The macroevolutionary consequences of trait correlations

地球上生命的多样性是巨大的。几百万年来，进化产生了种类繁多的物种。然而，当我们更仔细地观察时，我们开始注意到一些令人惊讶的模式。有些种群的物种比其他种群多得多，例如，甲虫的种类超过35万种，但大象的种类很少。其他类群表现出不同的形状和大小，例如夏威夷蜜生爬行动物，而其他类群则相当一致，例如小鼠和大鼠。这些模式表明，物种的数量及其形式和功能的多样性是有限制的，即生物多样性并不均匀地分布在生命之树上。为什么会这样仍然没有得到解决，这是进化生物学中的一个基本问题。这里，我们重点解释这种多样性的不均匀分布：物种特征之间的相关性。物种特征几乎可以是身体外观(如身体大小或眼睛颜色)、行为或生活史(大小与后代数量)的任何方面。这些特征并不总是相互独立的，例如，投资更多的后代往往会导致更小的后代。在这种情况下，我们说所涉及的特征是相关的。在性状相关的地方，一种性状的进化可能促进或阻碍另一种性状的进化。不同物种群体间性状相关强度的差异可能决定了相关物种群体之间的差异。在性状之间相关性很强的地方，这可能会阻止非常不同的形状和大小的进化。因此，形态和功能的多样性可能较低，近缘物种的数量应该很少。经过数百万年的时间，这些特征的相关性可能会变得更弱，甚至消失，从而允许物种以新的方式进化或在数量上繁殖。这些观点表明，特质相关性对于我们理解生命的多样性很重要。我们的建议旨在了解特征相关性如何影响决定整个生命树的多样性模式(物种数量以及形式和功能)的过程。我们专注于与鸟类的食物类型和获取相关的特征。鸟喙的大小和形状可以反映饮食需求和觅食方式。当性状相关时，一些组合可能永远不会出现在一起，例如，可能不可能进化出同时像乌贼一样又长又弯，像鸭子一样宽和深的喙。为了测试特征相关性如何影响决定生命之树多样性模式的过程，我们需要测量来自许多不同物种的许多个体。在战场上做到这一点几乎是一个不可能的挑战。相反，我们使用的是博物馆的标本。世界各地的博物馆收藏了数十亿件标本，通常来自100多年前的历史收藏品。伦敦自然历史博物馆拥有约8000万件标本。这些收集是关于几乎所有鸟类种类(超过10,000种)的丰富信息来源。我们将从这些收集中产生数据，包括对鸟喙的3D扫描。然后，我们将使用这些数据，以及最先进的分析方法，来评估物种之间和物种内的特征相关性，以及更高的分类类群之间的相关性，如科和目。通过使用博物馆标本和对特征相关性重要性的新观点，我们的研究将为生命如何以及为什么多样化提供新的见解。这项研究不仅将解决进化生物学中的一场根本性辩论，还将为未来的研究人员创建有价值的鸟类数据集，并突出自然历史收集对尖端研究的重要性。

The diversity of Life on Earth is immense. Over millions of years, evolution has generated a staggering variety of species. When we look more closely, however, we begin to notice some surprising patterns. Some groups have far more species than others, for example there are over 350,000 species of beetle but only a few species of elephant. Other groups exhibit diverse shapes and sizes, for example the Hawaiian honeycreepers, whereas others are fairly uniform, for example mice and rats. These patterns suggest that there are constraints, or limitations, on the numbers of species and their variety of form and function, i.e. biological diversity is not evenly distributed across the tree of life. Why this is so remains unresolved and is a fundamental question in evolutionary biology.Here we focus on a previously unexplored explanation for this uneven distribution of diversity: correlations among species traits. Species traits can be almost any aspect of physical appearance (such as body size or eye colour), behaviour, or life history (size vs. number of offspring). These traits are not always independent of one another, for example investing in more offspring often results in smaller offspring. In such cases we say that the traits involved are correlated. Where traits are correlated, evolution in one trait may promote or prevent evolution in another. Differences in the strength of trait correlations in different groups of species might determine variation between and among groups of related species. Where correlations among traits are strong these may prevent evolution of very different shapes and sizes. Therefore the diversity of form and function might be low, and the number of closely-related species should be few. Over millions of years these trait correlations might become weaker or even disappear, allowing species to evolve in new ways or multiply in number. These ideas suggest that trait correlations are important in our understanding of the diversity of life. Our proposal aims to understand how trait correlations can affect the processes that determine patterns of diversity (both in species number and in form and function) across the tree of life. We focus on traits related to food type and acquisition in birds. The size and shape of bird beaks can reflect dietary requirements and foraging methods. When traits are correlated, some combinations may never occur together, for example, it may not be possible to evolve beaks that are simultaneously long and curved like a curlew, and wide and deep like a duck. To test how trait correlations can affect the processes that determine patterns of diversity across the tree of life we need to measure many individuals from many different species. It would be a near-impossible challenge to do this in the field. Instead we use specimens from museums. Around the world museums house billions of specimens, often from historical collections over 100 years old. The Natural History Museum, London has around 80 million specimens. These collections are a rich source of information on nearly all species of birds (over 10,000 species). We will generate data from these collections, including 3D scans of bird beaks. We will then use these data, along with state-of-the-art analysis methods, to evaluate trait correlations among and within species, and among higher taxonomic groupings such as families and orders. By using museum specimens and a novel view of the importance of trait correlations, our research will provide new insights into how and why life diversifies. Not only will this research address a fundamental debate in evolutionary biology, but it will also create valuable bird datasets for future researchers, and highlight the importance of natural history collections for cutting edge research.