The Rise of Evolutionary Biology From: EVOLUTION by Mark Ridley

CONTENTS THE RISE OF EVOLUTIONARY BIOLOGY. MOLECULAR AND MENDELIAN GENETICS. THE EVIDENCE FOR EVOLUTION. NATURAL SELECTION AND VARIATION.

Evolutionary biology and contrasts it with some related but different concepts. Evolution is a big theory in biology. Evolution means change in living things by descent with modification. Evolutionary biology is a large science, and is growing larger. Evolutionary biologists now carry out research in some sciences, like molecular genetics: that are young and move rapidly morphology and embryology: that have accumulated their discoveries at a more stately speed over a much longer period. work with materials as diverse as naked chemicals in test tubes, animal behavior in the jungle, and fossils collected from barren and inhospitable rocks. And evolution by natural selection can be scientifically tested in all these fields. It is one of the most powerful ideas in all areas of science, and is the only theory that can seriously claim to unify biology.

As Theodosius Dobzhansky, one of the twentieth centurys most eminent evolutionary biologists, remarked in an often quoted but scarcely exaggerated phrase, nothing in biology makes sense except in the light of evolution

What is Evolution?? Evolution means change, change in the form and behavior of organisms between generations. The forms of organisms, at all levels from DNA sequences to macroscopic morphology and social behavior, can be modified from those of their ancestors during evolution. However, not all kinds of biological change are included in the definition. Developmental change within the life of an organism is not evolution in the strict sense, and the definition referred to evolution as a change between generations in order to exclude developmental change. A change in the composition of an ecosystem, which is made up of a number of species, would also not normally be counted as evolution.

Darwin defined evolution as descent with modification, and the word descent refers to the way evolutionary modification takes place in a series of populations that are descended from one another. Recently, Harrison (2001) defined evolution as change over time via descent with modification.

Adaptation: Adaptation is another of evolutionary theorys crucial concepts. Indeed, it is one of the main aims of modern evolutionary biology to explain the forms of adaptation that we find in the living world. Adaptation refers to design in life to those properties of living things that enable them to survive and reproduce in nature.

The concept is easiest to understand by example. Camouflage is particularly clear, example of adaptation. Camouflaged species have color patterns and details of shape and behavior that make them less visible in their natural environment. Camouflage assists the organism to survive by making it less visible to its natural enemies. Camouflage is adaptive. In humans, hands are adapted for grasping, eyes for seeing, the alimentary canal for digesting food, legs for movement. all these functions assist us to survive. Although most of the obvious things we notice are adaptive, not every detail of an organisms form and behavior is necessarily adaptive.

Adaptation by Natural Selection: Darwin regarded adaptation as the key problem that any theory of evolution had to solve. In Darwins theory as in modern evolutionary biology the problem is solved by natural selection. Natural selection means that some kinds of individual in a population tend to contribute more offspring to the next generation than do others. Provided that the offspring resemble their parents, any attribute of an organism causing it to leave more offspring than average will increase in frequency in the population over time. The composition of the population will then change automatically.

History of evolutionary biology We shall begin with a brief sketch of the historic rise of evolutionary biology, in four main stages: 1. Evolutionary and non-evolutionary ideas before Darwin. 2. Darwins theory (1859). 3. The eclipse of Darwin (c. 1880–1920). 4. The modern synthesis (1920s to 1950s)

1:Evolution before Darwin The history of evolutionary biology really begins in 1859, with the publication of Charles Darwins On the Origin of Species. However, many of Darwins ideas have an older pedigree. The French scientist Maupertuis discussed evolution, as did encyclopédistes such as Diderot. Charles Darwins grandfather, Erasmus Darwin, is another example. However, none of these thinkers put forward anything we would now recognize as a satisfactory theory to explain why species change. They were mainly interested in the factual possibility that one species might change into another. The question was brought to an issue by the French naturalist Jean- Baptiste Lamarck (1744–1829). The crucial work was his Philosophie Zoologique(1809), in which he argued that species change over time into new species. The way in which he thought species changed was importantly different from Darwins and our modern idea of evolution.

