© Copyright, Princeton University Press. No part of this book may be distributed, posted, or reproduced in any form by digital or mechanical means without prior written permission of the publisher. I.1 What Is Evolution? Jonathan Losos OUTLINE Natural Selection. The process in which individuals with a particular trait tend to leave more offspring in the 1. What is evolution? next generation than do individuals with a different 2. Evolution: Pattern versus process trait. 3. Evolution: More than changes in the gene pool Approximately 375 million years ago, a large and vague- 4. In the light of evolution ly salamander-like creature plodded from its aquatic 5. Critiques and the evidence for evolution home and began the vertebrate invasion of land, setting 6. The pace of evolution forth the chain of evolutionary events that led to the birds 7. Evolution, humans, and society that fill our skies, the beasts that walk our soil, me writing this chapter, and you reading it. This was, of course, just Evolution refers to change through time as species be- one episode in life’s saga: millions of years earlier, plants come modified and diverge to produce multiple descen- had come ashore, followed soon thereafter—or perhaps dant species. Evolution and natural selection are often simultaneously—by arthropods. We could go back much conflated, but evolution is the historical occurrence of earlier, 4 billion years or so, to that fateful day when the change, and natural selection is one mechanism—in most first molecule replicated itself, an important milestone in cases the most important—that can cause it. Recent the origin of life and the beginning of the evolutionary years have seen a flowering in the field of evolutionary pageant. Moving forward, the last few hundred million biology, and much has been learned about the causes and years have also had their highs and lows: the origins of consequences of evolution. The two main pillars of our frogs and trees, the end-Permian extinction when 90 knowledge of evolution come from knowledge of the percent of all species perished, and the rise and fall of the historical record of evolutionary change, deduced directly dinosaurs. from the fossil record and inferred from examination of These vignettes are a few of many waypoints in the phylogeny, and from study of the process of evolutionary evolutionary chronicle of life on earth. Evolutionary change, particularly the effect of natural selection. It is biologists try to understand this history, explaining how now apparent that when selection is strong, evolution can and why life has taken its particular path. But the study proceed considerably more rapidly than was generally of evolution involves more than looking backward to try envisioned by Darwin. As a result, scientists are realizing to understand the past. Evolution is an ongoing process, that it is possible to conduct evolutionary experiments in one possibly operating at a faster rate now than in times real time. Recent developments in many areas, including past in this human-dominated world. Consequently, molecular and developmental biology, have greatly ex- evolutionary biology is also forward looking: it includes panded our knowledge and reaffirmed evolution’s central the study of evolutionary processes in action today— place in the understanding of biological diversity. how they operate, what they produce—as well as in- vestigation of how evolution is likely to proceed in the GLOSSARY future. Moreover, evolutionary biology is not solely an Evolution. Descent with modification; transformation academic matter; evolution affects humans in many of species through time, including both changes that ways, from coping with the emergence of agricultural occur within species, as well as the origin of new pests and disease-causing organisms to understanding species. the workings of our own genome. Indeed, evolutionary © Copyright, Princeton University Press. No part of this book may be distributed, posted, or reproduced in any form by digital or mechanical means without prior written permission of the publisher. 4 Introduction science has broad relevance, playing an important role in per breeding event. The logic behind natural selection is advances in many areas, from computer programming unassailable. If some trait variant is causally related to to medicine to engineering. greater reproductive success, then more members of the population will have that variant in the next generation; continued over many generations, such selection can 1. WHAT IS EVOLUTION? greatly change the constitution of a population. Look up the word “evolution” in the online version of the But there is a catch. Natural selection can occur with- Oxford English Dictionary, and you will find 11 defini- out leading to evolution if differences among individuals tions and numerous subdefinitions, ranging from math- are not genetically based. For natural selection to cause ematical (“the successive transformation of a curve by evolutionary change, trait variants must be transmitted the alteration of the conditions which define it”) to chem- from parent to offspring; if that is the case, then offspring ical (“the emission or release of gas, heat, light, etc.”) to will resemble their parents and the trait variants possessed military (“a manoeuvre executed by troops or ships to by the parents that produce the most offspring will in- adopt a different tactical formation”). Even with ref- crease in frequency in the next generation. erence to biology, there are several definitions, including However, offspring do not always resemble their “emergence or release from an envelope or enclosing parents. In some cases, individuals vary phenotypically structure; (also) protrusion, evagination,” not to men- not because they are different genetically, but because tion “rare” and “historical” usage related to the concept they experienced different environments during growth of preformation of embryos. Even among evolutionary (this is the “nurture” part of the nature versus nurture biologists, evolution is defined in different ways. For debate; see chapters III.10 and VII.1). If, in fact, varia- example, one widely read textbook refers to evolution as tion in a population is not genetically based, then se- “changes in the properties of groups of organisms over lection will have no evolutionary consequence; in- the course of generations” (Futuyma 2005), whereas dividuals surviving and producing many offspring will another defines it as “changes in allele frequencies over not differ genetically from those that fail to prosper, and time” (Freeman and Herron 2007). as a result, the gene pool of the population will not One might think that—as in so many other areas of change. Nonetheless, much of the phenotypic variation evolutionary biology—we could look to Darwin for within a population is, in fact, genetically based; con- clarity. But in the first edition of On the Origin of Spe- sequently, natural selection often does lead to evolu- cies, the term “evolution” never appears (though the last tionary change. word of the book is “evolved”); not until the sixth edi- But that does not mean that the occurrence of evo- tion does Darwin use “evolution.” Rather, Darwin’s lutionary change necessarily implies the action of nat- term of choice is “descent with modification,” a simple ural selection. Other processes—especially mutation, phrase that captures the essence of what evolutionary genetic drift, and immigration of individuals with dif- biology is all about: the study of the transformation of ferent genetic constitutions—also can cause a change in species through time, including both changes that occur the genetic makeup of a population from one generation within species, as well as the origin of new species. to the next (see Section IV: Evolutionary Processes). In other words, natural selection can cause adaptive evo- lutionary change, but not all evolution is adaptive. 2. EVOLUTION: PATTERN VERSUS PROCESS These caveats notwithstanding, 150 years of research Many people—sometimes even biologists—equate evo- have made clear that natural selection is a powerful force lution with natural selection, but the two are not the responsible for much of the significant evolutionary same. Natural selection is one process that can cause change that has occurred over the history of life. As the evolutionary change, but natural selection can occur chapters in Section II: Phylogenetics and the History of without producing evolutionary change. Conversely, Life, and Section III: Natural Selection and Adaptation, processes other than natural selection can lead to demonstrate, natural selection can operate in many evolution. ways, and scientists have correspondingly devised many Natural selection within populations refers to the sit- methods to detect it, both through studies of the phe- uation in which individuals with one variant of a trait notype and of DNA itself (see also chapter V.14). (say, blue eyes) tend to leave more offspring that are healthy and fertile in the next generation than do in- 3. EVOLUTION: MORE THAN CHANGES IN THE dividuals with an alternative variant of the trait. Such GENE POOL selection can occur in many ways, for example, if the variant leads togreater longevity,greaterattractiveness to During the heyday of population genetics in the middle members of the other sex, or greater number of offspring decades of the last century, many biologists equated © Copyright, Princeton University Press. 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