Darwin's Impact
The immediate result of Darwin’s theory was that most naturalists in the English- and German-speaking worlds accepted evolution, but withheld judgment on natural selection, for want of evidence that fulfilled the standards of "positivist science." Even T. H. Huxley, the ardent defender of Darwin, wanted to see evolution confirmed in a laboratory setting. Most evolutionists of the late 19th century were Darwinians only in a rhetorical sense.
Because there was no theory of heredity before the simultaneous "rediscovery" of Mendel’s laws of genetics by three biologists in 1900, natural selection (i.e., Darwinism) went into eclipse in the 1890s and was not fully revived—in its neo-Darwinian form—until the 1920s. Most American evolutionists of this period were neo-Lamarckians: that is, they thought that evolution was owing to the inheritance of acquired characteristics pushed by some drive internal to the organism. Some of these men, such as Edward Drinker Cope, were also orthogeneticists, believing that evolutionary trends were regular, non-adaptive, and described a linear pattern.
Another variation on this theme was called orthoselection: once a species had begun to specialize for a particular way of life, selection could be expected to produce trends consistent with this advantageous direction. Orthogenesis was based on idealistic philosophies which held that formal morphological relationships took precedence over both function and environment. Various facets of paleontology inclined its practitioners toward orthogenesis: you could ignore possible side branches and regard your evidence as complete and could also regard any odd characteristics of an extinct species as non-adaptive. So orthogenesis looked for and ostensibly found predetermined, regular patterns.
Early Mendelians were not population-oriented. For them, mutations created new species. Field naturalists and paleontologists alike were convinced of the gradual nature of evolution and hence were hostile to genetics until such a time when mutationism lost its influence. All of these alternative theories were ways of looking for order in the face of diversity. They were non-traditional in that all opposed the old essentialist concept of organic form (morphology) and realized that it could not serve as a mechanism for evolution. So in the period when Darwinism was in eclipse, all evolutionists looked for mechanisms that transcended the view—dating back to Cuvier at least—that organic forms were based on a narrow range of predetermined morphological types.
One of the crucial steps in the resurgence of Darwinism was the recognition that the genetic structure of populations is much more complex than had first been thought, and that selection for adaptive advantage might increase the frequency of some genes within the population at the expense of others. This development arose with the three founders of theoretical population genetics, R. A. Fisher and J. B. S. Haldane in England, and Sewall Wright in the United States, who worked out statistical techniques for evaluating the nature of genetic variation in "gene pools." This work was done in the 1920s.
Meanwhile, since 1910, T. H. Morgan had been experimenting with Drosophila and perfecting laboratory techniques for studying mutations and investigating the nature of "microevolution" (evolution within a species). Darwin’s studies of artificial selection as practiced by breeders convinced him that selection was most effective when acting on small continuous variation, but early Mendelians thought that the genetically most important variations were discontinuous and caused by mutations (mutation theory of Hugo de Vries). Fisher was among the first to deny that Mendelism was necessarily associated with discontinuous inheritance. Thus, although early Mendelians thought Darwinism incompatible with Mendelian genetics, by 1918 many had come to view the two as complementary.
Fisher worked on the mathematical parameters of variation and showed that small variations acted just as Darwin said they would. At the same time—in the 1920s—Sewall Wright studied interactions of systems of genes, not just single genes, and demonstrated that selection operates on entire interactive systems. The random drift of genes caused by inbreeding was important for the creation of novel interactive systems. Fisher concluded that natural selection acted most efficiently in large populations because more variant genes were stored there, while Wright thought selection acted most effectively in smaller systems. The third father of theoretical population genetics was J. B. S. Haldane, who quantified natural selection, demonstrating that it was a reality and that it selects Mendelian genes.
Then Theodosius Dobzhansky extended Morgan’s techniques to the study of wild populations of Drosophila and showed that (contrary to the "classical" hypothesis of population genetics) natural selection acts to maintain a fund of variability within a species that can be exploited by natural selection when conditions change.
A second arena of Darwin’s continuing influence is ecology. It is fair to argue that before Darwin, there was no field of ecology and nothing that we would recognize as being as comprehensive as the ecological awareness of present times. The term "ecology" is post-Darwinian; it was coined by the German evolutionist Ernst Haeckel in 1866. The term that was used previously in English was "the economy of nature," to indicate interaction of all the Earth’s organisms. The difference between the two terms is, first, there was no agreement as to what were the rules governing the interaction of such an economy: it was at best a metaphor suggesting the give-and-take of economic transactions. Darwin’s approach was different: his notion of adaptation required interaction between organisms and their niches, and it also provided a framework not only for understanding the mechanism involved, but also a guideline for future research programs.
Darwin’s understanding of adaptation was originally inspired by theologian William Paley’s notion of "perfect adaptation" (that God creates each organism perfectly adapted to its environment), which is well known, but also from his reading of Linnaeus’s book on the economy of nature which he read in English translation in the 1840s. The notion that different species have an "allotted place" or "proper business"—terms that appear in Darwin’s writing after his reading of Linnaeus—was a powerful legitimation of his developing concept of adaptation and natural selection. It has been argued that post-Darwinian ecology offered a way to escape from an increasingly fragmented biological science and to recover the lost academic wholeness that had characterized Natural History—only now in the spirit of Darwin.