Lamarcks conception of evolution, and how it differs from Darwins concept:

Lamarck had a two-part explanation of why species change. The principal mechanism was an internal force a some sort of unknown mechanism within an organism causing it to produce offspring slightly different from itself, such that when the changes had accumulated over many generations the lineage would be visibly transformed, perhaps enough to be a new species Lamarcks second (and possibly to him less important) mechanism is the one he is now remembered for: the inheritance of acquired characters. Biologists use the word character as a short-hand for characteristic. A character is any distinguishable property of an organism; it does not here refer to character in the sense of personality.

Lamarckian inheritance: Lamarck did not invent the idea of the inheritance of acquired characters. The idea is ancient it was discussed in ancient Greece by Plato. However, most modern thinking about the role of the process in evolution has been inspired by Lamarck, and the inheritance of acquired characters is now conventionally, if unhistorically, called Lamarckian inheritance.

Cuviers Views: Cuviers studied the anatomy of animals to discover the various fundamental plans according to which the different types of organism were designed. Cuvier in this way established that the animal kingdom had four main branches : vertebrates, articulates, mollusks, radiates. A slightly different set of main groups is recognized in modern biology, but the modern groupings do not radically contradict Cuviers four- part system. Cuvier also established, contrary to Lamarcks belief, that species had gone extinct. By the first half of the nineteenth century, most biologists and geologists had come to accept Cuviers view that each species had a separate origin, and then remained constant in form until it went extinct.

2:Charles Darwin Meanwhile, Charles Darwin was forming his own ideas. Darwin, after graduating from Cambridge, had traveled the world as a naturalist on board the Beagle(1832–37). He then lived briefly in London before settling permanently in the country.His father was a successful doctor, and his father-in-law controlled the Wedgwood china business; Charles Darwin was a gentleman of independent means. The crucial period of his life, for our purposes, was the year or so after the Beagle voyage (1837–38). As he worked over his collection of birds from the Galápagos Islands, he realized that he should have recorded which island each specimen came from, because they varied from island to island. He had initially supposed that the Galápagos finches were all one species, but it now became clear that each island had its own distinct species. How easy to imagine that they had evolved from a common ancestral finch! He was similarly struck by the way the ostrich-like birds called rheas differed between one region and another in South America. These observations of geographic variation probably first led Darwin to accept that species can change.

The important step was to invent a theory to explain why species change. The notebooks Darwin kept at the time still survive. They reveal how he struggled with Several ideas, including Lamarckism, but rejected them all because they failed to explain a crucial fact - adaptation.

His theory would have to explain not only why species change, but also why they are well designed for life. Because of the struggle for existence, forms that are better adapted to survive will leave more offspring and automatically increase in frequency from one generation to the next. As the environment changes through time (for example, from humid to arid), different forms of a species will be better adapted to it than were the forms in the past. The better adapted forms will increase in frequency, and the now poorly adapted forms will decrease in frequency. As the process continues, eventually. Darwins theory of evolution by natural selection explains evolutionary change and adaptation.

3:Darwins reception The reactions to Darwins two connected theories a evolution and natural selection differed. The idea of evolution itself become controversial mainly in the popular sphere only, rather than among biologists. Evolution seemed to contradict the Bible, in which the various kinds of living things are said to have been created separately. In Britain, Thomas Henry Huxley particularly defended the new evolutionary view against religious attack. Evolution was less controversial among professional scientists. Many biologists came almost immediately to accept evolution.

The leading anatomists were by now mainly German.Carl Gegenbauer, one of the major figures, had soon reorientated his work to the tracing of evolutionary relationships between animal groups. The famous German biologist Ernst Haeckel vigorously investigated the same problem, as he applied his biogenetic law the theory of recapitulation to reveal phylogenetic pedigrees. Although some kind of evolution was widely accepted among biologists, probably few of those biologists shared Darwins own idea of it. In Darwins theory, evolution is not inherently or automatically progressive. The local conditions at each stage mainly determine how a species evolves. The species does not have an inherent tendency to rise to a higher form.

Objections to Darwins theory It lacked a satisfactory theory of heredity. There were various theories of inheritance at that time, and all of them are now known to be wrong. Darwin preferred a blending theory of inheritance, in which the offspring blend their parental attributes. A second objection was that gaps exist between forms in nature gaps that could not be crossed if evolution was powered by natural selection alone. The anatomist St George Jackson Mivart, for instance, in his book The Genesis of Species, listed a number of organs that would not (he thought) be advantageous in their initial stages. In Darwins theory, organs evolve gradually, and each successive stage has to be advantageous in order that it can be favored by natural selection.

