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Author Biography: Ernst Mayr is Professor Emeritus at the Zoology Department of Harvard University, and for twenty-three years served as Curator at the American Museum of Natural History in New York. He lives in the Boston area.
IN WHAT KIND OF A WORLD DO WE LIVE?
Mankind apparently has always had an urge to explain and understand that which is unknown or puzzling. The folklore of even the most primitive human tribes indicates that they had given some thought to questions about the origin and history of the world. They had thought about such questions as: Who or what gave rise to the world? What will the future bring? How did we humans originate? Numerous answers to these questions were given in tribal myths. Most often the existence of the world was simply taken for granted, as was the belief that it had always been as it is now, but there were innumerable stories about the origin or creation of man.
Later on the founders of religions, as well as the philosophers, also tried to find answers to these questions. When one studies these answers, one can sort them into three classes: (1) a world of infinite duration, (2) a constant world of short duration, and (3) an evolving world.
(1) A world of infinite duration
The Greek philosopher Aristotle believed that the world had always been in existence. Some philosophers thought that this eternal world had never changed, that it was constant; others thought that it was going through different stages ("cycling") but would ultimately always return to an earlier stage. However, such a belief in an infinite age of the world was never very popular. There seems to have been an urge to account for a beginning.
(2) A constant world of short duration
Thiswas, of course, the Christian view, as presented in the Bible. It was the prevailing view of the Western world in the Middle Ages and up to the middle of the nineteenth century. It was based on a belief in a supreme being, an all-powerful God, who had created the entire world as well as the human species, as described in the two stories of creation in the Bible (Genesis).
The belief that the world was created by an Almighty God is called creationism. Most of those who hold this belief also believe that God designed his creation so wisely that all animals and plants are perfectly adapted to each other and to their environment. Everything in the world today is still as it was when it was created. This was an entirely logical conclusion based on the known facts at the time the Bible was written. Some theologians, on the basis of the biblical genealogy, calculated that the world was quite recent, having been created in 4004 B.C., that is, about 6,000 years ago.
The beliefs of creationism are in conflict with the findings of science, and this has resulted in a controversy between creationists and evolutionists. This book is not the place to settle their arguments and we refer to the extensive literature on this subject listed in Box 1.1 and the bibliography. For the source of the creation stories in Genesis, see Moore (2001).
More or less similar creation stories are found in the folklore of peoples all over the world. They filled a gap in mankind's desire to answer the profound questions about this world that we humans have asked ever since there has been human culture. We still treasure these stories as part of our cultural heritage, but we turn to science when we want to learn the real truth about the history of the world.
THE RISE OF EVOLUTIONISM
Beginning with the Scientific Revolution in the seventeenth century, more and more scientific observations were in conflict with the biblical story. Its credibility was gradually being weakened by a series of discoveries. The Copernican Revolution was the first development to demonstrate that not every statement in the Bible could be interpreted literally. The newly developing science was at first primarily concerned with astronomy, that is, with the sun, the stars, the planets, and other physical phenomena. It was inevitable that in due time the early practitioners of science would feel compelled to find explanations for many other phenomena in the world.
Discoveries in other sciences also raised new puzzling questions. The research of geologists in the seventeenth and eighteenth centuries revealed the immense age of the Earth, while the discovery of extinct fossil faunas undermined the belief in the constancy and permanence of the Creation. Even though more and more evidence contradicted the assumption of the constancy of the world and its short duration, even though more and more voices were heard among scientists and philosophers questioning the validity of the biblical story, and even though the naturalist Jean-Baptiste de Lamarck had proposed in 1809 a full-fledged evolutionary theory, a more or less biblical worldview prevailed up to 1859, not only among laypeople but also among natural scientists and philosophers. It provided a simple answer to all questions about the world: God had created it and he had designed his created world so wisely that every organism was perfectly adapted to its place in nature.
During this transitional period of conflicting evidence, all sorts of compromises were attempted to cope with these contradictions. One such attempt was the so-called scala naturae, the Great Chain of Being (Fig. 1.1), in which all entities in this world were arranged in an ascending ladder, beginning with such inanimate objects as rocks and minerals, rising through lichens, mosses, and plants, and through corals and other lower animals to higher animals, and from them through mammals up to primates and man. This scala naturae was held to be never changing (constant) and simply to reflect the mind of the creator who had ordered everything in a sequence that led toward perfection (Lovejoy 1936).
Eventually, the evidence for the conclusion that the world is not constant but is forever changing became so overwhelming that it could no longer be denied. The result was the proposal and eventual adoption of a third worldview.
