Copyright © 1999 Massachusetts Institute of Technology
In its simplest form, the theory of evolution is just the idea that life has changed over time, with younger forms descending from older ones. This idea existed well before the age of Charles DARWIN, but he and his successors developed it to explain both the diversity of life, and the adaptation of living things to their circumstances. Ernest Mayr (1991) argues that this developed conception of evolution combines five main ideas:
Darwin, Wallace, and others rapidly convinced their scientific contemporaries of the fact of evolution (Darwin 1859/1964; Wallace 1870). They persuaded that community of the existence of the tree of life. Darwin himself was a gradualist, thinking that tiny increments across great periods of time accumulate as evolutionary change, and he thought that the main agent of that change was natural selection. But his views on gradual change and on the importance of selection were not part of the biological consensus until the synthesis of population genetics with evolutionary theory by Fisher, Wright, Haldane, and others in the 1930s (Depew and Weber 1995). The importance of isolation in the generation of new species remained controversial even longer. It became part of the consensus view on evolution only after Mayr's postwar work on speciation and evolution (Mayr 1942, 1976, 1988).
The biological world confronted Darwin, Wallace, and their successors with two central problems. The world of life as we know it is fabulously diverse, even though today's life is only a tiny fraction of its total historical diversity. We tend to underestimate that diversity, because most large animals -- animals that we notice -- are vertebrates like us. But many organisms are weirdly different from us and from one another, and weird not just in finished adult form, but also in their developmental history. Humans do not undergo major physical reorganizations during their growth from children, whereas (for example) many parasites' life cycles take them through a number of hosts, and in their travels they experience complete physical transformation. Yet though life is diverse, that diversity is clumped in important ways. Arthropods have jointed, segmented bodies with various limbs and feelers attached, the whole covered with an exoskeleton. They are very different from anything else, from vertebrates, worms, and other invertebrates. Before Darwin, the differences between arthropods, vertebrates, worms, and other great branches of life seemed so vast as to rule out evolutionary transitions between them. They are so distinctive that even after the universal acceptance of the evolutionary descent of life, arthropod affinities remain controversial. Thus one task of evolutionary biology is in the explanation of diversity and its clumping, both on the large scale of different kinds of organism, and on the smaller scale of the difference between species of the same general kind.
If diversity is important, so too is ADAPTATION. The structured complexity of organisms, and their adaptation to their environment, is every bit as striking as the diversity of organisms through their environment. Perceptual systems are classic examples of complex, fine-tuned adaptation. Bat ECHOLOCATION requires mechanisms that enable bats to produce highly energetic sound waves. So they also have mechanisms that protect their ears while they are making such loud sounds. They have elaborately structured facial architectures to maximize their chances of detecting return echoes, together with specialized neural machinery to use the information in those echoes to guide their flight to their target. But there are many other examples of complex adaptation. Many parasites, for example, manufacture chemicals that they use to manipulate the morphology and behavior of their host.
Darwin's greatest achievement was to give a naturalistic explanation of adaptation. His key idea, natural selection, can explain both adaptation and diversity. Imagine the population ancestral to the Australasian bittern. Let us suppose that this population, like current bitterns, lived in reeds adjacent to wetlands and sought to escape predation by crouching still when a threatening creature was near. It is quite likely that the color and pattern of the plumage of this ancestral population varied. If so, some birds were favored. Their plumage made them somewhat harder to see when they froze among the reeds. They were more likely to survive to breed. If the plumage patterns of their offspring were like those of their parents, the plumage patterns of the descendant generation would be somewhat different from that of the ancestral generation. Over time, the colors and patterns characteristic of the population would change. Thus we could reach today's superbly well-concealed bitterns. Natural selection selects fitter organisms, and the heritability of their traits ensures a changed descendant population. Evolutionary change depends on variation in a population, fitness differences in the population consequent on that variation and heritability. Adaptive change takes place despite the fact that the mechanisms that generate variation in the population are decoupled from the adaptive needs of the population. But it depends on more than those principles. The adaptive shift to good camouflage took place gradually, over many generations. It depended on cumulative selection. If selection is to explain major adaptation it must be cumulative. Innovation is the result of a long sequence of selective episodes rather than one, for the chances of a single mutation producing a new adaptation are very low.
Thus evolution under natural selection can produce adaptation. At the same time, it can produce diversity, as populations become adapted to different local environments, and thus diverge from one another.
Evolutionary biology has developed a consensus on the broad outline of life's history. There is agreement on important aspects of the mechanism of evolution. Everyone agrees that selection is important, but that chance and other factors play an important role too. No one doubts the importance of isolation in generating diversity. But important disagreements remain. The nature of species and speciation remains problematic. Although everyone agrees that selection, chance, history, and development combine to generate life's history, the nature of that combination remains controversial. Though all agree that selection matters, the mode of its action remains contested. Dawkins and others think of selection as primarily selecting lineages of genes in virtue of their differing capacities to get themselves replicated (Dawkins 1982). Others -- for example, Gould (1989) and Sober (1984) -- conceive of selection as acting on many different kinds of entities: genes, organisms, colonies and groups, and even species. Finally, some -- David Hull (1988) being one -- think of evolution in biology as just a special case of a general mechanism of change involving undirected variation and selective retention. These controversial ideas lead to attempts to give evolutionary accounts of scientific and cultural change.
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Williams, G. C. (1992). Natural Selection: Domains, Levels and Challenges. Oxford: Oxford University Press.
Copyright © 1999 Massachusetts Institute of Technology