Darwin’s explanation of design: From natural theology to natural selection § Infection, Genetics and Evolution

power of evolution arises from a distinctive interaction between chance and necessity, between random mutation and natural selection.

function or design, we look for its author. Whenever we see a watch, we know that there is a watchmaker. Similarly, the structures, organs and behaviors of living beings are directly organized to serve certain purposes or functions. The functional design of organisms and their features would therefore seem to argue for the existence of a Designer. It was Darwin's greatest accomplishment to show that the complex organization and functionality of living beings can be explained as the result of a natural process-natural selection-without any need to resort to a Creator or other external agent. The origin and adaptation of organisms in their profusion and wondrous variations were thus brought into the realm of science. Darwin accepted that organisms are ''designed'' for certain purposes, that is, they are functionally organized. Organisms are adapted to certain ways of life and their parts are adapted to perform certain functions. Fish are adapted to live in water, kidneys are designed to regulate the composition of blood, the human hand is made for grasping. But Darwin went on to provide a natural explanation of the design. The seemingly purposeful aspects of living beings could now be explained, like the phenomena of the inanimate world, by the methods of science, as the result of natural laws manifested in natural processes.

Darwin's explanation of design
Darwin is deservedly given credit for the theory of evolution. The evolution of organisms was commonly accepted by naturalists in the middle decades of the 19th century. In The Origin of Species, Darwin accumulated overwhelming evidence demonstrating the evolution of organisms. However, Darwin accomplished something much more important for intellectual history than demonstrating evolution. Indeed, accumulating evidence for common descent with diversification may very well have been a subsidiary objective of Darwin's masterpiece. Darwin's The Origin of Species is, first and foremost, a sustained argument to solve the problem of how to account scientifically for the design of organisms. Darwin seeks to explain the adaptations of organisms, their complexity, diversity, and marvelous contrivances as the result of natural processes. Darwin brings about the evidence for evolution because evolution is a necessary consequence of his theory of design.
It is my contention that Darwin's most revolutionary achievement is that he extended the Copernican revolution to the world of living things, much more so than his demonstration of the evolution of organisms. Henceforward, the origin and design of organisms could be explained, like the phenomena of the inanimate world, as the result of natural laws manifested in natural processes. Early in his Notebooks of 1837-1839, which he had started shortly after returning from a five-year trip around the world in the HMS Beagle, Darwin registers his discovery of natural selection and thereafter, over the years, he repeatedly refers to it as ''my theory.'' From the late 1830s until his death in 1882, Darwin's life would be dedicated to substantiating natural selection and its companion postulates, mainly the pervasiveness of hereditary variation and the enormous fertility of organisms, which much surpassed the capacity of available resources. Natural selection became for Darwin ''a theory by which to work.'' He relentlessly pursued observations and performed experiments in order to test the theory and resolve presumptive objections.
Alfred Russel Wallace (1823-1913) is famously given credit for discovering, independently of Darwin, natural selection as the process accounting for the evolution of species. On June 18, 1858, Darwin wrote to Charles Lyell that he had received by mail a short essay from Wallace such that ''if Wallace had my [manuscript] sketch written in [1844] he could not have made a better abstract.'' Darwin was thunderstruck.
Wallace's independent discovery of natural selection is remarkable. Wallace, however, was not interested in explaining design, but rather in accounting for the evolution of species, which he saw as a sustained and progressive process, as indicated in his paper's title: ''On the Tendency of Varieties to Depart Indefinitely from the Original Type.'' Wallace thought that evolution proceeds indefinitely and is progressive. Darwin, on the contrary, did not accept that evolution would necessarily represent progress or advancement, nor did he believe that evolution would always result in morphological change over time; rather, he knew of the existence of ''living fossils,'' organisms that had remained unchanged for millions of years. For example, ''some of the most ancient Silurian animals, as the Nautilus, Lingula, etc., do not differ much from living species.'' (We now know that the Silurian geological period lasted from 444 to 416 million years ago.) In 1858, Darwin was at work on a multivolume treatise, intended to be titled ''On Natural Selection.'' Wallace's paper stimulated Darwin to write The Origin of Species, which would be published the following year. Darwin intended this as an abbreviated version of the much longer book he had intended to write. As I have noted earlier, Darwin's focus, in The Origin as elsewhere, was the explanation of design, with evolution playing the subsidiary role of supporting evidence.