(3) An evolving world
According to this third view, the world is of long duration and is forever changing; it is evolving. Even though this may seem strange to us moderns, the concept of evolution was at first alien to Western thought. The power of the Christian fundamentalist dogma was so strong that it required a long series of developments in the seventeenth and eighteenth centuries before the idea of evolution became fully acceptable. As far as science is concerned, the acceptance of evolution meant that the world could no longer be considered merely as the seat of activity of physical laws but had to incorporate history and, more importantly, the observed changes in the living world in the course of time. Gradually the term "evolution" came to represent these changes.
What Kinds of Change?
Everything on this Earth seems to be in a continuous flux. There are highly regular changes. The change from day to night and back again, caused by the rotation of the Earth, is such a regular cyclical change. So are the changes of the sea level in the tides, caused by the lunar cycle. Even more pervasive are the seasonal changes due to the annual circling of the Earth around the sun. Other changes are irregular, such as the movements of the tectonic plates, the severity of the winter from year to year, or aperiodic climatic changes (El Niño, ice ages), as well as periods of prosperity in a given nation's economy. Irregular changes are largely unpredictable, being subject to various stochastic processes.
There is, however, one particular kind of change that seems to keep going continuously and to have a directional component. This change is referred to as evolution. The first widespread feeling that the world was not static as implied by the story of Creation, but rather was evolving, can be traced to the eighteenth century. Eventually it was realized that the static scala naturae could be converted into a kind of biological escalator, leading from the lowest organisms to ever higher ones and finally to man. Just as gradual change in the development of an individual organism leads from the fertilized egg to the fully adult individual, so it was thought that the organic world as a whole moved from the simplest organisms to ever more complex ones, culminating in man. The first author to articulate this idea in detail was the French naturalist Lamarck. One even took the word evolution, which originally had been applied by Charles Bonnet to the development of the egg, and transferred it to the development of the world of life. Evolution, one said, consists of a change from the simple to the complex and from the lower to the higher. Evolution, indeed, was change, but it seemed to be a directional change, a change toward ever greater perfection, as it was said at that time, not a cyclical change like the seasons of the year or an irregular change like the ice ages or the weather.
But what is it that is actually involved in this continuing change of the organic world? This question was at first quite controversial, even though Darwin already knew the answer. Finally, during the evolutionary synthesis (see below), a consensus emerged: "Evolution is change in the properties of populations of organisms over time." In other words, the population is the so-called unit of evolution. Genes, individuals, and species also play a role, but it is the change in populations that characterizes organic evolution.
It is sometimes claimed that evolution, by producing order, is in conflict with the "law of entropy" of physics, according to which evolutionary change should produce an increase of disorder. Actually there is no conflict, because the law of entropy is valid only for closed systems, whereas the evolution of a species of organisms takes place in an open system in which organisms can reduce entropy at the expense of the environment and the sun supplies a continuing input of energy.
Evolutionary thinking spread throughout the second half of the eighteenth and the first half of the nineteenth century, not only in biology but in linguistics, philosophy, sociology, economics, and other branches of thought. Yet, on the whole, in science it remained for a long time a minority view. The actual shift from the belief in a static worldview to evolutionism was caused by the dramatic event of the publication of Charles Darwin's On the Origin of Species on the 24th of November in 1859.
DARWIN AND DARWINISM
This event represents perhaps the greatest intellectual revolution experienced by mankind. It challenged not only the belief in the constancy (and recency) of the world, but also the cause of the remarkable adaptation of organisms and, most shockingly, the uniqueness of man in the living world. But Darwin did far more than postulate evolution (and present overwhelming evidence for its occurrence); he also proposed an explanation for evolution that did not rely on any supernatural powers or forces. He explained evolution naturally, that is, by using phenomena and processes that everybody could daily observe in nature. In fact, in addition to the theory of evolution as such, Darwin proposed four theories about the how and why of evolution. No wonder the Origin caused such turmoil. It almost single-handedly effected the secularization of science.
