，还能创造新的功能性突变，这一观点几乎总是被忽视。但他对竹节虫的研究表明，这可能支持新生态型的进化。

他说：“在未来几年，试图理解基因组重排这两个方面的相对重要性，将成为研究的重点。”

生态型的起源
多年来，进化生物学家一直在争论生态型是否是新物种进化过程中的第一步。厚壳和薄壳海螺、海洋和淡水三刺鱼、有条纹和纯色的竹节虫，是否是物种正在走向分化的例子？

诺西尔说：“现在人们越来越认识到，生态型可能是形成新物种的一个步骤，但它们可能永远无法真正到达那一步。”

物种形成并不像系统发育树的分支那样清晰。进化生物学家越来越倾向于将其视为一个平滑延续的过程，而非具有离散步骤的进程。物种之间存在灰色区域，而生态型就位于其中。

例如，三刺鱼在海洋和淡水中的显著特征表明它们应该分化成两个不同的物种。然而，即使在数百万年后，它们也没有。相反，当环境条件从咸水变为淡水时，进化似乎会选择一种生态型或另一种，而不会丢弃另一套基因。

话虽如此，物种概念自达尔文时代起就备受争议。什么区分一个物种与另一个物种，或物种与亚种，或生态型与简单的变异，并没有具体标准。基因组学既澄清了问题，也使其复杂化。

斯坦科夫斯基说：“定义生态型的问题和定义物种的问题——实际上非常相似。”他补充说，“这确实归结为科学视角上的文化差异”，例如生物学家如何受训、他们提出什么问题，以及他们倾向于将物种归并还是拆分。

例如，许多分类学家认为加拉帕戈斯群岛的达尔文雀是不同的物种。然而，它们可以在几代内改变喙的形状以匹配可用的种子，并且它们之间都可以交配。按照斯坦科夫斯基和约翰内松的标准，达尔文雀“都将是单一物种的生态型”，他说。东非维多利亚湖的丽鱼科鱼也是如此，这是另一个快速进化辐射的著名例子。但这并不意味着我们需要重新思考《物种起源》。

斯坦科夫斯基说：“达尔文会对我们取得的进展感到震惊。”“生态型概念确实与他对物种的看法非常吻合。”

