Evolutionary theory has always had its critics. When Charles Darwin first published On the Origin of Species, he was vilified by prominent opponents from a wide variety of perspectives. Today, resistance to evolution remains steadfast in some corners of society, but is almost always grounded in a theological perspective. This has hindered proposed alternatives from attracting much support in the scientific community.
In scientific research, ideas are proposed as either hypotheses or theories (depending on their size and scope), and these make predictions that can be tested. An idea that survives the repeated testing of its predictions eventually gains acceptance and becomes an assumption on which more specific ideas are based. Those ideas are then tested, which simultaneously tests the underlying assumption, and the theory develops as evidence is obtained.
While Darwin’s ideas about natural selection have been substantially refined in the past century and a half, the basic premise has survived rigorous testing in all sub-disciplines of biology, and even in the adjacent disciplines of chemistry, biophysics, psychology, and even economics. It has become the unifying idea of all life science.
Most of the alternatives to evolution by natural selection cannot be tested by experimentation. That doesn’t make them false, per se. The disciplines of history, literature, and art exist almost entirely outside the methodology of science, but that doesn’t mean they’re worthless or unreliable. No one calls literary studies pseudoscience, because it doesn’t pretend to be science.
However, most critics of evolution present their alternatives as scientific and so, if they do not make testable predictions, or the predictions are refuted by evidence, they are rightly labeled pseudoscience. Most of these critics are not themselves scientists and their work attracts little attention. Enter Dr. Michael Behe.
A tenured full professor of biochemistry at Lehigh University in Pennsylvania, Behe is one of the very few truly credentialed biologists in public disagreement with the foundation of evolutionary theory. I normally avoid such snobbish credentialism, but the dearth of accomplished biologists in opposition to evolution speaks volumes. Many who present themselves as experts qualified to critique evolutionary theory are no such thing. The mark of a contributing expert is a terminal degree and a publishing record in the scientific literature.
The few evolution critics that hold doctorates and/or professorships are usually engineers, chemists, or physicists, but rarely biologists. This disciplinary mismatch is relevant. Just as biologists don’t have the knowledge or experience to critique quantum mechanics, engineers are out of their depth in critiquing evolution. But, as a biochemist, Behe has the relevant training and has contributed to the discipline that he’s critiquing — the evolution of biomolecular structures. Thus, he has “cred,” and his work warrants a response. In fact, as I write for Skeptic magazine, scientists ignore Behe at our own peril.
The basic critique
Behe burst onto the scene in the mid-1990s with his first book, Darwin’s Black Box, which advanced an idea he calls Irreducible Complexity. The idea is that some biological structures are so complex and have so many interacting parts, they could not have possibly evolved in an unguided, step-wise fashion. If the whole thing cannot function until all the parts are in place, none of the individual mutations along the way toward building it would be beneficial. Therefore, natural selection could not build a complex structure, so the argument goes.
The simple analogy that Behe puts forward is the mousetrap. While it only has a few individual components, this simple machine cannot work, even a little bit, until all of them are in place, fully formed. Therefore, there is no way for evolution to construct a mousetrap in a step-wise fashion because there is no partial function to give some selective advantage along the way.
Like all analogies, this one works because it’s simple. However, when the goal is to explain complexity, simplification may not be the best approach. After all, the building of a mousetrap bears no resemblance whatsoever to the fashioning of biomolecules. This debate is about origins, so an example that has an entirely different origin just isn’t relevant.
The biological examples of “irreducible complexity” that Behe employs are structures like the vertebrate eye. As I and many others have written, the step-wise evolution of the vertebrate eye is surprisingly well understood. Given that eyes don’t fossilize, one might imagine great difficulty in discovering past forms. However, fortunately for us, we have examples of earlier, simpler versions of the vertebrate eye among currently living animals.
These “living fossils” (a term I know I shouldn’t use but was too tempting here) demonstrate that Behe’s assertion that all of the components of a complex eye must be in place before it can function is simply not true. Sure, our eyes are breathtakingly complex now, but they didn’t evolve through the repeated addition of fully-formed, complex parts. Instead, the entire eye evolves as a unit, gradually moving from simplicity to complexity, but functional and beneficial to the organism at every step.
