Outline and Chapter notes to accompany chapter 5 EVOLUTION Dec., 2003 A. THE DARWINIAN PARADIGM REORGANIZED BIOLOGICAL THOUGHT PRE-DARWINIAN THOUGHT: THE SCALE OF BEING (Scala Naturae): Originally a non-evolutionary hierarchy of static, unchanging perfection, with people on top, animals below them, plants below animals; theological versions had angels above people. Among people, racist, sexist, and class-based ideologies put Europeans above other races, men above women, masters above slaves (as in Aristotle), and upper castes or classes above lower ones. People who were listed at lower levels were taught to accept their position as "natural" and unchangeable. Continuity of the scale was stressed, e.g., by Alexander Pope. Discoveries of the Renaissance and later centuries seemed to confirm the continuum because all newly discovered species could be made to fit into it somewhere. Gaps were explained by saying that new discoveries would fill them in. LAMARCK: Made the chain of being into a moving escalator which he called Nature's Parade (La Marche de la Nature). The lowest forms of life, such as bacteria, formed by spontaneous generation from lifeless matter, and each species would slowly change (i.e., evolve) into the next higher species on the scale. In order to account for local adaptation to the environment, Lamarck added as a secondary mechanism the Use and Disuse hypothesis: use would strengthen and enlarge any organ, while disuse would weaken an organ and allow it to atrophy. Lamarck did not realize that such ACQUIRED CHARACTERISTICS would not be inherited (it was commonly assumed in his day that they would be; non-inheritance of acquired characteristics was demonstrated in the 1870s by Weismann). Whenever ADAPTATION was discovered, Lamarck attributed it to the effects of use and disuse under each individual's voluntary control. Other French writers (Geoffroy, de Maupertius, Buffon) also stressed local adaptation to the environment, though sometimes by different mechanisms. WILLIAM PALEY AND NATURAL THEOLOGY: A British clergyman, Rev. William Paley, attributed all ADAPTATIONS to divine benevolence. He used the intricacy of adaptations as evidence that they had been consciously designed by a mind more powerful than our own: "design must have a designer, that designer must [be] ... God." Paley's ideas were the dominant theory in England during the early nineteenth century. THE DEVELOPMENT OF DARWIN'S IDEAS Darwin's ideas were formulated principally during the voyage of H.M.S. Beagle. Darwin saw many tropical habitats much richer in species than those he knew. He noticed that the same habitat often produced different species on different continents: the savannahs (grasslands) of East Africa and the pampas (grasslands) of Argentina have almost no species in common. This is contrary to Lamarck's and similar theories of environmental determinism. He also noticed that the closest relatives of animals were often found elsewhere on the same continent but in different habitats: the same group of South American rodents (the Caviomorpha, or guinea pig group) inhabited South American forests (but not African forests), South American plains, South American mountain regions, and so on. In other words, geography was more important than habitat in determining relationships (Fig. 5.2). He noticed that islands (like the Galapagos) always had inhabitants whose nearest relatives were on the nearest continent. For example, the Galapagos Islands had birds and plants related to those of South America, while the Cape Verde Islands (volcanic, geologically similar to the Galapagos) had species related to those of Africa and unlike those of the Galapagos. Also, nearby islands often had distinct but related species, as if they were descendants of a few original colonists. He noticed the absence of frogs and other amphibians on oceanic islands. In a few places, where humans had introduced them, amphibians flourished, meaning that the habitat was quite suited to them. Neither Lamarck's theory nor Paley's could explain why the environment (Lamarck) or God (Paley) had not produced amphibians on these islands, but Darwin said that they could never arrive as colonists across salt water. BRANCHING DESCENT WITH MODIFICATION Darwin's was not the first evolutionary theory, but it was the first that emphasized BRANCHING descent (in treelike patterns), which Darwin called "descent with modification". Descent with modification explained why classifications should have "groups within groups": families of related species, orders and classes made of related families, etc. (see also Ch. 6). The Darwinian paradigm was very productive of other research. NATURAL SELECTION: Upon return to