Much biology prior to Darwin was rooted in what may be termed typological thinking. Species represented fixed types, and though it was recognized that there could be variants on the theme of a given type (e.g., domestic dogs), it was the type itself, not the variants that were of crucial importance. Charles Darwin's great achievement [3] was to turn this idea on its head by emphasizing the importance of heritable variation in populations of interest, and to show how evolutionary mechanisms – natural selection in particular – could not only account for the exquisite adaptations seen in nature – adaptations hitherto attributed to providential intelligent design – but could also account for the origin of species – the very types that earlier generations had supposed were fixed and unchanging.
Part of Darwin's aim in his work on natural selection was to show how it was possible for populations of organisms, over successive generations, to adapt to problems confronting them in the environments with which they interacted. The classical Darwinian explanation of the evolution of organismal characteristics known adaptations rests on three basic ideas: (1) Populations of organisms show variation with respect to certain inherited characteristics of their members. (2) Individuals in such variable populations differ in rates of survival and reproduction by virtue of their characteristics, thereby manifesting differential reproductive success; and (3) The heritable characteristics that contribute to differential reproductive success will often be inherited by the progeny of successful individuals. In short, evolution occurs when different individuals leave behind different numbers of offspring. Over successive generations, other things being equal, those characteristics contributing to reproductive success will manifest themselves as adaptations. Following Sober [[4]:85], we may define adaptation as follows:
Characteristic c is an adaptation for doing task t in a population if and only if members of the population now have c because ancestrally there was selection for having c and c conferred a fitness advantage because it performed task t.
It should be noted that organisms have characteristics that are not, properly speaking, adaptations. Consider your blood. There was certainly selection for molecules capable of bearing oxygen. The redness of blood, however, is not an adaptation. It is simply a consequence of the chemistry of iron. Similarly, while there has been selection in the human lineage for large, problem-solving brains, the ability to do differential calculus is not an evolutionary adaptation – it is rather an accidental by-product of selection for other characteristics.
Darwin himself was aware of the distinction between the physical, abiotic environment and the biotic environment (predators, prey, pathogens and parasites, etc.). To this we may usefully add culture as a dimension of the biotic environment. Culture and a capacity for cultural evolution is not unique to the human species, yet humans have transformed the environment with which they (and other species) interact – and humans, along with other species of organisms, have in turn been transformed by the effects of cultural evolution. Cultural evolution is fast – just consider the changes that occurred in the course of the twentieth century – and it occurs within the lifetimes of long-lived organisms such as ourselves. As we will see below, cultural evolution has important medical implications. For organisms like us, with relatively long intervals between generations, rapid evolutionary responses to cultural changes are typically not possible, leading to the phenomenon of environment discord. For organisms with much shorter intervals between generations – every twenty minutes for bacteria such as Staphylococcus aureus, a major cause of wound infection – rapid, heritable, adaptive responses to such environmental/cultural products as antibiotics are not only possible, but have become a major medical problem. This is one of the major reasons why an understanding of the evolutionary phenomenon of host-parasite co-evolution is of vital importance.
Darwin knew virtually nothing about the mechanisms of inheritance and had precious little knowledge of organic chemistry (biochemistry was still a largely unformed intellectual fetus during his lifetime). After 1900, with the rediscovery of Mendel's insights about the particulate factors involved in the inheritance of characteristics, genetics emerged as a science in its own right. Of crucial importance here are events occurring from the 1920s through the 1950s – a period that gave rise to what historians of biology know as the new evolutionary synthesis. Here ideas about genetics were fused with ideas about evolution. The result was that population genetics – especially the study of the ways in which the relative frequencies of variant forms of genes (alleles) can change over successive generations – became the corner stone of modern evolutionary thought. In the course of this intellectual revolution, natural selection (resulting in adaptations) emerges as but one way in which allele frequencies can change. Other mechanisms that can shift allele frequencies include random genetic drift, gene flow (the effects of emigration and immigration), assortative mating, and a variety of linkage effects. As an understanding of bacterial evolution grew, it gradually became clear that, in addition to "vertical" gene transfers across the generations, there are occurrences of "horizontal" gene transfers (e.g., genes conferring resistance to various drugs can be exchanged between members of an extant population – there may even be cross-species horizontal transfers). Such transfers, when they occur, can permit extremely rapid evolution.
In the last quarter century enormous strides have been made as evolutionary biologists have learned the need to fuse their gene-based perspective on evolution with insights drawn from developmental biology. The resulting ideas – discussed under the rubric of evolutionary developmental biology – have come to constitute an intellectual revolution in their own right. The results of various genome projects have shown an enormous genetic similarity between humans, chimps, dogs and mice. At the level of the genes centrally involved in development (e.g., the so-called Hox genes), we are virtually identical. Notwithstanding this, humans and our evolutionary relatives are clearly very different types of organisms. It is now beginning to emerge that the key to understanding this diversity in the face of so much similarity is the study of gene regulation. For a crude analogy, two identical piano keyboards can play very different tunes – what matters is the order and timing with which the keys are played [see [5-10]].
As evolutionary biology has itself evolved, so too have its implications for the biomedical sciences and the practice of medicine. In the last twenty-five years, a growing number of evolutionary theorists have started to build bridges between evolutionary biology and the biomedical sciences [2,11]. This has culminated in the emergence of a new discipline called Darwinian Medicine. Darwinian Medicine is not offered as an alternative to existing branches of medical inquiry, but rather as a means of enriching our current understanding of biomedical phenomena [12]. It's a two-way street: as evolution enriches our understanding of medical phenomena, medicine enriches our understanding of evolutionary principles. For example, studies of the nature of humoral immunity [13,14] as well as cancer [15], have given evolutionary biologists valuable insights into the mechanisms of adaptive evolution as they shape the fates of populations of cells in multi-cellular organisms. Some of these examples are discussed later in this essay. We begin, however, with examples of the importance of evolutionary ideas to some of the biomedical sciences.
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