Scientists have also dismantled Behe’s claims about the bacterial flagellum, which likely evolved from a structure also used for secretion. Similarly, Kenneth Miller has articulated a relatively simple scheme through which the blood clotting cascade, complicated though it is, could have evolved in a step-wise fashion. Indeed, the laboratory of Russell Doolittle identified the evolutionary progenitor of a key clotting factor in sea cucumbers.
Mutations and more
In his new book, Darwin Devolves, Behe concentrates mostly on a key source of the diversity upon which natural selection acts: mutations. It turns out that Behe accepts common descent, the notion that all living things on Earth are descended from universal ancestors. He’s not an evolution denier! Behe also accepts that the Earth is billions, not thousands, of years old. He’s not a young earth creationist, either! He also understands how differential survival and reproduction can lead to adaptation, which is the basic mechanism of natural selection, the idea first championed by Darwin. Therefore, Behe is out of step with most of the people who support him. So what exactly is his disagreement with the scientific consensus?
Behe’s critique of modern evolutionary theory is actually quite narrow. He believes that random, unguided mutations (and other genetic alterations) cannot possibly be the source of all the variation that natural selection acts upon to shape organisms. Instead, he believes that an intelligent agent must provide an influx of new genetic information at key steps during evolution. This intelligently-provided information is the only source of innovation, according to Behe. He is skeptical of nature’s ability to provide even modest gains-of-function to genes or proteins.
Behe threads the needle carefully, accepting that natural selection can shape organisms, but only through minor modifications involving the destruction or diminishing of a gene. He even documents a few cases where random mutations diminish proteins function but benefit the organism in doing so, including in polar bears. By diminishing the deposition of pigment into their fur, harmful mutations helped the species thrive in the almost purely white background of the arctic. Behe offers other examples, such as bacteria evolving to a laboratory environment and finches in the Galápagos surviving and then recovering from drought.
He gets some pretty important things wrong about all three of those, but even if he didn’t, he is absolutely correct that loss-of-function mutations sometimes actually help the organism. Behe calls this phenomenon devolution, hence the name of the book, Darwin Devolves. I’m guessing that Behe accepts this particular point about natural selection because the evidence is clear, unequivocal, and overwhelming.
However, Behe goes further and claims that destroying function is all that mutations can ever do, and this is where he goes off the scientific rails. As many evolution critics are wont to do, Behe draws an imaginary line between “microevolution,” the diversification of populations and species, and “macroevolution,” the diversification of higher-order classification groups (primates, bony fishes, etc.). To be taxonomically precise, Behe drew his imaginary line between the level of family and order in his previous book, The Edge of Evolution. He now revises this and puts the line between genus and family.
To put this in terms familiar to creationists, he allows that natural selection can tweak and trim a bit, but not effect a “change in kind” when, for example, our reptile-like ancestors evolved into mammals. For that kind of transition, a supernatural force with “an intelligent mind” is needed.
The science that Behe ignores
According to Behe, the scientific community disagrees with his analysis because we refuse to allow room for the actions of a divine mind in the realm of the natural world, but there’s a simpler reason. We don’t agree with his position because there is no evidence for it. Moreover, there is a great deal evidence against it. He attempts to rebut some of this evidence, but most of it he outright ignores.
Devolves fails to mention the word exaptation even once. Exaptation is the re-tooling of existing structures for new purposes, like the evolution of forelimbs into wings in birds, bats, and pterosaurs. Exaptation is one of the key processes that explains Behe’s “irreducible complexity,” both at the molecular and anatomical scales.
So many things in our bodies used to be other things. The unique bones of our middle ears, for example, evolved from the jaw bone of our reptilian ancestors, and we have discovered a variety of intermediate fossils along this surprising evolutionary path (called the “Evolution Slam Dunk” by Rulon Downard). While Behe can’t be expected to cover everything, it’s not too much to ask that he engage with the very mechanism that scientists believe explains the thing that Behe feels can’t be explained.
