CHAPTER 7

ADAPTEDNESS AND NATURAL SELECTION: ANAGENESIS

How can we explain why organisms are so remarkably well adapted to the environment in which they live? When preoccupied with other thoughts, we take this adaptedness very much for granted. Of course, a bird has wings to fly with and other attributes that are needed for its aerial existence. Of course, a fish has a streamlined shape and fins to enable it to swim; it has gills to take up the needed oxygen. So it is with all the properties of adapted organisms. But when you think about this more deeply, you begin to wonder how this admirable world of life could have reached such astonishing perfection. By perfection I mean the seeming adaptedness of each structure, activity, and behavior of every organism to its inanimate and living environment. Examples of such seeming perfection are structures such as the eye of vertebrates and insects, or the annual migrations of birds to their tropical winter quarters and their return with extraordinary precision to the spot from which they had started the previous autumn, or the admirable cooperation of the members in a colony of social insects, such as ants or bees.

As far back as we have written records, an occasional thinker or founder of a religion would ask these questions of why and how. Before the rise of science, only revealed religion could give an answer. Indeed, during the seventeenth and eighteenth centuries, adaptations were considered by the pious to be proof for the existence of a wise creator who had designed every created organism with the appropriate structures and behaviors needed for its particular place in nature (e.g., William Paley). Natural theology, the study of the work of the creator, was considered a branch of theology. This interpretation of the design of the living world is still defended in this age of science by the creationists.

Yet the claims of natural theology ran into considerable difficulties. Yes, wolves kill sheep, but it was argued that the creator had created sheep specifically so that the wolves would not die of starvation. A closer study of living nature, however, revealed an alarming amount of brutality and waste. As scientists came to understand more and more about the natural world, the credibility of perfect design by a benign creator further declined. Consideration of how God could have carried out his task of Creation raised even more serious difficulties. The manifold adaptations of structure, activity, behavior, and life cycle for each of the millions of species of organisms were far too specific to be explained by general laws. On the other hand, it seemed quite unworthy of the creator to believe that he personally arranged every detail in the traits and life cycles of every individual down to the lowest organism. The analysis of parasitism and other seemingly rather cruel aspects of the living world added to the loss of credibility. It came as a considerable relief for the thinking naturalists of the nineteenth century when they were able to replace the supernatural explanation of natural theology by a naturalistic explanation. However, to find a workable naturalistic explanation turned out to be a very difficult task.

The process of adaptation fitted very well with the thinking of natural theology and with Aristotle’s belief in a “final cause.” Adaptation in the non-Darwinian orthogenetic theories of evolution was attributed to intrinsic final causes. Even after 1859 many antiselection-ist evolutionists still considered adaptation to be a more or less finalistic process. Actually there is no trace of any finalistic factor in the Darwinian explanation of the process of adaptation.

Darwin proposed an explanation of adaptation, based on population thinking, that succeeded in refuting all attacks since made against it. This was the application of the theory of natural selection to the process of adaptation (see Chapter 6), in which a character of an organism is an adaptation when among the variable populations of the ancestors it had been favored by nonelimination. The process of eliminating the less well adapted organisms results in the survival of the better-adapted individuals. Since this is equally true for the offspring of every set of parents in the population, the population as a whole remains well adapted or, perhaps, even increases in adaptedness.

DEFINITION OF ADAPTATION

There must be literally hundreds of definitions of adaptation in the literature. Ultimately, most agree that a trait is adaptive if it enhances the fitness (however defined) of an organism, that is, if the trait contributes to the survival and/or better reproductive success of an individual or social group. Or: an adaptation is a property of an organism, whether a structure, a physiological trait, a behavior, or any other attribute, the possession of which favors the individual in the struggle for existence. We believe that most such traits were acquired by natural selection or, if they arose by chance, their maintenance was favored by selection.

In determining what qualifies as an adaptation, it is the here and now that counts. It is irrelevant for the classification of a trait as an adaptation whether it had the adaptive quality from the very beginning, like the external skeleton of the arthropod, or acquired it by a change of function, like the swimming paddle of a dolphin or a Daphnia. One must always remember that adaptation is not a teleological process, but the a posteriori result of an elimination (or of sexual selection). Being an a posteriori process, the earlier history of a piece of the phenotype is therefore of little relevance for its adaptive value. The recognition of an adaptation is facilitated if it also occurs in other preferably unrelated organisms living in a similar environment, or if the adaptive quality of the character can be modified by appropriate experiments. One way to assess adaptations is by studying the variation of the adaptive character in variable natural populations. For an analysis of the problem of how to define adaptation, see West-Eberhard (1992) and Brandon (1998).

