THEORIES OF EVOLUTION: LAMARCK VS DARWIN The theory of evolution is simply a statement of species transformation over time with no mechanism proposed. A variety of mechanisms behind evolution have been suggested and each constitutes a theory of evolution. Modern evolutionary theory, on the other hand, is a complex of those specific theories which have yet to be falsified by testing and contribute something to our understanding of the processes behind evolution. Charles Darwin's name is universally associated with the concept of evolution, but he did not discover this concept. Some scholars have suggested that the concept of evolution goes all the way back to the natural, pre- Socratic philosophers of ancient Greece. Anaximander and Empedocles wrote about successive changes in species, but these changes can be considered a series of creative acts rather than actual change in a species over time. At any rate, it is clear that during Darwin's time most intellectuals were questioning the concept of the fixity of species and were leaning toward an evolutionary interpretation of the origin of species. What Darwin did was to present a mountain of evidence supporting this concept and propose a plausible mechanism for the process of evolution: his theory of natural selection - one of a number of theories of evolution. We will not survey all of the different theories of evolution which have been proposed in the past, but will focus on Darwin's theory which is the centerpiece of modern evolutionary theory and contrast it with another prominent theory proposed by Lamarck. Biological diversity was ordered into a taxonomic system by Carl von Linne (Linnaeus) who, although a creationist, proposed a scheme for classifying species which itself evolved to our present system of classification. Linnaeus proposed that individual species be grouped together in a larger category (the genus) and named by referring to both the genus and species - his binomial system of nomenclature. Thus, our own species is named Homo sapiens after the genus Homo which includes other species (now extinct) from which we are distinguished by our species epithet sapiens. Genera were clumped together on the basis of similarity into larger groups (higher taxonomic categories), e.g., families which in turn were grouped into orders and so on up to the most inclusive of the Linnaean categories - the kingdom. Analysis of this classification system suggested a trend in increasing complexity from one-celled organisms to multicellular invertebrates to non-mammalian vertebrates to mammals with ourselves being the most complex. This ordering suggested a ladder of life similar to the older idea of a Scala Naturae or the Great Chain of Being envisioned by the ancient Greeks. A creationist interpretation of this ladder suggested a divine plan for creation. However, other interpretations with an evolutionary flavor are possible. One such evolutionary interpretation was suggested by Georges Louis Leclerc de Buffon (1707-1788) who proposed a degenerative form of evolution for species within a genus. A very generalized species, which embodied the ideal characteristics of the genus, could be modified through evolution to produce variations on this theme through specialization. The major force producing these specializations was the environment. Comte de Buffon's idea of degenerative evolution is remarkably similar to that held by scientific creationists (to be discussed later in the course) who claim that the creator need not have created each different species, only the genus from which the component species evolved by modification (degeneration) of some of the ideal characteristics of the originally created type, e.g., an unblemished, uniform color could be modified with stripes, blotches or spots. These creationists allow a limited amount of "within type" evolution but deny that one type can evolve into another. Lamarckism Jean Baptiste de Lamarck (1744-1829) attempted to reconcile the Great Chain of Being with species change over time and suggested that each species climbs the ladder of life. Lamarck denied extinction and viewed evolution as a progression along a fixed path to greater complexity. Because mammals are more complex than fish and occupy a higher rung on the ladder of life, they must have arisen earlier in time. Given sufficient time fish will evolve into amphibians which in turn will evolve into reptiles and so on until they advance to the mammal stage. To account for the continued existence of the simplest, one- celled animals, Lamarck postulated the spontaneous generation of life to fill the vacancy created when these species evolved to occupy the next rung of the ladder. Lamarck's view of evolution, published in 1809 under the title Philosophie Zoologique, was more philosophical than scientific. What determined the path taken in evolution? Lamarck claimed that species were driven by an internal drive to climb the Scala Naturae, but what was the nature of this drive? He suggested that this internal drive was a form of volition or desire mediated by a nervous fluid; but if this were true, how did plants which lack a nervous system evolve and how could this idea be tested? His contemporaries did not take kindly to his theory which was based more on metaphysical speculation than on physical mechanism and so provided no basis for research. Lamarck did propose his use and disuse theory to explain how change might occur and this theory drew on the idea of inheritance of acquired characteristics now firmly linked to his name. Lamarck's theory can be summarized by the following steps. 