Most intelligent design theorists accept evolutionary biology as providing a partial explanation for the diversity and complexity of life. Some, including prominent ID supporters like biochemists Michael Behe and Michael Denton, even accept the thesis of universal common descent. However, arguments for intelligent design in biology have challenged the second major thesis of evolutionary theory, namely, the claim that natural evolutionary mechanisms are sufficient (apart from any mind-directed causes) to explain the evolution of life. ID theorists maintain that some features of living things—such as irreducibly complex biochemical machinery, gene-regulatory networks, and at least some of the digital information encoded in DNA—are best explained as the work of an intelligent mind rather than as products of mindless evolutionary processes. Several lines of argument have been advanced to support that conclusion.
Before summarizing a few of the main arguments for design in biology, two caveats are in order. First, as I emphasized previously, arguments for design are not the same as design hypotheses. If an argument succeeds in persuading us that some biological features were designed, this might motivate us to develop and investigate hypotheses about when, where, or how the designer(s) produced those designs; but the argument itself need not include any such hypothesis. So, an argument for design could be accepted without committing oneself to any particular design hypothesis. Conversely, a design hypothesis could be accepted by someone who does not find any philosophical arguments for design convincing. The arguments considered below are, in that sense, independent of design hypotheses. (We’ll encounter numerous design hypotheses later in this chapter.)
Second, it’s possible to reject both design arguments and design hypotheses while still believing that living things were, in fact, designed. Many evolutionary creationists repudiate all of the arguments and hypotheses propounded in the ID movement, but this does not mean they deny that God designed life. Evolutionary creationists believe God designed living creatures by creating and ordaining natural processes to carry out His intentions, and they typically embrace mainstream evolutionary theory as the best scientific account of how God did so. For this reason, most evolutionary creationists dismiss design arguments that allege explanatory deficiencies in evolutionary theory, and most also reject hypotheses that posit supernatural miracles in the origin and evolution of life.There are exceptions to this generalization. Some self-described evolutionary creationists do take design arguments and supernatural design hypotheses seriously. Moreover, many regard the creation of the universe (at the time of the Big Bang) as a supernatural miracle, and some think the creation of the very first living organism(s) may have been miraculous as well. For example, in his book The Works of His Hands: A Scientist’s Journey from Atheism to Faith, evolutionary creationist Sy Garte writes: “I could not fathom how the laws of nature that governed the universe from the time of the big bang would have possibly allowed for the emergence of life. I now believe that the origin of life was the second major creation event, and I have found no science to contradict that belief.” (Grand Rapids: Kregel Publications, 2019), 102. (Not all design hypotheses posit miracles, however, as we’ll see.) Nevertheless, evolutionary creationists agree with other biblical creationists that God designed all living creatures.
The ongoing debates surrounding arguments for design in biology are complicated, so I cannot do justice to the intricacy and subtlety of the relevant issues here. My aim in the following summary is merely to acquaint readers with several popular arguments for design, and to convey some of the main objections raised against them.
The Argument from Irreducible Complexity
One of the simplest and most intuitive arguments for biological design is Michael Behe’s argument from irreducible complexity, expounded in his 1996 book Darwin’s Black Box. As we saw in Chapter 10, Behe coined the term ‘irreducibly complex’ to describe any complex system whose function depends on all of its components working together. In other words, all parts of an irreducibly complex system are indispensable to its function: if any part is eliminated, the system fails. A striking example is the bacterial flagellar motor—an electricThe flagellar motor is powered by a flow of protons instead of electrons, and its operating principles differ from artificial electric motors, which typically use a combination of electromagnets and permanent magnets for propulsion. motor that spins a tail-like propeller called a flagellum in some species of bacteria, enabling them to swim through water. The motor consists of dozens of specialized protein components, all of which are essential to its function.The flagellum as a whole does not satisfy Behe’s definition of irreducible complexity, since some of its protein parts can be eliminated without completely destroying its function; but the proteins that comprise the motor are indispensable, so the motor itself is irreducibly complex. Its efficiency exceeds that of man-made electric motors, and its performance is stunning: it can spin up to 10,000 revolutions per minute, stop within a quarter turn, and instantly begin spinning the opposite direction at 10,000 RPMs.Behe mentions these impressive details in an interview with Lee Strobel, quoted in Strobel’s book The Case for a Creator: A Journalist Investigates Scientific Evidence That Points Toward God (Grand Rapids: Zondervan, 2004), 217.
