Constraints on Evolution
The many examples of adaptations that we have just discussed are testimony to the power of natural selection, but evolution is limited by a serious constraint: Evolutionary changes must be based on modifications of previously existing traits, which may come to serve new functions. Engineers are able to design a completely new type of engine (jet) to power an airplane that can replace a previous type (propeller), but evolutionary changes cannot happen that way. A striking example of such constraints on evolution is provided by the evolution of fish that spend most of their time resting on the sea bottom. One lineage, the bottom-dwelling skates and rays, is beautifully symmetrical. These fishes are descended from sharks whose bodies were already somewhat flattened; therefore, skates and rays are able to lie on their bellies. Plaice, sole and flounders, on the other hand, are bottom dwelling descendants of deep-bodied, laterally flattened ancestors. Unlike sharks, these fishes cannot lie on their bellies; they must flop over on their sides. During development, the eyes of plaice and sole are grotesquely twisted around to bring both eyes to one side of the body. Small shifts in the position of one eye probably helped ancestral flatfishes see better resulting in the form found today.
Traits can evolve by natural selection only if they are at least partly heritable. However, individuals may acquire new traits via cultural evolution— that is, by learning them from other individuals. Cultural evolution is most highly developed in humans, whose language and remarkable learning abilities enable new innovations to spread and be adopted at rapid rates. But the only requirement for traits to evolve via cultural evolution is that individuals have the ability to learn them. Birds, for example, copy the songs of other individuals, resulting in the evolution of song “dialects.” Many behaviors of the apes (chimpanzees, gorillas, gibbons, and orangutans) are transmitted via learning. In one study, investigators compared the behavior of four orang-utan populations on the island of Borneo and two on Sumatra. The investigators identified 24 behaviors that are restricted to a single population. These behaviors are not correlated with any differences in the environments in which the populations live. Ten of the behaviors are specialized feeding techniques, including tool use. Six are alternative forms of social signals, such as kiss-squeaks. Thus, orangutan populations develop cultural distinctions as individuals copy the behavior of other individuals.
Short-Term versus Long-Term Evolution
The short-term changes in allele frequencies within populations that we have emphasized in this chapter are an important focus of study for evolutionary biologists. These changes can be observed directly, they can be manipulated experimentally, and they show us the actual processes by which evolution occurs. By themselves, however, they do not enable us to predict — or, more properly, “postdict” (because they have already happened). The reason is that patterns of evolutionary change can be strongly influenced by events that occur so infrequently or so slowly that they are unlikely to be observed during shortterm studies. In addition, the ways in which evolutionary agents act may change with time; even among the descendants of a single ancestral species, different lineages may evolve in different directions. Therefore, additional types of evidence demonstrating the occurrence of rare and unusual events and trends in the fossil record, must be gathered if we wish to understand the course of evolution over billions of years. “Postdiction” problems are not unique to evolutionary studies. For example, seismologists know the physical principles that explain how earthquakes occur, and they can pinpoint regions that are prone to earthquakes; but they cannot predict when or where an earthquake will happen. In subsequent chapters, we will discuss the kinds of information that biologists assemble to study long-term evolutionary changes and infer the processes that led to them.
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