1. Environmental change imposes new demands upon an organism which must be met if the organism is to survive. 2. The environmental demand produces a desire in the organism to change and the organism responds by a behavioral change through use or disuse of existing organs. 3. Use and disuse modify the organism's appearance (habit modifies form) and this change becomes heritable and so is passed on to offspring (inheritance of acquired characteristics). Like Buffon, Lamarck also saw the environment as an important factor in evolution and added the idea that characters acquired by an individual during its lifetime in response to environmental change became heritable and so were passed on to offspring. For Lamarck, the environment ultimately induced genetic change in individuals as these characters acquired by use and disuse developed a genetic base. August Weismann (1834-1914) falsified this idea by demonstrating that removing the tail of newborn mice over many generations had no effect on tail size in succeeding generations. He proposed his germplasm theory which suggested that inheritance passes from one germ line to another through the gonads or reproductive organs and that there is no interaction between somatic (non-reproductive) tissue and germplasm (reproductive tissue). Curiously, Darwin proposed a theory of heredity which explained the inheritance of acquired characteristics by claiming an interaction between somatic and germ line tissue. His theory of pangenesis suggested that the gonads receive pangenes from each part of the body rather than forming them themselves. According to pangenesis theory, if a somatic body part were modified during the life of an individual, its pangenes would be modified and the modified pangenes would then migrate to the gonads and be passed on to offspring. In this manner acquired characteristics could be inherited. Needless to say this idea was also discredited by Weismann. Darwinism Darwin's great contribution was not the discovery of evolution; rather, he convinced a scientific community already receptive to an evolutionary origin of organic diversity that evolution and not creation was in fact the reason for the origin of species. In addition, he proposed a mechanism to explain the process behind evolution, namely, his theory of natural selection. In this latter endeavor he was less successful. Darwin's proposed mechanism behind evolution follows logically from a series of observations and deductions: 1. Organisms produce more offspring than can survive. (An observation amply documented.) 2. Adult population size remains constant from generation to generation. (An observation which enjoys empirical support for some, but not all, species.) 3. There is a struggle for existence among off- spring. (This is a logical deduction from the first two observations. If there only exists a fixed number of adults in each generation and more offspring are produced than needed to replace them in the next generation, then it follows logically that many offspring will die in struggling to become the next generation's adults.) 4. Individuals vary in their adaptations to the environment. (This is an observation that has been carefully documented.) 5. These individual variations in adaptation are inherited, i.e., they are under genetic control. (This was an assumption because Darwin was unable to demonstrate the hereditary basis for individual variation, later confirmed by Mendel). 6. On average those offspring with the better adaptations will survive: "survival of the fittest." (This conclusion is derived from deduction #3 and observation #4, but will only result in evolution if assumption #5 is true.) The process of natural selection Natural selection is neither a thing nor a theory; it is a process deduced from the rationale provided above. The reality of this process depends upon the truth of these propositions and the logical integrity of the conclusions. If it could be demonstrated that propositions 1, 2, 4 and 5 are not valid and/or not sufficient to deduce conclusions 3 and 6, then the process of natural selection would not be an adequate description of how nature operates. So far, however, these propositions have been empirically verified and seem logically sufficient to deduce the critical conclusions. As is apparent from conclusion 6, Darwin viewed natural selection as a process of differential mortality; but what really counts in evolution is reproduction, not survival. No change will occur in a population in the next generation if some individuals outlive others but do not reproduce. Darwin was well aware of this but used the term sexual selection for the evolution of traits which specifically contribute to the reproduce success of their possessors. Thus, Darwin saw two forms of selection: (1) natural selection which