As we saw in Chapter 10, the existence of irreducibly complex biochemical machinery poses a challenge for evolutionary theory, which seems to predict that natural selection will eliminate any functionless partial system before it can evolve into the complete, functionally-integrated form.See the discussion on this page. However, note the objection below. It is difficult to imagine how the flagellar motor could have evolved from simpler structures, for example, since a partly-formed motor would have been a useless encumbrance to the evolving bacteria and should have been eliminated by natural selection. (A rebuttal to this challenge will be considered below.) However, Behe did not merely question the adequacy of evolutionary explanations. He argued that there is a better explanation for the existence of structures like the flagellar motor.
Behe points out that human inventors routinely create irreducibly complex systems. There is reason to think an intelligent mind was responsible for the irreducible complexity of biochemical machinery as well, he argues. Minds can plan and anticipate the function of a system that does not yet exist, so if a mind was somehow involved in the creation and evolution of life, it could direct nature toward those functional ends. On the other hand, we should not expect unguided processes, or even processes “guided” by natural selection, to produce irreducibly complex systems. So, the existence of such biochemical systems can be explained more adequately by (some) design hypotheses, and counts as evidence favoring such hypotheses.
Despite its simplicity, I think Behe’s argument is more reasonable than many critics acknowledge. Contrary to a common critique, it is not a mere “argument from ignorance.” Behe doesn’t conclude that irreducibly complex systems must have been designed because we don’t know how evolutionary processes could have produced them. Rather, his argument rests on the premise that we do know of a process that often produces irreducible complexity in other (non-biological) cases: namely, intelligent design (by human inventors). This gives us reason to suspect that something similar has occurred in biology, given that no other known causes besides intelligent activity—not even mutation and natural selection—seem likely to have produced them.
The logical structure of the argument can be characterized in more than one way. Behe suggests that the inference to design can be viewed either as an argument by analogy or as an inference to the best explanation.Michael Behe, Darwin’s Black Box: The Biochemical Challenge to Evolution (New York: Free Press, 1996), 218-219. (The latter type of reasoning, also called abductive inference, is routinely employed in the natural sciences. For further discussion of these kinds of inferences, see the chapter on Non-deductive Inferences in my ebook Skillful Reasoning: An Introduction to Formal Logic and Other Tools for Careful Thought.) Alternatively, the argument could be framed in terms of Bayesian confirmation theory, similar to the fine-tuning argument I outlined on the previous page.In particular, we could appeal to a Bayesian rule known as the likelihood principle, which says that if evidence E is more likely (more expected, or less surprising) assuming hypothesis H1 is true than if we assume H2 is true, then E is evidence in favor of H1 over H2. Behe can be understood as suggesting that the discovery of irreducibly complex biochemical machinery is less surprising given certain design hypotheses than given mainstream evolutionary theory; hence, this evidence favors design according to the likelihood principle. Either way, Behe’s argument does not commit any logical fallacies.
The argument is, however, susceptible to an important objection.Actually, it is susceptible to several objections, but I take the one described here to be most important. For a philosophically careful discussion of Behe’s argument and several objections raised against it, see Alvin Plantinga, Where the Conflict Really Lies: Science, Religion, and Naturalism (New York: Oxford University Press, 2011), 225-231. Critics contend that a crucial premise of Behe’s argument is false: irreducible complexity is not a surprising or unexpected outcome of evolutionary processes. Natural selection acting on random mutations can, plausibly, produce the sorts of systems Behe calls “irreducibly complex.” Evolutionary theorists have proposed abstract, hypothetical models showing how irreducibly complex structures might arise via evolutionary processes.For instance, T. Ryan Gregory’s essay “The Evolution of Complex Organs” (Evolution: Education and Outreach, 2008, volume 1, 358-389) borrows an example from philosopher Paul Draper to illustrate how, in principle, an “irreducibly complex organ” might arise. See especially Figure 3 (page 365) and the caption thereto. Although it is difficult to construct such models for real-world cases like the flagellar motor, the simplified hypothetical models show that it is at least possible, in principle, for a gradual sequence of structures (each of which is favored by natural selection, in changing environments) to result in a structure that satisfies Behe’s definition of irreducible complexity.
A key insight from those models is that parts or subsystems of an irreducibly complex system need not serve the same function in order to be favored by natural selection. Individual proteins and other molecular parts may evolve to serve various other functions, but then these parts might be co-opted into a new system with a different function—a process known as exaptation. For example, some of the proteins found in the bacterial flagellar motor are also found in structures that serve wholly different purposes in other bacteria species.See this article for details and further discussion. Perhaps the various parts of the motor evolved (together or separately) through a series of incremental stages, each of which served some function favored by natural selection, though only the last stage functioned as a motor. Thus, even if the motor is irreducibly complex in the sense Behe defines, this doesn’t prove that it couldn’t have evolved by natural selection acting on incremental modifications of other (mostly unknown) functional structures.