Darwin's Origin
The Origin of Species is usually characterized as the most important book ever published about the theory of evolution. This is correct, but not so much because of the numerous observations and facts magisterially gathered by Darwin demonstrating the evolution of organisms, but rather because with his theory of natural selection he advanced an explanation that accounted not only for the evolution of organisms, but also for their adaptations. The Introduction and Chapters I through VIII of The Origin of Species explain how natural selection accounts for the adaptations and behaviors of organisms, their ''design.'' The extended argument starts in Chapter I, where Darwin describes the successful selection of domestic plants and animals and, with considerable detail, the success of pigeon fanciers seeking exotic ''sports.'' The success of plant and animal breeders manifests how much selection can accomplish by taking advantage of spontaneous hereditary variations that occur in organisms but happen to fit the breeders' objectives. A sport (mutation) that first appears in an individual can be multiplied by selective breeding, so that after a few generations that sport becomes fixed in a breed, or ''race.'' The familiar breeds of dogs, cattle, chickens, and food plants have been obtained by this process of selection practiced by people with particular objectives.
The ensuing chapters (II-VIII) of The Origin of Species extend the argument to variations propagated by natural selection for the benefit of the organisms themselves, rather than by artificial selection of traits desired by humans. As a consequence of natural selection, organisms exhibit design, that is, exhibit adaptive organs and functions. The design of organisms as they exist in nature, however, is not ''intelligent design,'' imposed by God as a Supreme Engineer or by humans; rather, it is the result of a natural process of selection, promoting the adaptation of organisms to their environments. This is how natural selection works: Individuals that have beneficial variations, that is, variations that improve their probability of survival and reproduction, leave more descendants than individuals of the same species that have less beneficial variations. The beneficial variations will consequently increase in frequency over the generations; less beneficial or harmful variations will be eliminated from the species. Eventually, all individuals of the species will have the beneficial features; new features will arise over eons of time.
Organisms exhibit complex design, but this design is not, in current language, ''irreducible complexity,'' emerging all of a sudden in full bloom. Rather, according to Darwin's theory of natural selection, the design has arisen gradually and cumulatively, step by step, promoted by the reproductive success of individuals with incrementally more adaptive elaborations.
It follows from Darwin's explanation of adaptation that evolution must necessarily occur as a consequence of organisms becoming adapted to different environments in different localities, and to the ever-changing conditions of the environment over time; and as hereditary variations become available at a particular time that improve, in that place and at that time, the organisms' chances of survival and reproduction. The Origin of Species' evidence for biological evolution is central to Darwin's explanation of design, because this explanation implies that biological evolution occurs, which Darwin therefore seeks to demonstrate in most of the remainder of the book (chapters IX-XIII. In the sixth edition of The Origin of Species, these are chapters X-XIV, because Darwin had added a new chapter VII: ''Miscellaneous objections to the theory of natural selection.'').
In the concluding Chapter XIV of Origin, Darwin returns to the dominant theme of adaptation and design. In an eloquent final paragraph, Darwin asserts the ''grandeur'' of his vision: ''It is interesting to contemplate an entangled bank, clothed with many plants of many kinds, with birds singing on the bushes, with various insects flitting about, and with worms crawling through the damp earth, and to reflect that these elaborately constructed forms, so different from each other, and dependent on each other in so complex a manner, have all been produced by laws acting around us.. . . Thus, from the war of nature, from famine and death, the most exalted object which we are capable of conceiving, namely, the production of the higher animals, directly follows. There is grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved'' (emphasis added).

