212 © 1978 Scientific American

212 © 1978 SCIENTIFIC AMERICAN, INC Adaptation The manifest fit between organisms and their environment is a major outcome of evolution. Yet natural selection does not lead inevitably to adaptation; indeed, it is sometimes hard to define an adaptation by Richard C. Lewontin he theory about the history of life iors that appear to have been carefully These "organs of extreme perfection" that is now generally accepted, the and artfully designed to enable each or were only the most extreme case of a TDarwinian theory of evolution by ganism to appropriate the world around more general phenomenon: adaptation. natural selection, is meant to explain it for its own life. Darwin's theory of evolution by natural two different aspects of the appearance It was the marvelous fit of organisms selection was meant to solve both the of the living world: diversity and fitness. to the environment, much more than the problem of the origin of diversity and There are on the order of two million great diversity of forms, that was the the problem of the origin of adaptation species now living, and since at least chief evidence of a Supreme Designer. at one stroke. Perfect organs were a dif 99.9 percent of the species that have Darwin realized that if a naturalistic ficulty of the theory not in that natural ever lived are now extinct, the most con theory of evolution was to be successful. selection could not account for them but servative guess would be that two billion it would have to explain the apparent rather in that they were its most rigorous species have made their appearance on perfection of organisms and not simply test, since on the face of it they seemed the earth since the beginning of the their variation. At the very beginning of the best intuitive demonstration that a Cambrian period 600 million years ago. the Origin 01 Species he wrote: "In con divine artificer was at work. Where did they all come from? By the sidering the Origin of Species, it is quite time Darwin published On the Origin 01 conceivable that a naturalist ...might The modern view of adaptation is that Species in 1859 it was widely (if not uni come to the conclusion that each spe the external world sets certain versally) held that species had evolved cies ...had descended, like varieties, "problems" that organisms need to from one another, but no plausible from other species. Nevertheless, such a "solve," and that evolution by means of mechanism for such evolution had been conclusion, even if well founded, would natural selection is the mechanism for proposed. Darwin's solution to the be unsatisfactory, until it could be creating these solutions. Adaptation is problem was that small heritable varia shown how the innumerable species in the process of evolutionary change by tions among individuals within a species habiting this world have been modified, which the organism provides a better become the basis of large differences be so as to acquire that perfection of struc and better "solution" to the "problem," tween species. Different forms survive ture and coadaptation which most just and the end result is the state of being and reproduce at different rates depend ly excites our admiration." Moreover, adapted. In the course of the evolution ing on their environment, and such Darwin knew that "organs of extreme of birds from reptiles there was a succes differential reproduction results in the perfection and complication" were a sive alteration of the bones, the muscles slow change of a population over a peri critical test case for his theory, and he and the skin of the forelimb to give rise od of time and the eventual replacement took them up in a section of the chap to a wing; an increase in the size of the of one common form by another. Dif ter on "Difficulties of the Theory." He breastbone to provide an anchor for the ferent populations of the same species wrote: "To suppose that the eye, with all wing muscles; a general restructuring of then diverge from one another if they its inimitable contrivances for adjusting bones to make them very light but occupy different habitats, and eventual the focus to different distances, for ad strong. and the development of feathers ly they may become distinct species. mitting different amounts of light, and to provide both aerodynamic elements Life forms are more than simply mul for the correction of spherical and and lightweight insulation. This whole tiple and diverse, however. Organisms chromatic aberration, could have been sale reconstruction of a reptile to make a fit remarkably well into the external formed by natural selection, seems, I bird is considered a process of major world in which they live. They have freely confess, absurd in the highest de adaptation by which birds solved the morphologies, physiologies and behav- gree." problem of flight. Yet there is no end to adaptation. Having adapted to flight. some