Around the turn of the century, Weismann was a highly influential figure, but few biologists shared his belief in natural selection. Some, such as the British entomologist Edward Bagnall Poulton, were studying natural selection. However, the majority view was that natural selection needed to be supplemented by other processes. By this time, Mendels theory of heredity had been rediscovered. Mendelism has been the generally accepted theory of heredity since the 1920s, and is the basis of all modern genetics. Mendelism eventually allowed a revival of Darwins theory, but its initial effect (around 1900–20) was the exact opposite. The early Mendelians, such as Hugo de Vries and William Bateson, all opposed Darwins theory of natural selection.

4:The modern synthesis By the second decade of the twentieth century, research on Mendelian genetics had already become a major enterprise. It was concerned with many problems, most of which are more to do with genetics than evolutionary biology. But within the theory of evolution, the main problem was to reconcile the atomistic Mendelian theory of genetics with the biometricians description of continuous variation in real populations. This reconciliation was achieved by several authors in many stages, but a 1918 paper by R.A. Fisher is particularly important. Fisher demonstrated there that all the results known to the biometricians could be derived from Mendelian principles.

The next step was to show that natural selection could operate with Mendelian genetics. The theoretical work was mainly done, independently, by R.A. Fisher, J.B.S. Haldane, and Sewall Wright (Figure 1.8). Their synthesis of Darwins theory of natural selection with the Mendelian theory of heredity established what is known as neo- Darwinism, or the synthetic theory of evolution, or the modern synthesis, after the title of a book by Julian Huxley, Evolution: the Modern Synthesis (1942). The old dispute between Mendelians and Darwinians was ended. Darwins theory now possessed what it had lacked for half a century: a firm foundation in a well tested theory of heredity.

The ideas of Fisher, Haldane, and Wright are known mainly from their great summary works all written around Fisher published his book The Genetical Theory of Natural Selection in Haldane published a more popular book, The Causes of Evolution, in 1932; it contained a long appendix under the title A mathematical theory of artificial and natural selection, summarizing a series of papers published from 1918 onwards. Wright published a long paper on Evolution in Mendelian populations in 1931; unlike Fisher and Haldane, Wright lived to publish a four-volume treatise (1968–78) at the end of his career. These classic works of theoretical population genetics demonstrated that natural selection could work with the kinds of variation observable in natural populations and the laws of Mendelian inheritance. No other processes are needed. The inheritance of acquired characters is not needed. Directed variation is not needed. Macromutations are not needed. This insight has been incorporated into all later evolutionary thinking, and the work of Fisher, Haldane, and Wright is the basis for much of the material in Chapters 5–9

The reconciliation between Mendelism and Darwinism soon inspired new genetic research in the field and laboratory. Theodosius Dobzhansky (Figure 1.9), for example, began classic investigations of evolution in populations of fruitflies (Drosophila) after his move from Russia to the USA in Dobzhansky had been influenced by the leading Russian population geneticist Sergei Chetverikov (1880–1959), who had an important laboratory in Moscow until he was arrested in Dobzhansky, after he had emigrated, worked both on his own ideas and collaborated with Sewall Wright. Dobzhanskys major book, Genetics and the Origin of Species, was first published in 1937 and its successive editions (up to 1970 (retitled)) have been among the most influential works of the modern synthesis. We shall encounter several examples of Dobzhanskys work with fruitflies in later chapters.

E.B. Ford (1901–88) began in the 1920s a comparable program of research in the UK. He studied selection in natural populations, mainly of moths, and called his subject ecological genetics. He published a summary of this work in a book called Ecological Genetics, first published in 1964 (Ford 1975). H.B.D. Kettlewell (1901–79) studied melanism in the peppered moth Biston betularia, and this is the most famous piece of ecological genetic research (Section 5.7, p. 108). Ford collaborated closely with Fisher. Their best known joint study was an attempt to show that the random processes emphasized by Wright could not account for observed evolutionary changes in the scarlet tiger moth Panaxia dominula. Julian Huxley (Figure 1.10a) exerted his influence more through his skill in synthesizing work from many fields. His book Evolution: the Modern Synthesis (1942) introduced the theoretical concepts of Fisher, Haldane, and Wright to many biologists, by applying them to large evolutionary questions.