Natural History had encompassed botany, zoology, and geology, conjoined because all were descriptive. The reorganization of the life sciences in the wake of Darwin forced the merger of zoology and botany into a comprehensive field of "biology" (a term coined by Lamarck—another evolutionist—but which had no institutional repercussions until after Darwin). In the first decades of the 20th century, subfields of ecology appeared: one such was "autoecology" a highly technical laboratory approach to ecology, inasmuch as its focus was on such topics as phototropism, which required the microscopic study of plant physiology at the cellular level. That was not, however, the main thrust of ecology, described by Victor Shelford in 1919 as the "science of communities." It could be reckoned one of the academic homes for what Ernst Mayr has called "population thinking," which, of course, descended from Darwin.
With biologists like the geneticist Theodosius Dobzhansky, one the founders of the Synthetic Theory, Darwinian population thinking underlay the conceptualization of population genetics, whose approach was to use techniques developed in the laboratory to study the genetics of the fruit fly, for example, in wild populations, to see evolutionary processes, like natural selection, firsthand. This technique Dobzhansky refined in Brazil in the 1940s and ’50s, among other places, and while he was there he invented tropical ecology. For a Darwinian biologist like Dobzhansky, genetics and ecology were a continuum of closely related problems.
Evolutionary Ecology, a relatively recent field, deals with the intersection of evolution and ecology, which amounts to the study of adaptation. The study of "mutualisms" is a typical interest of this field and deals with the co-evolution of species interacting in the same environment.
Medical Evolution
Evolution, genetics, and medicine share a long and distinguished tradition. Although evolutionary biology and genetics merged during the grand synthesis of the mid-20th century, and are now inextricably linked, medicine has largely remained isolated from the synthesis. Paradoxically, the early evolutionary insights of pioneering physicians such as Archibald Garrod and Wilhelm Weinberg laid the foundation for much of present-day medical and human genetics, prior to the grand synthesis. Friedrich Meischer’s discovery of DNA and Oswald Avery’s identification of its function further revolutionized genetics and enriched many aspects of evolutionary biology: constructing evolutionary lineages across kingdoms of living organisms, testing many evolutionary hypotheses, and identifying genes underlying many human disorders.
Classical evolutionary ideas such as adaptive landscape, polygenic variation, maintenance of sex ratios, effective population sizes, founder effects, population structure, neutrality, coalescent theory, gametic/linkage disequilibria, phylogenetic relationships, and haplotype diversity have collectively had a major impact on our understanding of human genetic variation and evolution. From a molecular perspective, the origins, maintenance, and consequences of the enormous amount of variation emerging from the genome sequences need to be interpreted. Epigenetic mechanisms—still incompletely understood and identified but explained by evolutionary ideas—appear to govern the expression and timing of many human disorders.
Newer ideas such as the evolution of life history traits (fitness components), evolution in age-structured populations, reaction norms, units of selection, multivariate selection on phenotypes, and genetic robustness have had little impact on medicine despite their great impact on evolutionary anthropology. These new ideas and data are pointing toward the ubiquitous role of evolutionary forces in understanding human development and reproduction, longevity and health, as well as in the classically understood evolution of drug resistance and virulence in pathogens and drug response in humans.
Evolutionary thinking can be used in many ways to understand human health and disease. Does the history of human colonization with its unique exposure of populations to particular infectious agents shed light on the incidence of diseases, our capacity to process drugs, and our resistance to pathogens? How do the magnitude and pace of environmental change interact with evolved developmental programs in the etiology of complex diseases? Applications of evolutionary principles can help to predict the future course of human diseases and to treat patients as products of selection and evolutionary history. In its turn, medicine can contribute to evolution, for evolutionary models can be tested with the vast amount of data emerging from human genetic and genomic research. These results will illuminate both evolutionary biology and human health.
One of the most newsworthy research programs in evolutionary medicine is the observation that evolutionary forces such as natural selection, mutation, recombination and genetic drift account for the diversity of disease-causing pathogens, including viruses, particular the HIV and bacteria. Disease-causing pathogens are a fecund area of research into evolutionary mechanisms because the evolutionary process in microorganisms is speeded up due to their short generation cycle relative to their human and other hosts. While both genetic and environmental factors influence the genetic diversity of pathogens, the host immune system destroys a substantial amount of the novel forms of pathogens in the infected individuals. Very low frequencies of resistant strains, however, survive both the action of immune systems and modern drugs. Evolution of numerous vaccine-resistant pathogens such as HIV, tuberculosis, malaria, etc. is a sober reminder of the central role of evolution in public health and medicine.
Such rapid replication and the huge populations of the organisms studied means that large numbers of mutation will be generated: viruses mutate constantly because the DNA/RNA replication process is prone to error. And the large number of mutant DNA/RNA ensures not all mutants will be selected out. The environment that does the selecting is the immune system of the infected individual. The immune system detects and eliminates what it can, but many mutant forms survive to be "selected" by their human host, which is, of course, a paradoxical outcome that explains the tenacity of this virus. The HIV virus has the capacity to mutate in the face of a drug that threatens its existence.
The rapid emergence of medical evolution as a dynamic new specialty demonstrates the undiminished power that Darwin’s theory has to generate novel research programs, a trend that is most likely to surface repeatedly as the life sciences continue to mature and broaden their scope.
