- Darwin and the first evolutionary synthesis: Its grandeur, constraints, and difficulties
- Genetics and the "black day" of Darwinism
- Population genetics, Fisher's theorem, fitness landscapes, drift, and draft
- Positive and purifying (negative) selection: Classifying the forms of selection
- Modern Synthesis
- Recommended further reading
In his book On the Origin of Species, Charles Darwin meticulously collected evidence of temporal change that permeates the world of living beings and proposed for the first time a plausible mechanism of evolution: natural selection. Evolution by natural selection certainly is one of the most consequential concepts ever developed by a scientist and even has been deemed the single most important idea in human history (Dennett, 1996). Somewhat paradoxically, it is also often branded a mere tautology, and when one thinks in terms of the survival of the fittest, there seems to be some basis for this view. However, considering the Darwinian scenario as a whole, it is easy to grasp its decidedly nontautological and nontrivial aspect. Indeed, what Darwin proposed is a mechanism for the transformation of random variation into adaptations that are not random at all, including elaborate, complex devices that perform highly specific functions and so increase the fitness of their carriers. Coached in physical terms and loosely following Erwin Schroedinger's famous treatise, Darwinian evolution is a machine for the creation of negentropy—in other words, order from disorder. I submit that this was the single key insight of Darwin, the realization that a simple mechanism, devoid of any teleological component, could plausibly account for the emergence, from random variation alone, of the amazing variety of life forms that appear to be so exquisitely adapted to their specific environments. Viewed from that perspective, the "invisible hand" of natural selection appears almost miraculously powerful, and one cannot help wondering whether it is actually sufficient to account for the history of life. This question has been repeatedly used as a rhetoric device by all kinds of creationists, but it also has been asked in earnest by evolutionary biologists. We shall see in the rest of this book that the answers widely differ, both between scientists and between different situations and stages in the evolution of life.
Of course, Darwinism in its original formulation faced problems more formidable and more immediate than the question of the sufficiency of natural selection: Darwin and his early followers had no sensible idea of the mechanisms of heredity and whether these mechanisms, once discovered, would be compatible with the Darwinian scenario. In that sense, the entire building of Darwin's concept was suspended in thin air. The rediscovery of genetics at the beginning of the twentieth century, followed by the development of theoretical and experimental population genetics, provided a solid foundation for Darwinian evolution. It was shown beyond reasonable doubt that populations evolved through a process in which Darwinian natural selection was a major component. The Modern Synthesis of evolutionary biology completed the work of Darwin by almost seamlessly unifying Darwinism with genetics. As it matured, Modern Synthesis notably "hardened" through indoctrinating gradualism, uniformitarianism, and, most important, the monopoly of natural selection as the only route of evolution. In Modern Synthesis, all changes that are fixed during evolution are considered adaptive, at least initially. For all its fundamental merits, Modern Synthesis is a rather dogmatic and woefully incomplete theory. To name three of the most glaring problems, Modern Synthesis makes a huge leap of faith by extending the mechanisms and patterns established for microevolution to macroevolutionary processes; it has nothing to say about evolution of microbes, which are the most abundant and diverse life forms on Earth; and it does not even attempt to address the origin of life.