Adaptation and evolution
Darwin's Origin addresses the same issue as Paley: how to account for the adaptive configuration of organisms and their parts, which are so obviously designed to fulfill certain functions. Darwin argues that hereditary adaptive variations (''variations useful in some way to each being'') occasionally appear, and that these are likely to increase the reproductive chances of their carriers. The success of pigeon fanciers and animal breeders clearly shows the occasional occurrence of useful hereditary variations. In nature, over the generations, Darwin's argument continues, favorable variations will be preserved, multiplied, and conjoined; injurious ones will be eliminated. Evolution affects all aspects of an organism's life-morphology (form and structure), physiology (function), behavior, and ecology (interaction with the environment). Underlying these changes are changes in the hereditary materials. Hence, in genetic terms, evolution consists of changes in the organisms' hereditary makeup. Darwin formulated natural selection primarily as differential survival. The modern understanding of the principle of natural selection is formulated in genetic and statistical terms as differential reproduction. Natural selection simply implies that some genes and genetic combinations are transmitted to the following generations more frequently than their alternates. Favored genes will become more common in every subsequent generation, and their alternates less common. Natural selection is a statistical bias in the relative rate of reproduction of alternative genes.
Evolution can be seen as a two-step process. First, hereditary variation arises by mutation; second, selection occurs by which useful variations increase in frequency and those that are less useful or injurious are eliminated over the generations. ''Useful'' and ''injurious'' are terms used by Darwin in his definition of natural selection. The significant point is that individuals having useful variations ''would have the best chance of surviving and procreating their kind.'' As a consequence, useful variations increase in frequency over the generations, at the expense of those that are less useful or injurious.
Natural selection is much more than a ''purifying'' process, for it is able to generate novelty by increasing the probability of otherwise extremely improbable genetic combinations. Natural selection in combination with mutation becomes, in this respect, a creative process. Moreover, it is a process that has been occurring for many millions of years, in many different evolutionary lineages and a multitude of species, each consisting of a large number of individuals. Evolution by mutation and natural selection has produced the enormous diversity of the living world with its wondrous adaptations. Several hundred million generations separate modern animals from the early animals of the Cambrian geological period (542 million years ago). The number of mutations that can be tested, and those eventually selected, in millions of individual animals over millions of generations is difficult for a human mind to fathom, but we can readily understand that the accumulation of millions of small, functionally advantageous changes could yield remarkably complex and adaptive organs, such as the eye.
Natural selection does not operate as a sieve that retains the rarely arising useful genes and lets go the more frequently arising harmful mutants; at least, not only. Natural selection acts in the filtering way of a sieve, but it is much more than a purely negative process, for it is able to generate novelty by increasing the probability of otherwise extremely improbable genetic combinations. Natural selection is thus a creative process. It does not ''create'' the entities (mutations) upon which it operates, but it produces adaptive (functional) genetic combinations that could not have existed otherwise.
Critics have sometimes alleged as evidence against Darwin's theory of evolution examples showing that random processes cannot yield meaningful, organized outcomes. It is thus pointed out that a series of monkeys randomly striking letters on a typewriter would never write The Origin of Species, even if we allow for millions of years and many generations of monkeys pounding at typewriters. This criticism would be valid if evolution would depend only on random processes. But natural selection is a nonrandom process that promotes adaptation by selecting combinations that ''make sense,'' i.e., that are useful to the organisms. The analogy of the monkeys would be more appropriate if a process existed by which, first, meaningful words would be chosen every time they appeared on the typewriter; and then there would also be typewriters with previously selected words rather than just letters in the keys, and again there would be a process to select meaningful sentences every time they appeared in this second typewriter. If every time words such as ''the,'' ''origin,'' ''species,'' and so on, appeared in the first kind of typewriter, they each became a key in the second kind of typewriter, meaningful sentences would occasionally be produced in this second typewriter. If such sentences became incorporated into keys of a third type of typewriter, in which meaningful paragraphs were selected whenever they appeared, it is clear that pages and even chapters ''making sense'' would eventually be produced. The end product would be an ''irreducibly complex'' text.
We need not carry the analogy too far, since the analogy is not fully satisfactory, but the point is clear. Evolution is not the outcome of purely random processes, but rather there is a ''selecting'' process, which picks up adaptive combinations because these reproduce more effectively and thus become established in populations. These adaptive combinations constitute, in turn, new levels of organization upon which the mutation (random) plus selection (non-random or directional) process again operates. The complexity of organization of animals and plants is ''irreducible'' to simpler components in one or very few steps, but not through the millions and millions of generations and the multiplicity of steps and levels made possible by eons of time.