England, Darwin began studying the methods of animal breeders who had changed many domesticated species in the preceding 100 years or so. They had done so by always selecting the best of their flocks or breeds, a practice called ARTIFICIAL SELECTION. Darwin also read the works of the economist Malthus, who convinced Darwin that populations tend to overreproduce and overreach the available resources, only to be held in check by starvation, predation, and similar forces. From these ideas, he concluded that a continual "struggle for existence" (metaphorical) always exists in nature, not usually including actual combat, but, "what is more important, success in leaving progeny". Darwin concluded that the struggle for existence brings about NATURAL SELECTION by a process similar to artificial selection among domestic species. Variations that lessen the ability to survive and reproduce will not be passed on to future generations; variations that increase the ability to survive and reproduce will be passed on; this difference in the contributions to future generations is called NATURAL SELECTION. NATURAL SELECTION DEFINED: Consistent differences in the relative contribution of different genotypes to future generations. Agents of selection can include predators, diseases, environmental extremes, ability to obtain food, and potential mates (of the opposite sex). Selection by potential mates is called SEXUAL SELECTION. FITNESS DEFINED: The relative number of viable offspring left by each genotype. B. A GREAT DEAL OF EVIDENCE SUPPORTS DARWIN'S IDEAS Darwin's theory became accepted because it explained the available evidence better than any previous theory. Natural selection can explain MIMICRY while earlier theories could not: many species survive because they resemble other, unrelated species that predators avoid. Selection by predators perpetuates the best mimics and eliminates the less effective ones. Industrial melanism among moths demonstrates natural selection: The frequency of dark-colored moths varies geographically with levels of soot pollution. Experiments with bird predators confirms that predators eat the non-camouflaged moths much more often than those which resemble their background. Branching descent with modification explained the facts of geographic distribution much better than any previous theory. The theory also explained HOMOLOGIES (Fig. 5.6), structures which resembled one another in their construction among related species, despite differences in adaptive use in many cases; earlier theories could not explain homologies so well. Some homologies include embryonic characters; others include functionless VESTIGIAL organs. Darwin's theory also explained CONVERGENT resemblances reflecting similar adaptations (ANALOGIES). The fossil record was poorly known in Darwin's time, but fossils discovered since then have in most cases fit well into branching patterns of descent with modification. The ages of fossils are determined by both relative and absolute dating methods. As an example: mollusks of the class Cephalopoda (squids, octopus, extinct ammonites, etc.) all fit into a pattern of branching descent, and their shared adaptations and anatomical features are all consistent with this pattern of descent. Other evidence supporting branching descent includes the patterns of resemblance among embryos (comparative embryology) and among gene sequences (comparative genomics). Post-Darwinian evolutionary thought has added various details and secondary theories to Darwin's paradigm. For example, theories of species formation, mutualism, genetic drift, and puctuated equilibria have all become included in the paradigm. C. CREATIONISTS CHALLENGE EVOLUTIONARY THOUGHT Most scientists before Darwin were devout. Many were clergymen, and most believed that living species had been created by God. BIBLE-BASED CREATIONISM This type of creationist belief centers around the literal reading of the bible, and is often hostile to scientific ideas that appear to contradict scripture. Starting around 1890 in the United States, certain Protestants who called themselves Fundamentalists sought to outlaw the teaching of evolution because they feared that the teaching of evolution was eroding people's faith in divine creation and in religious faith more generally. In 1925, John Scopes was convicted in a Tennessee court for teaching evolution in violation of a state law. The law remained on the books until many decades later. Since about 1960, creationists have changed tactics by insisting only that creationism be taught side by side with evolution and be given "equal time". Religious concepts and quotations from the Bible pervade their writings. Leading creationist writers have proclaimed