produced adaptations by conferring a survival advantage on those individuals better able to resist predators, competitors or physical environmental forces, e.g., weather, and (2) sexual selection which did not produce adaptations but did favor (reproductively) individuals more attractive to the opposite sex or more successful in competing for mates with other members of their own sex. Since what really matters in both forms is that the selected individuals leave more offspring than others in the population, modern biologists define natural selection as a process of differential reproduction. The theory of natural selection Darwin's theory of evolution by natural selection is not a statement of the reality of natural selection as a process; rather it is the hypothesis that the process of natural selection causes evolution, i.e., that it changes the genetic composition of a population between successive generations. Hence, as an evolutionary force, natural selection acts only within single populations (not between populations or species) and the unit of natural selection (what is selected) is the individual. The result of natural selection is evolution, through the production of adaptation. Adaptations are aspects of the phenotype which meet the needs imposed by the environment and they are the result of gradual environmental selection of individual variants within a population. To be selected, individuals need not be perfectly adapted to their environment - only better adapted than others in their population. Thus, natural selection results in directed evolution, not random changes in a species. For this reason natural selection is considered to be the most important of all evolutionary forces. Comparison of Lamarckism and Darwinism Both Darwin's and Lamarck's theory of evolution are two- factor theories calling for a role of the environment and hereditary material (genes). They differ, however, in the way they view the action of the environment on genes. For Lamarck, the environment induced genetic changes in individuals which resulted in change in the appearance of the next generation. For Darwin, the environment did not change genes, but only selected individuals which varied genetically. The end result was the same - change in the appearance and genetic composition of the next generation. These two theories can be contrasted by the way they explain how giraffes evolved a long neck. According to Lamarck at one time all giraffes possessed short necks. An environmental change, such as a prolonged drought, resulted in a food shortage which induced a desire in giraffes to browse on leaves in trees rather than graze on the ground. This new manner of feeding caused giraffes to stretch their necks to obtain leaves and so lengthen their necks. The longer necks acquired through stretching were passed on to offspring born with longer necks than were their parents. This process continued each generation until all giraffes possessed the long necks which characterize them today. Darwin assumed that in the beginning giraffes varied in neck size but the average size was much shorter than at present. When the environment changed to produce a shortage of food, those giraffes with long necks could browse on leaves and so had a higher probability of surviving the drought. The selected long-necked giraffes reproduced on average more offspring than short-necked ones, and since variation in neck size is inherited, over a number of generations average neck size in the giraffe population increased. Neo-Darwinism Several reasons prevented Darwin's colleagues from accepting the mechanism of natural selection. First and foremost was the prevailing view that the Earth was not old enough to allow a process as slow as natural selection to account for contemporary diversity. This objection came not from creationists but from Lord Kelvin, the famed physicist, who argued from his calculations of the physics of a cooling body that the Earth could not be more than about 100 million years old. Secondly, he himself recognized that traits must be heritable for his theory to work and he was unable to explain the mechanism of heredity. The prevailing view of blending inheritance was inimical to his theory because the characters of any variant which enjoyed a survival advantage over other variants in a population would be diluted when that individual mated. With each subsequent generation the trait would be diluted until it was eventually blended out of existence. For this reason he proposed his pangene theory which postulated particles, not fluids which could blend. Finally, although many could see how natural selection could remove unfit individuals from a population, they could not understand how this process could create the fit, i.e., produce adaptations, and so concluded that it must be of minor significance in causing evolution. The discovery of radiometric dating techniques, the rediscovery and acceptance of Mendelian genetics and demonstration of the creatveed in terms of modern genetics and population biology. Because Darwin's theory is based so heavily on the idea of heritable individual variation, we will next discuss the mechanisms responsible for producing this variation.