Chapter 14: Reconstructing Evolutionary TreesOverview |
The evolutionary history of a group is called its phylogeny. A phylogenetic tree is a graphical summary of this history. The tree describes the pattern, and in some cases the timing, of events that occurred as species diversified. It records the sequence that lineages appeared and documents which taxa are more closely or distantly related. In Chapter 1 we introduced how phylogenies are used by asking whether HIV was transmitted from humans to chimps or from chimps to humans. In Chapter 2 we introduced “tree thinking” more formally and used a phylogeny to explore how leopards got their spots. But in these and other instances, we have been analyzing trees without asking how they were put together.
In Chapter 14 of your textbook you will begin understand how phylogenies are estimated, so we can interpret them critically and make independent judgments about their quality.
14.1 The Logic of Phylogeny Inference At its most basic level, the logic of estimating evolutionary relationships is simple: The most closely related taxa should have the most traits in common. Naively, we would say that any traits that are heritable and that vary among the taxa involved can help us reconstruct who evolved from whom. Many types of characters could qualify: the sequence of nucleotides in a particular gene, the presence or absence of specific skeletal elements or flower parts, or the mode of embryonic or larval development.
Turn to section 14.1 of your textbook to explore the ways species are grouped and how that information can help us understand evolutionary relationships.
14.2 The Phylogeny of Whales The whales, dolphins, and porpoises share an array of features that are unusual for mammals, the most spectacular of which is a lack of posterior limbs. Whales are so highly adapted to aquatic life, in fact, that it is difficult to figure out which mammal group is their closest living relative. The oldest fossils that can be recognized as whales come from rocks in the Himalayas that are about 53.5 million years old. These whales had hind limbs and resembled an extinct group of amphibious mammals called the mesonychians (Thewissen and Hussain 1993; Thewissen et al. 1994; Bajpai and Gingerich 1998).
Refer to section 14.2 of your textbook to learn more about how the fossil record supports the hypothesis that whales evolved from ancestors that had hind limbs
14.3 Using Phylogenies to Answer Questions The first two sections of this chapter have focused on methods. Their goal was to show that estimating evolutionary relationships requires a series of careful decisions about which data are appropriate for the task and how they should be analyzed.
Turn to section 14.3 of your textbook to explore how evolutionary trees can be used to answer interesting questions.