The Road to Modern Evolutionary Biology Although the roots of modern evolutionary biology are firmly entrenched in Charles Darwin's 1859 publication On the Origin of Species by Means of Natural Selection, the historical development of evolutionary biology can be traced back much further than this. Prior to Darwin there were numerous non-evolutionary views of the natural world, many of which hindered any attempts at developing an evolutionary world view. Presented below is a brief sketch of the historical rise of evolutionary biology, culminating in what is referred to as the modern synthesis of evolutionary theory. The Ancient Greeks Natural Theology Darwinian Precursors The Modern Synthesis Non-evolutionary Views of Nature Prior to Darwin's Theory Influence of the Greeks During the 7th, 6th, and 5th centuries B.C., the Greeks realized that many phenomena previously ascribed to the gods could be explained naturally. It stood to reason that one could ask questions and possibly explain things like the origin of matter, the earth, and life. Two aspects characterized the concepts of world origins of the early Greek philosophers. First, the natural world was not created by some supernatural force, but instead was the result of the generative power of nature. Second, origins were viewed in a nonteleological context. The natural world was result of chance or of an irrational necessity. The early Greeks also envisioned an eternal world without significant change or with cyclical changes. The world was thought to exist in a steady state. But even though time was unlimited (eternal), it was of little consequence for the Greek world view. It certainly did not require a replacement of a world of origins by an evolving world. Origins were of interest to the Greeks, but little thought was given to subsequent change. From the early Greek philosophers came promising notions for evolutionary thinking (i.e., unlimited time, spontaneous generation, etc.) But the Greeks developed these ideas no further. Greek philosophy soon changed its direction rather drastically, owing to the influence of the Pythagoreans of southern Italy. Greek philosophy moved toward abstract metaphysics and was influenced by mathematics, especially geometry. Unfortunately, this is the first of countless of times in the history of biology where mathematics exerted a harmful influence on its development. A preoccupation with geometry led to the search for unchanging realities underlying the fleeting flux of appearances in the natural world. Variation was ignored or was thought to have little meaning. Plato's philosophy of idealism or essentialism Plato (427-347 B.C.) introduced the concept of the eidos, the unchanging ideal forms that underlie all the variable phenomena of the world. Plato suggested there were 2 worlds. 1) A real world that was ideal and eternal. 2) An illusionary world of imperfection that we perceived through our senses. Plato observed variations in plant and animal populations, but suggested that they were merely incomplete and imperfect manifestations of the ideal forms. Only the perfect forms were real. Plato's philosophy ruled out evolution thinking mainly because evolution requires variation (also see above). Moreover, evolution would be counterproductive in a world where ideal organisms were already perfectly adapted to their environments. Aristotle Aristotle (384-322 B.C.) questioned Plato's philosophy of dual worlds. Aristotle contended that there was gradation in the natural world because organisms ranged from being relatively simple to extremely complex. He suggested that nature passed from inanimate objects to plants and then to animals such that all living things could be arranged on a scale of increasing complexity. Aristotle's view eventually became converted this into the grand concept of the scala naturae or Great Chain of Being. This concept suggests there is a "ladder of life" and each species has an allotted rung along this ladder. According to this view of life, species are fixed or permanent and therefore do not evolve. Aristotle's world view was entrenched in the minds of western philosophers for some two thousand years. The Influence of Natural Theology During the eighteenth century and first half of the nineteenth century the natural sciences in western Europe and America were dominated by natural theology, a philosophy dedicated to unveiling the God's plan by studying his creation. According to this philosophy, species were individually created and designed by God for a purpose. The idea that species were designed and permanent became so firmly ingrained in the minds of western naturalists that any thoughts or suggestions to the contrary (that life evolved from simple forms, for instance!) were regarded as purely scandalous. During the reign of natural theology, numerous ideas were proposed that reflected an evolutionary world view. None of these ideas was able to overcome the doctrine of fixed species. Linnaeus One of the greatest natural theologians of his time was the 18th century Swedish physician and botanist Carolus Linnaeus (1701-1778). Linnaeus is often regarded as the "Father of Taxonomy" and he is perhaps best known for developing a two part system (genus and species) of naming organisms and developing a system of grouping species into a hierarchy of increasingly general categories. Linnaeus firmly believed that species were permanent creations and he dedicated his life to classifying and cataloging organisms in an effort to reveal the plan of God's creation. Darwinian Precursors Although many factors contributed to our understanding of the natural world which in turn made Darwin's theory of evolution possible, perhaps the most significant came from discoveries in the geological sciences. Geologists came to recognize that sedimentary rocks had been laid down at different times and as a consequence suggested that the earth might be very old. In fact, the French geologist Buffon (1707-1778) boldly suggested that the Earth might be as much as 168,000 years old! The French zoologist and paleontologist Georges Cuvier (1769-1832) documented the succession of fossil species in the Paris Basin. He noted the progressiveness of the fossil record, with each layer of sedimentary rock containing a unique suite of fossil organisms. Cuvier was, nonetheless, adamantly