英文来源：

How Ecotypes Harbor the Genetic Memory of a Species’ Past
Introduction
When she was a graduate student in the 1970s, the evolutionary biologist Kerstin Johannesson regularly walked the shores of a Swedish archipelago, scanning the ground for pebbles that moved: marine snails. Her adviser, a taxonomist, had tasked her with describing the species present there by documenting their traits. She noticed that snails with thicker shells stayed on the shore, while those with thinner shells seemed to prefer wave-battered rocks, and in between the two habitats were snails with intermediate shell thickness. While they seemed like distinct species, Johannesson couldn’t help but wonder whether these snails might instead be different types of the same one.
Around 50 years earlier, the botanist Göte Turesson had had a similar revelation in a similar setting. As he walked Sweden’s shores, he noticed that saltbush plants from different stretches of coastline had distinct traits — earlier or later flowering times, or shorter or longer stalks — and between habitats, those traits fell somewhere in the middle. He bred the plants in his home garde and found that these distinct traits had a genetic basis even though they arose from the same species. In 1922, he published his results and coined the term “ecotype” to describe a subpopulation of a species adapted to a hyperlocal habitat.
At that time, the definition of a species was even less clear than it is today. Genes were still theoretical, and the structure of DNA wouldn’t be discovered for another 30 years. Turesson “struggled to be accepted,” said Johannesson, now the director of Tjärnö Marine Laboratory at the University of Gothenberg. How can a species contain multiple distinct phenotypes — or sets of traits — without separating into two species? “He had quite a job to try to convince his colleagues that there were inherited differences and local adaptation within species,” she said.
It wasn’t until the early 2000s, when whole-genome sequencing became accessible to evolutionary biologists, that Turesson’s ideas about ecotypes could be tested. By comparing the DNA sequences of ecotypes across the tree of life — from marine snails to stickleback fish to stick insects and more — scientists can study adaptation and speciation, the process by which new species form, at a molecular level.
“It’s by far the most exciting time to be a biologist, ever, in my opinion — maybe with the exception of going right back to Darwin,” said Sean Stankowski, an evolutionary geneticist at University College London. “Even when we understood that organisms were programmed by genetic code, we could really never access that. Now, we’re looking at every single [nucleotide molecule] — A, T, G, and C [adenine, thymine, guanine, and cytosine] — in the genome.”
An analysis of genomic ecotypes by Johannesson, Stankowski, and other researchers explains how some species can maintain the DNA sequences for multiple adaptations, allowing evolutionary processes to effectively select among ecotypes as environmental conditions change — sometimes within only a few generations. The data also suggests that some canonically diverse groups of species, including Darwin’s finches, may not be separate species at all, but rather different ecotypes of the same species.
Ecotypes represent a kind of “genetic memory,” Stankowski said, that reflects a species’ history of survival in different habitats. “Genomics has just taught us everything that’s going on under the hood,” he said. Once scientists looked there, they found an engine for adaptation.
Genetic Memory
In March 1964, the largest earthquake ever documented in North America uprooted the Gulf of Alaska. Within four minutes, some of the gulf’s islands were lifted 38 feet in the air, rivers were closed off from the ocean, and freshwater lakes were created. A decade later, scientists were surprised to find three-spined sticklebacks thriving there.
The fish species typically lives in the salty ocean. Biologists might have expected it to die out in a freshwater lake. Instead, something more interesting happened. The marine sticklebacks, which are armed against ocean predators with bony plates, started to look and behave like their freshwater cousins, which have fewer plates. This change unfolded within decades, too fast for a new species to have formed.
What could have driven these rapid phenotypic changes? A 50-year study of genomic data, published in 2015, revealed that, tucked into genomes across the population, marine sticklebacks contained the genes necessary to survive in freshwater environments. These alternate genes occur sparingly across roughly 100 regions of the genome, said Catherine Peichel, an evolutionary geneticist at the University of Bern who studies the fish species.
The presence of this kind of genetic diversity — having multiple forms of the same gene that harbor different traits — is known as standing variation. Even in low numbers, those alternate genes have a chance to be expressed, as if natural selection could reach into the past and redeploy those genes when needed. Research from a different lab has shown that transplanted marine sticklebacks can switch to the freshwater ecotype in as few as 20 to 30 years. The emergence of and selection for a novel trait, on the other hand, would likely take far longer than that — if the fish even survived the initial shock of navigating a totally different habitat.
“It’s almost like populations have a genetic memory of their time spent in different environments,” Stankowski said.
Back in Sweden, Johannesson’s marine snails seemed to harbor this genetic memory, too. In 1988, a few years after she completed her doctorate, a rare algal bloom blanketed Scandinavian coastlines with a lime-green slime that killed almost all marine life. “All my snails, they were gone,” she recalled. But she turned the tragedy into an opportunity. Many isolated rock islands were left vacant, so she ran a natural experiment to see if she could trigger her snails to switch ecotypes.
Large, thick-shelled snails had armor to protect them from predatory crabs onshore; smaller snails could more easily cling to wave-battered rocks. With the help of her then-3-year-old daughter, Johannesson collected hundreds of large snails and placed them on empty rocks exposed to the sea. As the years turned into decades, and as generations of snails came and went, the entire population became smaller, with thinner shells. In less than 30 years, the wave-battered ecotype had been selected for across the population.
A few years later, she would uncover the genomic features that made the snails’ rapid adaptation possible.
Shuffling Chromosomes
A given ecotype might require the expression of hundreds of genes. So how can selection act on all of them at once? Genomic studies have found explanations in chromosomal architecture.
During egg and sperm formation, genes can be shuffled between chromosomes (highly compact packages of DNA) in a process known as recombination. Some portions of DNA can be deleted or inserted. Chromosomes can break into two, or fuse into one. And entire segments of DNA can be flipped, in what is called an inversion.