Charles Darwin was born on February 12, 1809, the second son of a physician in a small English country town (Fig. 1.2). From his boyhood on, he was an ardent naturalist, particularly passionate about beetles. At his father's wish, he studied medicine in Edinburgh for a while, but was so appalled, particularly by the operations, that he soon gave it up. The family then decided he should study for the ministry, and this seemed a perfectly natural education for a young naturalist, for nearly all leading naturalists of his time were ordained ministers. Although Darwin conscientiously did all the required reading in the classics and in theology, it was really natural history that he pursued with single-minded devotion. After obtaining his degree at Cambridge University (Christ College), he received through one of his teachers at Cambridge the invitation to join one of the Navy's survey ships, HMS Beagle, for a survey of the coasts of South America, particularly the harbors. The Beagle left England at the end of December 1831. On the five-year cruise of the Beagle, Darwin shared a cabin with the commander, Captain Robert Fitzroy. While the ship surveyed the coast of Patagonia in the east, the Strait of Magellan, and parts of the western coast and adjacent islands, Darwin had abundant opportunity to explore the mainland and the biota of the islands. Throughout the trip, he not only made significant collections of natural history specimens, but more importantly he asked endless questions about the history of the land and its fauna and flora. This was the foundation on which his evolutionary ideas grew.
After his return to England in October 1836, he devoted his time to the scientific study of his collections and to the publication of scientific reports, at first on some of his geological observations. After a few years, he married his cousin Emma, the daughter of the famous potter Wedgwood, bought a house near London (Down House), and lived there until his death on April 19, 1882, at the age of 73. It was at Down House that he wrote all of his major papers and books.
What made Darwin such a great scientist and intellectual innovator? He was a superb observer, endowed with an insatiable curiosity. He never took anything for granted but always asked why and how. Why is the fauna of islands so different from that of the nearest mainland? How do species originate? Why are the fossils of Patagonia basically so similar to Patagonia's living biota? Why does each island in an archipelago have its own endemic species and yet they are all much more similar to each other than to related species in more distant areas? It was this ability to observe interesting facts and to ask the appropriate questions about them that permitted him to make so many scientific discoveries and to develop so many highly original concepts.
Darwin also saw clearly that there are two aspects of evolution. One is the "upward" movement of a phyletic lineage, its gradual change from an ancestral to a derived condition. This is referred to as anagenesis. The other consists of the splitting of evolutionary lineages or, more broadly, of the origin of new branches (clades) of the phylogenetic tree. This process of the origin of biodiversity is called cladogenesis. It always begins with an event of speciation, but the new clade may become, in time, an important branch of the phylogenetic tree by diverging increasingly from the ancestral type. The study of cladogenesis is one of the major concerns of macroevolutionary research. Anagenesis and cladogenesis are largely independent processes (Mayr 1991).
Already in the 1860s knowledgeable biologists and geologists accepted that evolution was a fact, but Darwin's explanations of the how and why of evolution faced protracted opposition, as we shall show in later chapters. But let us first review some of the evidence for the actual occurrence of evolution that has been gathered since 1859.
Box 1.1 Anticreationist Books
Berra, Tim M. 1990. Evolution and the Myth of Creationism. Stanford: Stanford University Press.
Eldredge, Niles. 2000. The Triumph of Evolution and the Failure of Creationism. New York: W. H. Freeman.
Futuyma, Douglas J. 1983. Science on Trial: The Case for Evolution. New York: Pantheon Books.
Kitcher, Philip. 1982. Abusing Science: The Case Against Creationism. Cambridge, Mass.: MIT Press.
Montagu, Ashley (ed.). 1983. Science and Creationism. New York: Oxford University Press.
Newell, Norman D. 1982. Creation and Evolution: Myth or Reality? New York: Columbia University Press.
Peacocke, A. R. 1979. Creation and the World of Science. Oxford: Clarendon Press.
Ruse, Michael. 1982. Darwinism Defended. Reading, Mass.: Addison-Wesley.
Young, Willard. 1985. Fallacies of Creationism. Calgary, Alberta, Canada: Detrelig Enterprises.
Excerpted from WHAT EVOLUTION IS by Ernst Mayr. Copyright © 2001 by Ernst Mayr. Excerpted by permission. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
|Pt. I||What is Evolution?|
|1||In What Kind of a World Do We Live?||3|
|2||What Is the Evidence for Evolution on Earth?||12|
|3||The Rise of the Living World||40|
|Pt. II||How Are Evolutionary Change and Adaptedness Explained?|
|4||How and Why Does Evolution Take Place?||73|
|7||Adaptedness and Natural Selection: Anagenesis||147|
|Pt. III||Origin and Evolution of Diversity: Cladogenesis|
|8||The Units of Diversity: Species||161|
|Pt. IV||Human Evolution|
|11||How Did Mankind Evolve?||233|
|12||The Frontiers of Evolutionary Biology||265|
|App. A||What Criticisms Have Been Made of Evolutionary Theory?||269|
|App. B||Short Answers to Frequently Asked Questions About Evolution||274|