When reading clever, carefully argued examples like this one, it’s easy to get caught up in the excitement, to think that behind every human quirk or malfunction is a truly adaptive strategy. Underpinning such examples is a bold premise: that optimization is the inevitable outcome of evolution. But optimization is not an inevitable outcome of evolution, just a possible one. Some apparent bugs may turn out to be advantages, but — as the spine and inverted retina attest — some bugs may be genuinely suboptimal and remain in place because evolution just didn’t find a better way.
Natural selection, the key mechanism of evolution, is only as good as the random mutations that arise. If a given mutation is beneficial, it may propagate, but the most beneficial mutations imaginable sometimes, alas, never appear. As an old saying puts it, “Chance proposes and nature disposes”; a mutation that does not arise cannot be selected for. If the right set of genes falls into place, natural selection will likely promote the spread of those genes, but if they don’t happen to occur, all evolution can do is select the next best thing that’s available.
To think about this, it helps to start with the idea of evolution as mountain climbing. Richard Dawkins, for example, has noted that there is little chance that evolution would assemble any complex creature or organ (say, the eye) overnight — too many lucky chance mutations would need to occur simultaneously. But it is possible to achieve perfection incrementally. In the vivid words of Dawkins,
you don’t need to be a mathematician or physicist to calculate that an eye or a hemoglobin molecule would take from here to infinity to self-assemble by sheer higgledy-piggledy luck. Far from being a difficulty peculiar to Darwinism, the astronomic improbability of eyes and knees, enzymes and elbow joints and the other living wonders is precisely the problem that any theory of life must solve, and that Darwinism uniquely does solve. It solves it by breaking the improbability up into small, manageable parts, smearing out the luck needed, going round the back of Mount Improbable and crawling up the gentle slopes, inch by million-year inch.
And, to be sure, examples of sublime evolution abound. The human retina, for example, can detect a single photon in a darkened room, and the human cochlea (the hair cell containing the part of the inner ear that vibrates in response to sound waves) can, in an otherwise silent room, detect vibrations measuring less than the diameter of a hydrogen atom. Our visual systems continue, despite remarkable advances in computer power, to far outstrip the visual capacities of any machine. Spider silk is stronger than steel and more elastic than rubber. All else being equal, species (and the organs they depend upon) tend, over time, to become better and better suited to their environment — sometimes even reaching theoretical limits, as in the aforementioned sensitivity of the eye. Hemoglobin (the key ingredient in red blood cells) is exquisitely adapted to the task of transporting oxygen, tuned by slight variations in different species such that it can load and unload its oxygen cargo in a way optimally suited to the prevailing air pressure — one method for creatures that dwell at sea level, another for a species like the bar-headed goose, an inhabitant of the upper reaches of the Himalayas. From the biochemistry of hemoglobin to the intricate focusing systems of the eye, there are thousands of ways in which biology comes startlingly close to perfection.
But perfection is clearly not always the way; the possibility of imperfection too becomes apparent when we realize that what evolution traverses is not just a mountain, but a mountain range. What is omitted from the usual metaphor is the fact that it is perfectly possible for evolution to get stuck on a peak that is short of the highest conceivable summit, what is known as a “local maximum.” As Dawkins and many others have noted, evolution tends to take small steps.[4] If no immediate change leads to an improvement, an organism is likely to stay where it is on the mountain range, even if some distant peak might be better. The kluges I’ve talked about already — the spine, the inverted retina, and so forth — are examples of just that, of evolution getting stuck on tallish mountains that fall short of the absolute zenith.
In the final analysis, evolution isn’t about perfection. It’s about what the late Nobel laureate Herb Simon called “satisficing,” obtaining an outcome that is good enough. That outcome might be beautiful and elegant, or it might be a kluge. Over time, evolution can lead to both: aspects of biology that are exquisite and aspects of biology that are at best rough-and-ready.
Indeed, sometimes elegance and kluginess coexist, side by side. Highly efficient neurons, for example, are connected to their neighbors by puzzlingly inefficient synaptic gaps, which transform efficient electrical activity into less efficient diffusing chemicals, and these in turn waste heat and lose information. Likewise, the vertebrate eye is, in many respects, tremendously elegant, with its subtle mechanisms for focusing light, adjusting to varied amounts of lighting, and so forth. Though it operates with more sophistication than most digital cameras, it’s still hobbled by the backward retina and its attendant blind spot. On the highest peak of evolution, our eyes would work much as they do now, but the retina would face forward (as it does in the octopus), eliminating those blind spots. The human eye is about as good as it could be, given the backward retina, but it could be better — a perfect illustration of how nature occasionally winds up notably short of the highest possible summit.
There are a number of reasons why, at any particular moment, a given creature might have a design that is less than optimal, including random chance (sheer bad luck), rapid environmental change (for example, if there’s a major meteor hit, an ice age, or another cataclysmic event, it takes time for evolution to catch up), or the influence that will animate much of this book: history, as encapsulated in our genome. History has a potent — and sometimes detrimental — effect because what can evolve at any given point is heavily constrained by what has evolved before. Just as contemporary political conflicts can in part be traced to the treaties following the world wars, current biology can be traced to the history of earlier creatures. As Darwin put it, all life is the product of “descent with modification”; existing forms are simply altered versions of earlier ones. The human spine, for example, arose not because it was the best possible solution imaginable, but because it was built upon something (the quadruped spine) that already existed.
This gives rise to a notion that I call “evolutionary inertia,” borrowing from Newton’s law of inertia (an object at rest tends to stay at rest, and an object in motion tends to stay in motion). Evolution tends to work with what is already in place, making modifications rather than starting from scratch.
Evolutionary inertia occurs because new genes must work in concert with old genes and because evolution is driven by the immediate. Gene-bearing creatures either live and reproduce or they don’t. Natural selection therefore tends to favor genes that have immediate advantages, discarding other options that might function better in the long term. Thus the process operates a bit like a product manager who needs his product to ship now, even if today’s cut corners might lead to problems later.
4
Emphasis on “tends to.” Strictly speaking, the steps taken by evolution may be of any size, but dramatic mutations rarely survive, whereas small modifications often keep enough core systems in place to have a fighting chance. As a statistical matter, small changes thus appear to have a disproportionately large influence on evolution.