It is hard to imagine how the flagellar motor could have evolved from simpler structures, but this may be a shortcoming of our limited imagination rather than a serious problem for evolutionary theory. We have little trouble envisaging a sequence of functional intermediates leading from simpler to more complex structures when the complexity is reducible. Indeed, evolutionary sequences have been proposed to explain the origins of some astoundingly complex (but not irreducibly complex) biological structures and organs. The human eye is a good example. Although some of the exquisite mechanisms and subsystems involved in human vision may be irreducibly complex by Behe’s definition, the complexity of the eye as a whole is not irreducible. Several parts of the eye could be removed without causing blindness: its function (vision) will be impaired but not lost completely.The human eye can retain some functionality even when parts of the iris, retina, choroid, and/or optic disc are missing—a condition known as coloboma. This reducibility makes it easier to imagine how the eye may have evolved, and evolutionary theorists have identified plausible sequences of simpler structures from which many crucial features of the human eye might have originated. In fact, some of the proposed intermediary stages of this evolutionary sequence are found in organisms living today, from relatively simple “eyespots” (light-sensitive organelles) in single-celled organisms, through cup-shaped photoreceptors (found in some sea snails) that can detect the direction of incoming light, to eyes with lenses and image-focusing systems of increasing complexity.T. Ryan Gregory’s essay “The Evolution of Complex Organs” (Evolution: Education and Outreach, 2008, volume 1, 358-389) provides a detailed yet admirably readable explanation of the leading theoretical models of eye evolution. His essay also provides an abstract, hypothetical example of an evolutionary sequence leading to an “irreducibly complex organ.” (Figure 3, page 365.) Darwin himself proposed an account of eye evolution along similar lines in the original 1859 edition of On the Origin of Species.Darwin wrote: “To suppose that the eye, with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberration, could have been formed by natural selection, seems, I freely confess, absurd in the highest possible degree. Yet reason tells me, that if numerous gradations from a perfect and complex eye to one very imperfect and simple, each grade being useful to its possessor, can be shown to exist … then the difficulty of believing that a perfect and complex eye could be formed by natural selection, though insuperable by our imagination, can hardly be considered real.” He went on to propose an evolutionary sequence similar to the modern theoretical models. On the Origin of Species By Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life (London: Murray, 1859), 186-188. Available online here.
No comparable evolutionary sequence has been proposed for the bacterial flagellar motor,This is not to say that no hypotheses have been proposed at all. One hypothesis suggests that the flagellar motor evolved from a slightly simpler, syringe-like organelle called an injectisome or type III secretion system, which incorporates many similar proteins. However, genetic analyses suggest that the motor existed before the injectisome. (For further discussion, see the section on “Darwin’s Black Box and Michael Behe” in Stephen Meyer’s article “A Scientific History and Philosophical Defense of the Theory of Intelligent Design.”) To my knowledge, no one has yet proposed any plausible evolutionary sequence leading to the flagellar motor, let alone a highly detailed sequence comparable to the theoretical models of eye evolution. nor for many other intricate biochemical machines found inside cells, such as those involved in copying and transcribing the information in DNA.The videos on this page and this page of Chapter 10 illustrate the stunning complexity of the molecular machines involved in processing genetic information. Nevertheless, for all we know, these remarkable specimens of nanotechnology might have evolved by circuitous routes that are harder to envisage. The mere fact that we can’t think of an evolutionary pathway leading to the flagellar motor doesn’t entail that no such process occurred; it may just reflect the limitations of our imagination.
This objection to Behe’s argument does weaken its persuasive force, in my opinion, though I still find his argument somewhat compelling—especially taken in conjunction with other arguments discussed in the following pages. His critics are surely right that evolutionary processes can, in principle, produce irreducibly complex structures, but the ubiquity of such structures in living systems seems (to me) to exceed what we can reasonably expect from evolutionary mechanisms. In other words, the prevalence of irreducible complexity in biochemical machinery is not predicted by the theory and therefore counts as evidence favoring alternatives, such as design hypotheses, that make irreducible complexity less surprising.My suggestion here can be expressed in Bayesian terms. A Bayesian rule known as the likelihood principle says that if evidence E is more likely (more expected, or less surprising) assuming hypothesis H1 is true than if we assume H2 is true, then E favors H1 over H2. Evolutionary theory seems to predict that irreducible complexity should be rare. Given the assumptions of the theory, it is surprising that we find so many irreducibly complex machines in every living organism, even in the simplest living cells. The ubiquity of irreducibly complex biochemical machinery is entirely unsurprising, in contrast, if we assume that an intelligent designer was involved in the origin and evolution of life.