WHAT IS THE MEANING OF THE TERM ADAPTATION?

Unfortunately, the word adaptation is used in the evolutionary literature for two entirely different subjects, one legitimate and the other not. This has created a great deal of confusion.

The legitimate use of the term adaptation is for a property of an organism, whether a structure, a physiological trait, a behavior, or anything else that the organism possesses, that is favored by selection over alternate traits. But the term also has been used quite incorrectly for the process (“adaptation”) by which the favored trait was actively acquired. This view can be traced back to the ancient belief that organisms had an innate capacity for improvement, for steadily becoming “more perfect.” Also, if one accepts an inheritance of acquired characters, activities such as the straining of the neck by giraffes “adapts” the neck to an improved construction. In this view, adaptation is an active process with a teleological basis. Some recent authors still seem to look at adaptation as such a process and therefore reject the whole concept of adaptation. But this is not defensible.

Adaptation is a completely a posteriori phenomenon for a Darwinian, that is, it is based on the inductive assessment of facts. In every generation, all individuals that survive the process of elimination are de facto “adapted” and so are their properties that enabled them to survive. Elimination does not have the “purpose” or the “teleological goal” of producing adaptation; rather, adaptation is a by-product of the process of elimination.

To avoid the ambiguity of the word adaptation, it is preferable to use the word adaptedness for the state of being adapted. There is, however, no reason not to use the term adaptation for a property acquired or maintained by natural selection because it provided superior survival chances in competition with other individuals. Many adaptations acquire a new role through change of function such as the swim bladder of fishes from lungs, or the middle ear bones of mammals from bones of the reptilian jaw articulation. The process of adaptation is a strictly passive one. Individuals that do not have as good an adaptation as others are eliminated, but the survivors do not contribute to the process of becoming better adapted by any special activities, as proposed in teleological theories of evolution. It is not particularly helpful to make a terminological distinction between adaptations that previously had a different role and those that originated as a consequence of the role they still fulfill. In addition to having specific adaptations, an organism as a whole is adapted to its environment.

Box 7.1 Low Fertility of the Large Albatrosses (Diomedea)

Characteristic	Albatross	Most birds
Number of eggs	1	2-10 plus
Age at first reproduction	7-9 years	1 year or less
Sexual cycle	2 years or more	1 year or less
Life expectancy	Estimated to be 60 years or more	Mostly less than 2 years

The adaptations for optimal reproductive success possessed by certain species are quite astonishing. The large albatrosses of the waters of the Southern Ocean have only a single young every second year and do not enter the age of reproduction until they are seven to nine years old. How could natural selection have led to such a reduction of fertility? It was found that only the most able and experienced birds can find enough food to raise their young in this zone of incessant, powerful storms. On the other hand, they have the advantage of being able to establish breeding colonies on predator-free islands without any serious competitors. Hence the delay in age of reproduction and a reduction in the number of offspring were of selective advantage. The breeding cycle of the Emperor Penguin is another example. These birds court and lay their single egg under the most adverse conditions at the beginning or in the middle of the Antarctic winter, a season of frequent blizzards. The advantage of this timing is that the young hatch at the beginning of the southern spring and are raised during the southern summer, when conditions for their survival and growth are at an optimum. Such drastic reduction of fertility in albatrosses and penguins is compensated by an increased longevity of the adults and by the absence of predators from their breeding colonies on islands or on the Antarctic ice. Adaptations of extreme specialists, such as parasites, are sometimes even more astonishing.

TO WHAT IS AN ORGANISM ADAPTED? WHAT IS A NICHE?

We commonly say that a species is adapted to its environment. But this is not a sufficiently precise answer. A species shares its environment with hundreds of other species. For a hummingbird of the tropical forest who feeds in the canopy and builds there her nest, it is irrelevant whether or not some rocks are lying on the forest floor. Every species is adapted to a rather restricted selection of properties of the environment. These properties are certain general conditions (mostly climatic), but also specific resources (food, shelter, etc.). This specific set of environmental properties provides a species with the required living conditions called its niche. There are two ways to define a niche. The classic way is to consider nature to consist of thousands and millions of potential niches occupied by the various species adapted to them. In this interpretation, the niche is a property of the environment. Some ecologists, however, consider the niche to be a property of the species that occupies it. For them the niche is the outward projection of the needs of a species.

