CHAPTER 23 INTRODUCTION

CORE CONCEPTS

23.1 A phylogenetic tree is a reasoned hypothesis of the evolutionary relationships of organisms.

23.2 A phylogenetic tree is built on the basis of shared derived characters.

23.3 The fossil record provides a direct glimpse of evolutionary history.

23.4 Phylogeny and fossils provide independent and corroborating evidence of evolution.

Nature displays, all around us, nested patterns of similarity among species. For example, as noted in Chapter 1, humans are more similar to chimpanzees than either humans or chimpanzees are to monkeys. Humans, chimpanzees, and monkeys, in turn, are more similar to one another than any of them is to a mouse. And humans, chimpanzees, monkeys, and mice are more similar to one another than any of them is to a catfish. This pattern of nested similarity was recognized more than 200 years ago and used by the great Swedish naturalist Carolus Linnaeus to classify biological diversity. A century later, Charles Darwin recognized this pattern as the expected outcome of a process of “descent with modification,” or evolution.

Evolution produces two distinct but related patterns, both evident in nature. First is the nested pattern of similarities found among species on present-day Earth. The second is the historical pattern of evolution recorded by fossils. Life, in its simplest form, originated more than 3.5 billion years ago. Today, an estimated 10 million species inhabit the planet. Short of inventing a time machine, how can we reconstruct those 3.5 billion years of evolutionary history in order to understand the extraordinary events that have ultimately resulted in the biological diversity we see around us today? These two great patterns provide the answer.

Darwin recognized that the species he observed were the modified descendants of earlier ones. Distinct populations of an ancestral species separate and diverge through time, again and again, giving rise to multiple descendant species. The result is the pattern of nested similarities observed in nature (see Fig. 1.17). Just as a tree sprouts new branches on old ones and adds rings to a thickening trunk, the pattern of nested similarities among species strongly indicates a process of descent with modification and the accumulation of change. This history of descent with branching is called phylogeny, and is much like the genealogy that records our own family histories.

The evolutionary changes inferred from the patterns of relatedness among present-day species make predictions about the historical pattern of evolution we should see in the fossil record. For example, groups with features that we infer to have evolved earlier than others should appear earlier in time as fossils. Paleontological research reveals that the history of life is indeed laid out in the chronological order predicted on the basis of comparative biology. How do we reconstruct life’s two great evolutionary patterns, and how do they compare?