opposed to the notion that species evolved over time. He argued that the "boundaries" between suites of organisms from different strata were caused by past catastrophic events such as floods or drought that may have destroyed many species in an area at once. Cuvier's view of the history of life in the fossil record is called catastrophism. The uniformitarians, including Charles Lyell (1797-1875), had an extremely important impact on the intellectual climate leading to Darwin's theory. Lyell and others suggested that various geological formations on Earth today result from the cumulative effects of slow but continuous processes over long expanses of time. For example, canyons were thought to have been formed by the slow erosive action of rivers over millions of years of geological time. In his celebrated book Principles of Geology, Lyell affirmed the Earth must be extremely old and that slow gradual changes over expansive amounts of time can yield substantial change. The understanding that the Earth has been transformed through eons of slow gradual changes prompted many naturalists to suggest that living organisms may have evolved along with the evolution of Earth. Many hypothesis and fanciful stories about the evolution of life were subsequently proposed. However, few among them offered a model with an explanatory mechanism. Evolution, Darwinism and the Neo-Darwinian paradigm Lamarckian Evolution Jean Baptiste Lamarck (1744-1829) suggested that species changed over time and developed a comprehensive model to explain how this transformation occurred. Lamarck examined numerous living and fossil organisms and noticed what appeared to be several lines of descent. He suggested that microscopic forms of life were spontaneously being generated from inanimate materials and these simple organisms were continually being transformed into more complex forms of life. In short, lineages of species persisted indefinitely, but changed from one form to another over time. According to Lamarck, evolution was driven by an innate tendency toward greater and greater complexity which he seemed to equate with perfection. And as organisms achieved perfection, they became better adapted to their environments. Lamarck believed that organisms evolved by responding to their sentiments interieurs, or "felt needs." Lamarck proposed a mechanism for the evolution of life that incorporated two important ideas: use and disuse and the inheritance of acquired characteristics. The concept of use and disuse suggests that those parts of the body that are used most to cope with the environment become larger and stronger over time, while those that are not used deteriorate. Inheritance of acquired characteristics represents Lamarck's concept of heredity. He suggested that the modifications an organism acquires during its lifetime can be passed along to its offspring. For example, the long neck of the giraffe evolved as a cumulative product of many ancestral generations stretching higher and higher. To date, there is no evidence to support the notion that acquired characteristics can be inherited Darwin's theory of evolution: Its successes, trials and tribulations During his voyages on the H.M.S. Beagle, Charles Darwin was exposed to a rich diversity of plants and animals. Darwin was particularly intrigued by how well adapted these organisms were to the environments in which they inhabited, and would later postulate that new species were conceived from ancestral forms by the gradual accumulation of adaptations to different environments. The strength of Darwin's theory of evolution relied on its explanation for why species change and why they are adapted to their surroundings. The mechanism of evolution proposed by Darwin was natural selection. He indicated that survival in the struggle for existence is not random, but depends in part on the heredity constitution of individuals. Those individuals possessing heredity characteristics best suited for their environment are likely to leave more offspring than those that are less fit. This unequal ability of individuals to survive and reproduce will lead to gradual change in a population, with favorable characteristics accumulating over time and eventually resulting in the formation of a new species. Darwin's theory of evolution was, for the most part, well received among his contemporaries. However, during the early 1900s, evolution by means of natural selection fell on hard times. One of the main reasons for this was because it lacked a satisfactory theory of heredity. Darwin supported the Blending Theory of Heredity which suggested that the heredity material of parents blended together to produce characteristics observed in the offspring. This theory was damaging to Darwinian evolution because it would invariably lead to a reduction in the genetic variation (including favorable variation) of a population from generation to generation. If favorable variation from one generation is lost in the next generation, how can it be preserved by selection and generate adaptations? In essence, natural selection cannot operate if heredity is blended because favorable variation will be lost by blending process. The modern synthesis theory of evolution The late 1920s and early 1930s witnessed the unification of Mendel's theory of heredity (which is the basis of modern genetics) and Darwinian evolution. The theoretical work was mainly performed independently by Ronald Fisher (1890-1962), John Haldane (1892-1964), and Sewell Wright (1889-1988). Their synthesis of Darwin's theory of NS with the Mendelian theory of heredity established what is known as neo-Darwinism (also known as the synthetic theory of evolution or the modern synthesis). Julian Huxley did a masterful job of communicating the significance and application of this unification in his book titled Evolution: The Modern Synthesis (1942). The coalition between Mendelism and Darwinism inspired new genetic research in both the field and laboratory. A number of very influential books on evolutionary theory were published in the 1930s-1940s, including Genetics and the Origin of Species by Theodosius Dobzhansky in 1937, Systematics and the Origin of Species by Ernst Mayr in 1942, Tempo and Mode in Evolution by George Gaylord Simpson in 1944, and Variation and Evolution in Plants by G. Ledyard Stebbins in 1950.