And the same goes for mutations. While mutations often come from simple DNA copying errors and small instances of DNA damage, there are more drastic genomic rearrangements that can occur. Gene duplication, genetic recombination, virus- and transposon-mediated “jumping DNA,” and horizontal gene transfer are all technical terms for molecular phenomena in which large-scale jumbling of DNA sequences can occur. These big changes are usually harmful, of course, but occasionally, very rarely, they can lead to astoundingly unlikely and creative new arrangements of DNA.
Many years ago, a complex called T-urf13 was discovered in corn plants that had been formed through a dramatic rearrangement of disparate bits of noncoding mitochondrial DNA into functional genetic elements that encode a multi-subunit gated ion channel. This is relevant because gated ion channels are something that Behe claimed could never evolve through natural means. This pretty clear example of natural selection doing what Behe says it can’t was presented to him over at the Panda’s Thumb blog in 2007. So far, he hasn’t responded.
In fact, nowhere in the body of Darwin Devolves do the terms recombination or horizontal gene transfer appear at all. As these are key elements of how random genetic events can lead to the evolution of unlikely things, Behe’s failure to discuss them is, again, damning.
Behe also seriously misrepresents the science of three of his biggest examples in the book. In an essay for Skeptic magazine, I explain what he gets wrong about the long-term E. coli evolution experiment, and what he misses about the diversification of Darwin’s finches on the Galápagos Islands. And on my own blog, Arthur Hunt and I dissect his discussion of the evolution of polar bear pigmentation.
Getting evolution wrong
For someone who has dedicated his career toward critiquing evolutionary theory, Behe makes some puzzling errors about how it actually works. As mentioned, he demands that natural selection can only work if every single step on an evolutionary path is advantageous. We know that’s not true.
He also frequently conflates evolution with natural selection, and uses the term Darwinism to sloppily refer to either of them. Natural selection is but one evolutionary force and must be understood in the context of others such as genetic drift, neutral theory, recombination, exaptation, sexual selection, punctuated equilibrium, frequency-dependent selection, and dozens of others. Behe constantly repeats his refrain that natural selection cannot account for everything we see in nature. Yeah, we know. And we’ve known that for a very long time.
The very title of his book seems to betray a fundamental misunderstanding about how evolution actually works. Behe uses the terms devolves and devolution to indicate a loss or diminishing of function, as though these were the opposites of evolve and evolution.
This is a serious error because evolution simply means change over time. Evolution is not a steady march of increased function or increasing complexity. In fact, evolution favors simplicity, efficiency, and streamlining as often as it favors complexity, if not more so. While it’s possible that Behe is using the term in an off-hand or jocular manner, there’s no indication of that, and he uses the term repeatedly throughout the book in that very specific way.
If Behe thinks that evolution is a steady march towards increasing complexity and perfection, it’s no wonder that he has such angst about it. The reality is that evolution is aimless, sloppy, and produces clunky solutions as often as it does elegant ones. Our own bodies are filled with glitches and goofs left over from the imprecision of evolution. This may be deeply unsatisfying to some, but nature cares little about our satisfaction.
The tendency to see intelligent design in nature is an old one, but science moved past it long ago. As François Jacob wrote in Science over 40 years ago, “natural selection does not work as an engineer works. It works like a tinkerer — a tinkerer who does not know exactly what he is going to produce but uses whatever he finds around him … to produce some kind of workable object.”
Because he is an accomplished biochemist, Behe’s writing gives the air of scientific authority. However, the answers to the questions he poses have been worked out for quite some time. In 1918, H. J. Muller proposed the two-step process of how natural selection can create complexity: “Add a part; make it necessary.” This model was further updated in 2012 to “Innovation, Amplification, and Divergence,” but Behe seemed not to notice. Unfortunately, that is the dominant pattern of Darwin Devolves.
Note: An earlier version of this article misrepresented Behe’s claims about the evolution of chloroquine resistance in malaria. The author regrets the error.
Nathan H. Lents is Professor of Biology at John Jay College of the City University of New York, and is the author of Not So Different: Finding Human Nature in Animals and Human Errors: A Panorama of our Glitches, from Pointless Bones to Broken Genes.
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