A critical point is that evolution by natural selection is an incremental process, operating over eons of time and yielding organisms better able to survive and reproduce than others, which typically differ from one another at any one time only in small ways; for example, the difference between producing more or fewer progeny or between having or lacking an enzyme able to catalyze the synthesis of one particular amino acid. Notice also that increased complexity is not a necessary outcome of natural selection, although complexity increases from time to time in some lineages of descent, so that, although rare, these lineages are very conspicuous over time's eons. That is, increased complexity is not a necessary consequence of evolution by natural selection, but rather emerges occasionally. The longest living organisms on Earth are the microscopic bacteria, which have continuously existed on our planet for three and a half billion years and yet exhibit no greater complexity than their old time ancestors. More complex organisms came about much later, without the elimination of their simpler relatives. For example, the primates appeared on earth some fifty million years ago and our species, Homo sapiens, came about two hundred thousand years ago.

Chance and necessity
An engineer has a preconception of what the design of a contrivance of structure is supposed to achieve, and will select suitable materials and arrange them in a preconceived manner so that it fulfills the intended function. On the contrary, natural selection has no foresight, nor does it operate according to some preconceived plan. Rather it is a purely natural process resulting from the interacting properties of physicochemical and biological entities. Natural selection is simply a consequence of the differential multiplication of living beings, as pointed out. It has some appearance of purposefulness because it is conditioned by the environment: which organisms reproduce more effectively depends on what variations they possess that are useful in the place and at the time where the organisms live. But natural selection does not anticipate the environments of the future; drastic environmental changes may be insuperable to organisms that were previously thriving. Species extinction is the common outcome of the evolutionary process. The species existing today represent the balance between the origin of new species and their eventual extinction. More than 99 percent of all species that ever lived on Earth have become extinct without issue. These may have been more than one billion species; the available inventory of living species has identified and described less than two million out of some ten million estimated to be now in existence.
The team of typing monkeys is a bad analogy of evolution by natural selection, because it assumes that there is ''somebody'' who selects letter combinations and word combinations that make sense. In evolution there is no one selecting adaptive combinations. These select themselves because they multiply more effectively than less adaptive ones.
The process of natural selection can explain the adaptive organization of organisms, as well as their diversity and evolution, as a consequence of their adaptation to the multifarious and everchanging conditions of life. The fossil record shows that life has evolved in a haphazard fashion. The radiations, expansions, relays of one form by another, occasional but irregular trends, and the ever-present extinctions, are best explained by natural selection of organisms subject to the vagaries of genetic mutation and environmental challenge. The scientific account of these events does not necessitate recourse to a preordained plan, whether imprinted from without by an omniscient and all-powerful designer, or resulting from some immanent force driving the process towards definite outcomes. Biological evolution differs from a painting or an artifact in that it is not the outcome of preconceived design.
Natural selection accounts for the ''design'' of organisms, because adaptive variations tend to increase the probability of survival and reproduction of their carriers at the expense of maladaptive, or less adaptive, variations. The arguments of Paley against the incredible improbability of chance accounts of the adaptations of organisms are well taken as far as they go. But neither Paley nor any other author before Darwin was able to discern that there is a natural process (namely, natural selection) that is not random, but rather is oriented and able to generate order or ''create.'' The traits that organisms acquire in their evolutionary histories are not fortuitous but determined by their functional utility to the organisms, ''designed'' as it were to serve their life needs.
Chance is, nevertheless, an integral part of the evolutionary process. The mutations that yield the hereditary variations available to natural selection arise at random, independently of whether they are beneficial or harmful to their carriers. But this random process (as well as others that come to play in the great theatre of life) is counteracted by natural selection, which preserves and multiplies what is useful and eliminates the harmful. Without hereditary mutation, evolution could not happen because there would be no variations that could be differentially conveyed from one to another generation. But without natural selection, the mutation process would yield disorganization and extinction because most mutations are disadvantageous. Mutation and selection have jointly driven the marvelous process that starting from microscopic organisms has yielded orchids, birds, and humans.
The theory of evolution conveys chance and necessity jointly intricated in the stuff of life; randomness and determinism interlocked in a natural process that has spurted the most complex, diverse, and beautiful entities in the universe: the organisms that populate the earth, including humans who think and love, endowed with free will and creative powers, and able to analyze the process of evolution itself that brought them into existence. This is Darwin's fundamental discovery, that there is a process that is creative though not conscious, a process that creates design without necessitating a Designer.