the Bible to be an infallible guide that is not subject to falsification. Arkansas public law 590, providing "Balanced Treatment" for "Evolution Science" and "Creation Science", was declared unconstitutional in 1981, as was a similar Louisiana law a few years later. INTELLIGENT DESIGN The many scientific allies of Rev. William Paley believed in the theory of SPECIAL CREATION, under which each species was the result of a separate ("special") divine act of creation. Rev. Paley and his supporters all used scientific methods, not biblical quotations, to support their claims. Foremost among their claims were adaptations so perfect in every detail that they said only God could have made them. Darwin and his supporters argued against Paley's theory by showing: * Adaptations seemingly limited by inherited patterns; such patterns would not constrain an omnipotent God. * Geographic distributions limited by opportunity to migrate; an omnipotent God would not be subject to such limitations. * Mimicry, a form of deceptive resemblance that always imitated a species living in the same area but never far away. Michael Behe has recently revived Paley's arguments and used cell biology and biochemistry to build new examples of "irreducible complexity" that he says must have been intelligently designed. All of his examples could, however, have evolved step by step by natural selection, especially when changes in function during evolution are considered. RECONCILING SCIENCE AND RELIGION Many scientists and theologians have sought and found ways to reconcile science and religious beliefs. Possibilities include: 1. declaring that science and religion exist in separate spheres (body versus soul), or that religion is the realm of the "ultimately unknowable". 2. accepting science as only a series of operational descriptions. 3. accepting that the laws of science are the way that God operates. Theistic evolution (evolution following God's laws or God's design) is an example of the third approach. D. SPECIES ARE CENTRAL TO THE MODERN EVOLUTIONARY PARADIGM POPULATIONS AND SPECIES A POPULATION consists of all members of a species inhabiting a given location. A population can also be defined as all individuals that freely interbreed with one another in nature. SPECIES are defined as reproductively isolated groups of naturally interbreeding populations. REPRODUCTIVE ISOLATION consists of all biological mechanisms (not mere geographical separation) that prevent the interbreeding of natural populations. REPRODUCTIVE ISOLATING MECHANISMS can act either before or after mating. These mechanisms include isolation by differences in ecology, differences in mating seasons, differences in behavior, differences which prevent sexual parts from fitting together, or incompatibilities that make gametes, fertilized eggs, embryos, larvae, or adult hybrids inviable or sterile. HOW NEW SPECIES ORIGINATE Most new species originate after a period of geographic separation by an extrinsic barrier. If the barrier lasts long enough for the populations on either side to diverge, then one or more reproductive isolating mechanisms will result. Evidence to support this theory comes from many cases where intermediate stages in the process can be observed. Especially revealing are cases of incomplete speciation in which populations disconnected for a time were brought back together before reproductive isolation between them had been completed. E. LIFE ON EARTH ORIGINATED BY NATURAL PROCESSES AND CONTINUES TO EVOLVE THE ORIGIN OF LIFE and its early evolution is covered in chapter 19. CONTINUING EVOLUTION WITHIN SPECIES There is good evidence that evolution continues to take place. Natural selection brings about seasonal fluctuations in the characteristics of fruit flies and Galapagos finches. Agricultural and industrial societies have greatly changed the selective forces operating on human populations. Evolution continues to take place wherever natural selection occurs, meaning whenever mortality differs according to genotype or phenotype. Examples include: * higher mortality among low birth weight babies (even with good hospitals), * higher susceptibility of certain blood groups to epidemic diseases, * higher incidence of certain cancers in people with certain genotypes * people with certain genetic conditions dying before reproduction * people with certain genetic conditions leaving fewer children ---------------------------------- Dec., 2003 PERMISSION IS HEREBY GRANTED to instructors who have adopted the book BIOLOGY TODAY for classroom use to download, modify, and use these notes as needed to aid them in in their teaching. Students of such instructors may likewise use and modify these notes as study aids.