birds reversed the process: the penguins adapted to marine life by ADAPTATION is exemplified by "industrial melanism" in the peppered moth (Biston betu changing their wings into flippers and laria). Air pollution kills the lichens that would normally colonize the bark of tree trunks. On their feathers into a waterproof cover the dark, lichenless bark of an oak tree near Liverpool in England the melanic (black) form is ing. thus solving the problem of aquatic better adapted: it is better camouflaged against predation by birds than the light, peppered existence. wild type (top photograph on opposite page), which it largely replaced through natural selec The concept of adaptation implies a tion in industrial areas of England in the late 19th century. Now quality is improving. On a air preexisting world that poses a problem nearby beech tree colonized by algae and the lichen Lecanora conizaeoides,which is itself par to which an adaptation is the solution. A ticularly well adapted to low levels of pollution, the two forms of the moth are equally conspic uous (middle). On the lichened bark of an oak tree in rural Wales the wild type is almost in key is adapted to a lock by cutting and visible (bottom), and in such areas it predominates. The photographs were made by J. A. Bish filing it; an electrical appliance is adapt op of the University of Liverpool and Laurence M. Cook of the University of Manchester. ed to a different voltage by a transform- 213 © 1978 SCIENTIFIC AMERICAN, INC REPTILES BIRDS BONE ., ....... ...... ..:.'.:.:: ... ' FORELIMB STERNUM BOTTOM VIEW SIDE VIEW SIDE VIEW BOTTOM VIEW SKIN COVERING EVOLUTION OF BIRDS from reptiles can be considered a process tured to become lighter but strong; the forelimb was lengthened (and of adaptation by which birds "solved" the "problem" of flight. At the its muscles and skin covering were changed) to become a wing; the reptilian sternum, or breastbone, was enlarged and deepened to an top of the illustration the skeleton of a modern pigeon (right) is com pared with that of an early reptile: a thecodont, a Triassic ancestor chor the wing muscles (even in Archaeopteryx,the Jurassic transi of dinosaurs and birds. Various reptile features were modified to be tion form between reptiles and birds whose sternum is pictured here, come structures specialized for flight. Heavy, dense bone was restruc- the sternum was small and shallow); scales developed into feathers. 214 © 1978 SCIENTIFIC AMERICAN, INC er. Although the physical world certain ly predated the biological one, there are I certain grave difficulties for evolution ary theory in defining that world for the 000 process of adaptation. It is the difficulty 1 of defining the "ecological niche." The . �'�O�� ecological niche is a multidimensional H-{.-...t- ; --.,_ �<'!rt-�-::::.. ::::::::::::t::::::::::::::::::::j::::::::::::::::::!::::::::::::::::::::::t::::::::::::::::::::1::::::::::::::::� --1 , I description of the total environment and -�----+-----r----4------- --'---� way of life of an organism. Its descrip \ \e.. i tion includes physical factors, such as temperature and moisture; biological factors, such as the nature and quantity of food sources and of predators, and factors of the behavior of the organism itself, such as its social organization, its pattern of movement and its daily and seasonal activity cycles. The first difficulty is that if evolution is described as the process of adaptation of organisms to niches, then the niches must exist before the species that are to fit them. That is, there must be empty niches waiting to be filled by the evolu tion of new species. In the absence of organisms in actual relation to the envi ... ronment, however, there is an infinity of ....... ... ways the world can be broken up into "''l!, arbitrary niches. It is trivially easy to - ""''''" -- ... - describe "niches" that are unoccupied. • nnn n OJ For example, no organism makes a liv o 300 400 ing by laying eggs, crawling along the 00 200 1 SURVIVAL TIME (MILLIONS OF YEARS) surface of the ground, eating grass and EXTINCTION RATES in blanyevolutionary lines suggest that natural selection does not nec living for several years. That is, there essarily improve adaptation. The data, from Leigh Van Valen of the University of Chicago, are no grass-eating snakes, even though show the duration of survival of a number of living (solid dots) and extinct (open circles) gen snakes live in the grass. Nor are there era of Echinoidea (black) and Pelecypoda (color), two classes of marine invertebrates. If natu any warm-blooded, egg-laying animals ral selection trulyfitted organisms to environments, the points should fall along concave curves that eat the mature leaves of trees, even (broken-line curves)indicating a lower probability of extinction for long-lived genera. Actual though birds inhabit trees.

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