Size change and the development of mammalian body form: a morphometric and ancient DNA study of island dwarfing

不同物种的动物在形态上的变异，必然是由有机体从卵到成虫的生长差异引起的。既然在更长的时间尺度上，动物物种的多样性是由进化过程产生的，那么，一种新的身体形式的进化必然伴随着个体生长模式的变化。在这个项目中，我们将研究进化的自然实验：岛屿上侏儒哺乳动物的起源。这是冰河时代（地球历史的最后200万年左右）的一种常见现象，当时世界各地许多岛屿上孤立的种群中出现了大象、鹿、河马和其他哺乳动物的侏儒物种。它们是大陆祖先的微缩复制品吗？还是它们的形状不同？如果是的话，是如何以及为什么？在马达加斯加岛上发现了一批令人瞩目的古代动物遗骸。其中包括100多个保存完好的侏儒河马头骨，它们是从大型非洲大陆物种进化而来的，这些物种在岛上已经被隔离。我们将参观博物馆，对这些头骨进行大量测量，以及对两种现存非洲河马的男性和女性头骨进行样本。这些头骨代表了从新生到成年的动物，通过计算机分析测量数据，我们将量化大陆和岛屿形式之间的大小和形式差异，并了解这些差异是如何在个体动物的生长过程中产生的。通过这种方式，我们将了解进化如何修改发育过程以创造新物种。一个特别的想法是，特别是对于快速进化的变化，生长变化是祖先形式的男性和女性之间发展差异的延伸。最近的技术进步意味着从化石骨骼中提取遗传物质（DNA）是可能的。我们将从多达50个马达加斯加侏儒河马的头骨上取下少量的骨头或牙齿样本，并将它们提交给一个生化程序，提取微量的DNA，并将分子片段复制成数百万个拷贝，以便确定DNA序列。将一个基因与两个现存物种的基因进行比较，通过确定它们之间的差异，可以确定它们在进化中分离的时间长度，从而确定矮化过程的持续时间。根据头骨解剖学，也应该有可能证实马达加斯加岛上有不止一种矮河马的建议。对骨头进行放射性碳年代测定将有助于这些推断。另一个基因，存在于性染色体上，可以用来确定个体头骨的性别，这很重要，因为在化石中，性别差异经常与物种之间的差异混淆。我们还将简要地看看其他岛屿矮化的例子，看看是否出现类似的模式。其中包括一只矮象和其他种类的矮河马，来自地中海的西西里。其中一些比马达加斯加形态进化的时间更长，所以看看它们的生长是否以不同的方式被修改是很有趣的。总的来说，这项研究将探索如何将普遍存在的尺寸变化现象与生物世界的多样性生产密切联系起来。

Variation in the form of animals of different species must, of necessity come about by differences in the growth of the organism from egg to adult. Since, on a much longer time-scale, the diversity of animal species arose by a process of evolution, it follows that the evolution of a new body form must have entailed changes in the pattern of individual growth. In this project, we will examine a natural experiment in evolution: the origin of dwarfed mammals on islands. This was a common phenomenon during the Ice Ages (the last two million years or so of Earth history), when dwarfed species of elephants, deer, hippopotami, and other mammals arose from isolated populations on many islands around the world. Are they miniature replicas of their full-sized continental ancestors, or do they differ in shape, and if so, how and why? A remarkable collection of ancient animal remains was discovered on the island of Madagascar. These include a hundred or more beautifully preserved skulls of dwarf hippopotamus, which evolved from animals of the large, mainland African species that had become isolated on the island. We will visit museums to take numerous measurements of these skulls, as well as a sample of male and female skulls of both species of living African hippopotamus. The skulls represent animals from new-born to adult, and by computer-based analysis of the measurement data, we will quantify the differences of size and form between the mainland and island forms, and learn how those differences came about in the growth of the individual animal. In this way we will understand how evolution modifies developmental processes to create new species. One particular idea is that, especially for rapid evolutionary changes, the growth changes are an extension of the differences in development between males and females of the ancestral form. Recent technological advances now mean that it is possible to extract genetic material (DNA) from fossil bones. We will take small samples of bone or tooth from up to 50 skulls of Madagascan dwarf hippo, and submit them to a biochemical procedure that extracts minute quantities of DNA and multiplies the molecular fragments up into millions of copies so that the DNA sequence can be determined. One gene will be compared to those of the two living species, and by determining how different they are, the length of time since they separated in evolution, and hence the duration of the dwarfing process, can be determined. It should also be possible to confirm the suggestions based on skull anatomy, that there was more than one species of dwarf hippo on Madagascar. Radiocarbon dating of the bones will help with these deductions. Another gene, present on the sex chromosomes, can be used to determine the sex of the individual skulls, of importance since in fossils, sexual differences are often confused with differences between species. We will also look briefly at other examples of island dwarfing, to see if similar patterns emerge. These include a dwarf elephant, and other species of dwarf hippo, from Sicily in the Mediterranean. Some of these had longer to evolve than the Madagascan form, so it will be interesting to see if their growth has been modified in different ways. Overall, the research will explore the ways in which the widespread phenomenon of size change is intimately linked to the production of diversity in the living world.