From population genetics, the modern synthesis spread into other areas of evolutionary biology. The question of how one species splits into two a the event is called speciation a was an early example. Before the modern synthesis had penetrated the subject, speciation had often been explained by macromutations or the inheritance of acquired characters. A major book, The Variation of Animals in Nature, by two systematists, G.C. Robson and O.W. Richards (1936), accepted neither Mendelism nor Darwinism. Robson and Richards suggested that the differences between species are non- adaptive and have nothing to do with natural selection. Richard Goldschmidt (1878–1958), most famously in his book on The Material Basis of Evolution (1940), argued that speciation was produced by macromutations, not the selection of small variants.

The question of how species originate is closely related to the questions of population genetics, and Fisher, Haldane, and Wright had all discussed it. Dobzhansky and Huxley emphasized the problem even more. They all reasoned that the kinds of changes studied by population geneticists, if they took place in geographically separated populations, could cause the populations to diverge and eventually evolve into distinct species (Chapter 14). The classic work, however, was by Ernst Mayr: Systematics and the Origin of Species (1942). Like many classic books in science, it was written as a polemic against a particular viewpoint. It was precipitated by Goldschmidts Material Basis but criticized Goldschmidt from the viewpoint of a complete and differing theory athe modern synthesis arather than narrowly refuting him and it therefore has a much broader importance. Both Goldschmidt and Mayr (Figure 1.10b) were born and educated in Germany and later emigrated to the USA. Mayr left in 1930 as a young man, but Goldschmidt was 58 and had built a distinguished career when he left Nazi Germany in 1936.

A related development is often called the new systematics, after the title of a book edited by Julian Huxley (1940). It refers to the overthrow of what Mayr called the typological species concept and its replacement by a species concept better suited to modern population genetics (Chapter 13). The two concepts differ in what sense theymake of variation between individuals within a species. Species, in the typological conception, had been defined as a set of more or less similar-looking organisms, where similarity was measured relative to a standard (or type) form for the species. A species then contains some individuals of the standard type, and other individuals who deviate from that type. The type individuals are conceptually privileged, whereas the deviants show some sort of error.

However, the concept of a species as type plus deviants was inappropriate in the theory of population genetics. The changes in gene frequencies analyzed by population geneticists take place within a gene pool a that is, a group of interbreeding organisms, who exchange genes when they reproduce. The crucial unit is now the set of interbreeding forms, regardless of how similar looking they are to each other. The idea of a type for a species is meaningless in a gene pool containing many genotypes. One genotype is no more of a standard form for the species than any other genotype. A gene pool does not contain one, or a few, type genotypes that are the standard forms for a species, with other genotypes being deviants from that type. No type form exists that could be used as a reference point for defining the species. Population geneticists therefore came to define the members of a species by the ability to interbreed rather than by their morphological similarity to a type form. The modern synthesis had spread to systematics.

A similar treatment was given to paleontology by George Gaylord Simpson in Tempo and Mode in Evolution (1944). Many paleontologists in the 1930s still persisted in explaining evolution in fossils by what are called orthogenetic processes a that is, some inherent (and unexplained) tendency of a species to evolve in a certain direction. Orthogenesis is an idea related to the pre-Mendelian concept of directed mutation, and the more mystical internal forces we saw in the work of Lamarck. Simpson argued that no observations in the fossil record required these processes. All the evidence was perfectly compatible with the population genetic mechanisms discussed by Fisher, Haldane, and Wright. He also showed how such topics as rates of evolution and the origin of major new groups could be analyzed by techniques derived from the assumptions of the modern synthesis

By the mid-1940s, therefore, the modern synthesis had penetrated all areas of biology. The 30 members of a committee on common problems of genetics, systematics, and paleontology who met (with some other experts) at Princeton in 1947 represented all areas of biology. But they shared a common viewpoint, the viewpoint of Mendelism and neo-Darwinism. A similar unanimity of 30 leading figures in genetics, morphology, systematics, and paleontology would have been difficult to achieve before that date. The Princeton symposium was published as Genetics, Paleontology, and Evolution (Jepsen et al. 1949) and is now as good a symbol as any for the point at which the synthesis had spread throughout biology. Of course, there remained controversy within the synthesis, and a counterculture outside. In 1959, two eminent evolutionary biologists a the geneticist Muller and the paleontologist Simpson a could still both celebrate the centenary of The Origin of Species with essays bearing (almost) the same memorable title: One hundred years without Darwinism are enough (Muller 1959;Simpson 1961a).