An inversion happens when a portion of DNA from the chromosome breaks off, rotates 180 degrees, and plugs back into the chromosome in the reverse orientation. After inversion, a block of genes sits in one orientation on one chromosome, and in the opposite orientation on the other. This effectively prevents recombination from happening again in that region, and locks that group of genes together in a block. If those genes are somehow related, this can create a supergene, or multiple genes that act as a single unit. The snail traits for thick shells and evasive behavior to hide from crabs, for instance, become linked so that they will be inherited together in subsequent generations.
“It’s like if you had a deck of cards,” said Patrik Nosil, an evolutionary geneticist at the French National Center for Scientific Research who studies speciation and ecotypes of stick insects. “With normal genetics, you shuffle that deck completely — all 52 cards. Whereas with these chromosomal inversions, you have a part of the deck that refuses to shuffle, so you can never change the order of the cards in there. That’s the part that controls the traits that make the ecotypes different.”
This is what Peichel suspects happened in the sticklebacks. Even though they still mate across ecotypes (marine sticklebacks return to freshwater inlets to breed), past inversions ensure that the freshwater genes stay together and that the marine genes do too, she said. This differentiates the two ecotypes while maintaining species-wide gene flow.
Peichel’s lab is getting closer to confirming this hypothesis. Using the gene-editing technology CRISPR, her lab can flip an inversion back to its original orientation. As these sticklebacks reproduce, the genes in this region will be able to shuffle again, perhaps disrupting the suite of traits that form the ecotypes. “This would be some of the first proof of this idea that inversions actually bring together, and hold together, adaptive [genes] that distinguish ecotypes,” Peichel said.
Inversions aren’t always beneficial. They come with risks, including reproductive failure. For example, thousands of inversions have been identified in the human genome, and some can cause pregnancies to fail. But when they successfully hold groups of traits together, the rewards can be high. Indeed, Johannesson and Stankowski’s review surfaced inversions associated with ecotypes across the tree of life, including in plants, birds, fish, mammals, marine invertebrates, and insects.
“No one predicted that [inversions] would be as abundant as they are” in ecotypes, Stankowski said. “In evolutionary genetics, it’s probably one of the biggest realizations of the last two decades.”
And some ecotypes contain many chromosomal inversions. By the mid-2010s, Johannesson and her colleagues had identified nearly 20 different ecotype-related inversions in the marine snail genome. Interestingly, these same groups of traits are found across populations — in Spain and the United Kingdom as well as Sweden — even though these populations are isolated and don’t interbreed. Further analysis showed that some of these inversions are millions of years old, likely having occurred in a common ancestor.
“These inversions, which are big chunks of the DNA, were not really obvious until we had a reference genome and a genetic map,” Johannesson said. “When this [result] came, it was like we got to the end of the story.”
To be clear, not all chromosomal inversions form ecotypes, and ecotypes aren’t necessarily formed by inversions. Indeed, another chromosomal recombination process that preserves blocks of genes can be found in California’s dry, shrubby chaparral mountains, where the walking stick insect Cristina’s timema (Timema cristinae) is made for camouflage.
One of its ecotypes has a stripe down its back and feeds on plants with narrow leaves; the other is bright green and stripeless, and feeds on broad leaves. Each ecotype blends in with its preferred food, which helps it avoid predators, Nosil said. In these stick insects, as in the sticklebacks, a chromosomal inversion keeps each ecotype’s group of traits together on the genome. But Nosil’s research has found that the chromosomal changes don’t stop there.
Translocation is another structural change to chromosomal DNA that can happen, by chance, during recombination. Genes from one end of a chromosome are relocated to the other end — an event that can get “really messy,” Nosil said. Some parts of the genome can disappear entirely, while other genes can be inserted into a new place.
There’s another explanation for why structural rearrangements may form ecotypes. Chromosome breaks are rarely tidy, and sometimes they can lead to beneficial new traits. “Then that trait is favored by natural selection,” Nosil said. This idea that structural changes can create new functional mutations and not just prevent recombination is almost always overlooked, he said. But his research into stick insects suggests that it can support the evolution of new ecotypes.
“Over the next years, that’ll become a big point of research,” he said, “to try to understand the relative importance of those two aspects of genomic rearranging.”
On the Origin of Ecotypes
Over the years, evolutionary biologists have debated whether ecotypes are a first step in the evolution of new species. Are thick- and thin-shelled snails, marine and freshwater sticklebacks, or striped and solid stick insects examples of species on their way to splitting into two?
“There’s been growing appreciation now that ecotypes might be a step in the direction of forming new species, but they might never actually get there,” Nosil said.
Speciation isn’t as clean-cut as the branches of a phylogenetic tree would have us believe. Evolutionary biologists increasingly view it as smoothly scrolling across a continuum, rather than a process with discrete steps. There is a gray area between species, and that’s where ecotypes sit.
The distinct marine and freshwater traits in sticklebacks, for example, suggest that they should diverge into two different species. Yet even after millions of years, they haven’t. Instead, when environmental conditions change from salt water to fresh water, evolution seems to select for one ecotype or the other without discarding the alternate set of genes.
Then again, the species concept has been controversial since the days of Darwin. What distinguishes one species from another, or a species from a subspecies, or an ecotype from simple variation, isn’t concrete. Genomics has both clarified and complicated the question.
“The problem with defining ecotypes and the issue of defining species — they’re actually quite analogous,” Stankowski said. “It really does boil down to cultural differences” in scientific perspectives, he added, such as how biologists are trained, what questions they ask, and whether they’re inclined to lump species together or split them up.
For instance, many taxonomists consider Darwin’s finches in the Galápagos to be distinct species. However, they can change their beak shape to match the available seeds within generations, and they can all mate with one another. By Stankowski and Johannesson’s criteria, Darwin’s finches “would all be ecotypes of a single species,” he said. The same goes for cichlid fish in east Africa’s Lake Victoria, another famous example of rapid evolutionary radiation. But that doesn’t mean we need to rethink On the Origin of Species.
“Darwin would be blown away at the progress that we’ve made,” Stankowski said. “The ecotype concept really does match quite closely with his view on species.”