Is there a way to determine which of the two concepts of the niche has greater validity? The following example may help us to make up our mind. The large Sunda Islands, Borneo and Sumatra, west of Wallace’s Line, each have about 28 species of woodpeckers. Even though the tropical rain forest of New Guinea, east of Wallace’s Line, is remarkably similar to that of the Sunda Islands, with many of the dominant trees even belonging to the same genera, there is not a single woodpecker in New Guinea. Does this mean that there is no woodpecker niche in New Guinea? Definitely not! If we make a detailed analysis of the niches of the Malayan woodpeckers, we find that many of them are matched by analogous constellations of environmental factors in New Guinea. It would be quite misleading therefore to say that there are no woodpecker niches in New Guinea. Actually, the open niches are virtually calling for them, but woodpeckers are notoriously poor in crossing water gaps, and they simply did not succeed in crossing the various large water gaps between Sulawesi and New Guinea. And none of the indigenous families of New Guinea birds initiated a “woodpecker” branch. Many other pieces of evidence show that the classic definition of the niche, as a property of the environment, is preferable to the one that considers it a property of the organism. Biogeographers know that every colonizing species has to become adapted to the prospective niches it encounters in a newly occupied area. The word environment itself is often used in two very different senses, for all the surroundings of a species or biota or only for the niche-specific components.

LEVELS OF ADAPTATION

It is useful to distinguish between different levels of adaptation—adaptation for broad adaptive zones and adaptation for species-specific niches. Adaptations are hierarchically organized at different levels. This makes a specialization for highly specific niches possible. Among birds, we recognize woodpeckers, tree creepers, raptors (diurnal and nocturnal), waders (of a great range of sizes), swimmers, divers, terrestrial runners (ostrich, roadrunner), fish eaters, carrion eaters, seed eaters, and nectar feeders. They all have special adaptations of their bills, tongues, legs, claws, sense organs, digestive organs, and other structures and behaviors. These are mostly related to their mode of feeding or locomotion. All of these are adaptations for the special niches that these different kinds of birds occupy. And all are compatible with the demands of the special adaptive zone that birds occupy, namely, the air space. They differ from reptiles, their ancestors, by numerous adaptations for flying. They have feathers and wings, have reduced their weight by the loss of teeth and the tail vertebrae, and have hollow, thin-walled bones. They are endothermic and have numerous physiological adaptations for flight.

General and Special Adaptations

When we study the lifestyle of any particular group of organisms, we are at once impressed by the presence of very specific adaptations that make this lifestyle possible. Every book on animals describes such adaptations. Birds, for instance, have wings, feathers, lost the heavy teeth, have hollow bones, lost the bony tail, are endothermal, and possess physiological adaptations for flight. However, as Darwin already emphasized, birds have a second set of characters, all of which they share with other vertebrates, and which they inherited from their ancestors. These are not special adaptations for flight but are aspects of their vertebrate body plan. The genes for this part of the avian phenotype are components of the basic developmental machinery of birds derived from their ancestors, and in its totality it is adaptive, but not reducible into separate characters.

During embryonic development the basic features of the body plan are laid down before the special adaptations for their niches begin to develop. This explains all cases of so-called recapitulation (remember the age-old mantra “ontogeny recapitulates phylogeny”), such as the development of teeth in whale embryos or of gill arches in terrestrial vertebrates. An organism has to be well adapted as a whole, but it also must be able at all times to cope with its ancestral genome. Not every part of an organism is an ad hoc adaptation for its present lifestyle. These ad hoc adaptations are superimposed on the basic body plan. Nothing illuminates this better than the fact that in the ocean one can find representatives of as many as 15 or 20 phyla happily coexisting in the same general area. The enormous differences in their body plans do not prevent their perfect adaptation to their environment.

THE ADAPTATIONIST PROGRAM: CAN ONE PROVE ADAPTEDNESS?

How can one prove that certain individuals, as well as their structures and behaviors, are truly well adapted? This is a valid and indeed a very important question. It can be answered mainly by the ever repeated and severe testing of the supposedly adaptive attributes of organisms. This is the so-called adaptationist program outlined below (Gould and Lewontin 1979). For a refutation of the Gould and Lewontin critique of the adaptationist program, see Mayr (1983), Brandon (1995), and West-Eberhard (1992).

In an adaptational analysis it is of particular importance to consider the numerous constraints (Mayr 1983) that usually prevent a component of the phenotype from reaching optimal adaptedness. It must always be remembered that the individual as a whole is the target of selection and that there is an interaction between the selection pressures on different aspects of the phenotype. This is well illustrated by Archaeopteryx, which first acquired the most immediately needed flight adaptations—feathers, wings, improved eyes, enlarged brain—but was still not yet fully flight adapted in the retention of some less important reptilian characters (teeth, tail).

