DARWIN'S THEORY OF NATURAL SELECTION As we learned from our previous discussion, four factors change gene frequency in a population and so cause evolution. Two (mutation and migration) do so largely by introducing new alleles into the population. The other two (natural selection and genetic drift) do so by removing some alleles from the population while preserving others. Mutation, migration (gene flow), and genetic drift act in a random fashion and so produce chance deviation from the Hardy-Weinberg equilibrium; only natural selection acts in a non-random or directed fashion and it does so by producing adaptation. For this reason natural selection is considered to be the centerpiece of modern evolutionary theory. Both drift and natural selection require genetically- based individual variation to effect evolution. What is the source of this variation in the population? The ultimate source of all genetic variation is mutation, but mutation does not occur frequently enough to be the major source of individual variation in any population. The proximate source of the genetic variation upon which drift and natural selection act in any one generation is sexual reproduction. Whereas mutation by itself can change gene frequency (and so is a causative factor of evolution), sexual reproduction cannot (see discussion of the Hardy-Weinberg law). That is not to say, however, that sexual reproduction is not important in evolution: it is extremely important as the major supplier of individual variation. Sexual reproduction, through Mendel's law of independent assortment and recombination, shuffles allelic variation (caused historically by past mutation) to produce the genotypic variation required by the theories of natural selection and genetic drift. Due to independent assortment and recombination no two sexually reproducing individuals are exactly alike. The same can be said for populations and species. For a long time, especially when taxonomy was the most influential of the biological disciplines, individual variation was dismissed as unimportant. Taxonomists searched for characters shared by all individuals in a species which would define the species to which they all belonged. This mentality of a common essence was inherited from Plato who argued that the real world consisted of unseen perfect forms which could be grasped by the intellect, and that the imperfect world revealed by the senses represented a mere shadow of the world of forms. Variation was a nuisance which clouded recognition of the perfect, immutable and eternal essence of each species. For these typologists, taxonomic characters shared by all individuals defined the species and these were unchangeable. Because variation was ignored, the concept of evolution was unthinkable! Only when individual variation was recognized as important and populational thinking replaced typological thinking was the possibility of evolution taken seriously. Without variation there could be no change over time; hence no evolution. The modern view of species defines them on the basis of reproductive compatability, not on shared morphological characters and recognizes individual variation as the key to understanding how species might change over time. The theory of evolution is simply a statement of species transformation over time with no mechanism proposed; hence it is the statement of an inductive pattern, as is the cell theory, rather than an explanatory theory. 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 to our understanding of the varied 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 to propose a plausible mechanism for the process of evolution: his theory of natural selection - the centerpiece of modern evolutionary theory. Darwinism 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 reproductive 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 (intersexual selection) or more successful in competing for mates with other members of their own sex (intrasexual selection). 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. This point is clear from a careful reading of Darwin's rationale presented above. 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. Assumptions of the theory of natural selection. Because the unit of natural selection is the individual, the theory of natural selection requires individual variation in phenotype, genotype and fitness. Without phenotypic variation in a population there is nothing for the environment to select. If phenotypic variation exists but is due to nongenetic factors which can modify phenotypes, e.g., nutritional differences, injury or disease, then selection can occur but will have no evolutionary significance since all individuals will be genetically identical. Differential reproduction of these phenotypic variants will not change gene frequency in the next gene pool. Nongenetic production of phenotypic difference results in environmental modification which must be clearly recognized as distinct from genetically-mediated phenotypic difference because the theory of natural selection requires genotypic variation underlying phenotypic variation. Finally, even if all individuals in a population differ phenotypically and genotypically, the process of natural selection will not cause evolution unless individuals differ in reproductive success (fitness). Hence, the theory of natural selection assumes individual variation in phenotype, genotype and fitness. If any one of the three forms of individual variation is missing, then evolution by the process of natural selection cannot occur. Natural selection and the production of adaptation. As we mentioned above the process of natural selection results in directed, rather than random, change in evolution and therefore is considered to be the most significant of the four evolutionary factors. In producing adaptation gradually over a number of generations, natural selection is a creative force in evolution - a point emphasized by Darwin but not fully appreciated by his critics. Most people appreciate how natural selection can remove the unfit, but fail to understand how it can produce the fit. For this reason Darwin's theory did not gain prominence until rather recently. After the rediscovery of Mendel's laws at the turn of the century and the development of genetics as a science, one would have thought that Darwin's theory would have gained immediate acceptance as the major force in evolution. After all, Mendel's work provided the information to vindicate Darwin's assumption of heritability behind adaptive variants (see point 5 in his rationale). Unfortunately, due to the influence of Hugo DeVries, one of the codiscoverers of Mendel's work, the early geneticists emphasized mutation as the mechanism behind the origin of species and relegated natural selection to the minor role of removing unfit individuals (the erosive action of natural selection). Darwin insisted, however, that natural selection created the fit rather than simply removing the unfit and so was responsible for gradual change in a species resulting in the origin of a new species. Natural selection acts as a creative force by preserving in a population individuals which are better adapted to that environment than others (the unfit which are removed). By preserving the better adapted individuals, selection indirectly preserves those genes which produced the adapted phenotypes, while eliminating genes which produced unfit phenotypes. Due to the process of sexual reproduction, parental genes are shuffled into new combinations among their offspring to produce a range of individual variation. Some offspring will therefore be less adapted than their parents, some equal in adaptation to their parents, but, and this is the important point, some will be better adapted. These better adapted offspring will then be selected as parents in the next generation and the process will be repeated. Gradually, adaptations will be improved by the process of natural selection so that after many generations it can be said that the adaptation was produced by the process of natural selection. Creationists often raise the objection that early stages in the origin of an adaptation via natural selection would have no benefit and so they argue that adaptations are created in their fully perfected form. How could a less than perfect eye benefit an individual?, they ask. A moment's reflection on this problem will provide the answer: a less than perfect eye is an improvement over no eyes at all! Thus, each stage in the evolution of an adaptation confers a reproductive advantage on its possessor over lesser adapted variants and so gradually over time natural selection directs evolution by producing adaptation.