Adaptation is the evolution of characteristics better suiting an organism to its environment. Or perhaps, in view of considerations to be dealt with further below, adaptation is best describable more vaguely as evolution better suiting whatever is evolving to its environment. [Of course, we can also speak of “an adaptation”, as a characteristic that strongly favors the survival of an organism in its environment, and even as “adaptation” as being the state reached by adaptive evolution, but I will try to stick to just the one meaning in this essay.]
Clearly, adaptation is close to just being evolution driven by natural selection, which in the longer term is the replacement of one favorable mutation by another even more favorable (in Monod’s (1972) terms, the action of necessity on the products of chance). The distinction is worth retaining because evolution can lead to lower fitness for a lineage or population (Blick, 1977; Crozier 1979; Paquin and Adams, 1983); I won’t discuss these cases here either, save to point out that Darwin, lacking knowledge of modern genetics, did not realise that selection could lead to lower fitness. Here especially we see further than Darwin, standing on his shoulders.
Darwin had, of necessity, to infer that adaptation occurs by documenting the end results and by considering the Malthusian pressures certain to bring it about. For humans to observe adaptation in progress, it has to be rapid; since Darwin’s time both strong selection and rapid adaptive evolution have been observed many times. It is worthwhile looking at some of these instances.
Most of the cases of rapid evolution that we know of have been driven by human activities. This happenstance is understandable from standard evolutionary theory, because a changing environment is the simplest cause of strong selection, and we are the major perturbing force on the planet today. Some of these cases involve introductions, such as of the house sparrow, Passer domesticus, into North America.
Winter stresses, particularly due to storms, form a major component of selection acting on North American sparrow populations (Bumpus, 1899; Fleischer and Johnston, 1982; Lande and Arnold, 1983). These winter stresses lead to both stabilizing and directional selection on the birds, and fall differently on males and females (Fleischer and Johnston, 1982).
From their reanalysis of Bumpus’s (1899) data, Lande and Arnold (1983) estimated that, in females, departure by one standard deviation from the mean in either direction for the first principal component led to a decrease in relative fitness by 45%, and directional selection showed similar but lesser strength for several morphological measures (Lande and Arnold, 1983; Fleischer and Johnston, 1982). We would therefore not be surprised to find that significant changes had emerged in house sparrow populations since their introduction, and indeed such changes have occurred. Not only have the various North American populations differentiated markedly since introduction (Johnston and Selander, 1971), but also similar (but not yet identical) trends in variation with climate have emerged to those seen in the ancestral European populations (Murphy, 1985; Figure 1).
Fig. 1. Body size of house sparrows (divided according to origin and sex) versus annual temperature range. Size was measured by the first principal component derived from 14 skeletal measurements. Each point plotted represents the mean of a locality at which at least ten individuals were measured. Taken from Murphy (1985), reprinted with permission.
Fig. 2. Evolution of resistance to insecticides by houseflies on Danish farms. The width of each bar reflects the extent of use, the inverted triangle symbol the date of the first confirmed case of resistance of a given insecticide, and the letter R the date when most populations were resistant. Open bars refer to residue sprays and solid ones to other methods of application. From Wood and Bishop (1981), reprinted by permission.
Fig. 3. Effects of radiation on inbred Drosophila serrata populations. An inbred population was split into three parts, two of which were irradiated. The resulting changes in population size are readily interpreted as indicating the induction of favorable mutations in the irradiated populations. Other experiments gave similar results. From Ayala (1966), reprinted by permission.
Although this does not appear to be so, the house sparrow case should be as famous as the text-book case of rapid evolution, that of the large-scale replacement of the normal white-peppered phenotype of the moth Biston betularia by a dominant allele for essentially black moths. The Biston case provides a further example of humaninduced changes, although this time through the modification of the environment: the killing by air pollution of encrusting lichens yielded black rather than light-colored tree-trunks, resulting in a shift in bird predation pressure in favor of the previously-unknown allele.
Many other cases are known of both strong selection and the response to it. Of course, the peppered moth and house sparrow cases are almost exclusively scientific in their appeal, but natural selection can be of practical importance as in the many cases of the evolution of resistance by insects to insecticides (Figure 2) or by bacteria to drugs. Resistance to biocides usually arises with dismaying speed, as perusal of the figure will show. The speed is both understandable (we are applying extremely strong selection pressures) and serious: we are in an evolutionary race between our ability to produce new compounds and that of the pest species to produce counteracting mutations in its degradative enzymes I’ll allow a somber thought to intrude here: as we become an evermore important part of the planet’s total biomass, we thereby increase the selection payoff for pests to attack us, as well as our crops.