There are theoretically two ways to supply proof for the adaptedness of a feature. First, one can try to show that the occurrence of the feature cannot possibly be explained by chance. But it is very difficult to succeed in this endeavor. Second, one can test the various possible adaptive advantages of the feature, and its adaptedness is confirmed when all attempts to disprove these advantages are unsuccessful. What must be tested is the adaptedness of the particular phenotypic feature in question.

Almost any feature of an organism can be and has been shown to be of selective significance. Cases that have been experimentally tested are industrial melanism, banding patterns in snails, mimicry, aspects of sexual dimorphism, and scores of others reported in the literature (Endler 1986). By contrast, it is virtually impossible to prove that any property of an organism is not of selective significance. One is therefore forced to apply the second method and adopt the chance explanation only when all endeavors to demonstrate a selective value of a feature have been failures.

Adaptedness Is Acquired Gradually

New adaptations are ordinarily acquired quite gradually. Archaeopteryx, a 145-million-year-old fossil bird, documents almost perfectly the intermediacy between reptiles and birds. It still had teeth, a long tail, simple ribs, and the separated ilia and ischia of a reptile, but also had the feathers, the wings, the eyes, and the brain of a bird. The fossil ancestors of whales document a similar intermediate state in their adaptation to two different media. Darwin marveled that such a wonderful structure as an eye could have evolved through natural selection, but the comparative anatomists have shown not only that eyes evolved in the animal series at least 40 times independently, but also that among the existing photosensitive organs every intermediate step is found between a simple light-sensitive spot on the epidermis and a perfect eye with all of its accessories. The same regulatory gene (Pax 6) occurs in all forms with eyes, but is also widespread in eyeless taxa. It is apparently a very old regulatory gene that has been coopted for vision whenever eyes were selected.

Convergence

Open ecological niches or zones are often repeatedly colonized by entirely unrelated organisms that, once adapted to these niches, become by convergence extremely similar. The outstanding example is the Australian fauna of marsupial mammals, which, in the absence of placental mammals, have evolved adaptive types corresponding to (and remarkably similar to) Northern Hemisphere placentals such as the flying squirrel, mole, mouse, wolf, badger, and anteater. Very similar but unrelated nectar-feeding birds have evolved in Australia (honeyeaters), Africa and India (sunbirds), Hawaii (honeycreepers), and the Americas (hummingbirds) (see Fig. 10.4); ratites, the flightless birds with rudimentary wings, in South America, Africa, Madagascar, Australia, and New Zealand; and tree creepers in Australia, the Philippines, Africa, the Holarctic, and South America. The unrelated American and African porcupines are so similar that until recently they were considered to be closely related. Similar cases of convergence can be found in almost all groups of animals and even in plants (e.g., American cactuses and African euphorbs, see Fig. 10.5). Even only distantly related animals, like sharks (fishes), ichthyosaurs (reptiles), and porpoises (mammals), have become superficially very similar to each other.

The ubiquity of adaptedness is also documented by plants, fungi, protists, and bacteria. Life-forms have an astonishing capacity to vary, to respond to natural selection, and to take advantage of ecological opportunities.

CONCLUSIONS

Evolution in sexually reproducing organisms consists of genetic changes from generation to generation in populations, from the smallest local deme to the aggregate of interbreeding populations in a biological species. Numerous processes, particularly mutation, contribute to these genetic changes to supply the phenotypic variation needed by selection. The most important factor is recombination, which is largely responsible for the virtually inexhaustible supply of new genotypes in every generation. Selection, then, is responsible for the elimination of all but on the average two of the offspring of two parents. Those individuals that are best adapted to the abiotic and biotic environment have the greatest chance to be among the survivors. This process favors the development of new adaptations and the acquisition of evolutionary novelties, thus leading to evolutionary advance, as stated in the language of evolutionary biology. Evolution, being on the whole a population turnover, is ordinarily a gradual process, except for certain chromosomal processes that may lead to the production of a new species-individual in a single step.

Genetic material (nucleic acids) is constant and impervious to any influence from the environment. No genetic information can be transmitted from proteins to nucleic acids, and so the inheritance of acquired characters is therefore impossible. This provides an absolute refutation of all Lamarckian theories of evolution. The Darwinian model of evolution, based on random variation and natural selection, explains satisfactorily all phenomena of evolutionary change at the species level, and in particular all adaptation.