There are two main features that distinguish natural selection from the concept of “design.” First, natural selection does not have any long-term “strategic plan” or ultimate goal. (It is not teleological.) Rather than striving toward some ideal of perfection, it simply tinkers by elimination of the less adapted with generation after generation, often changing direction or even resulting in the extinction of entire lineages. This is not what one would expect from a master designer. Second, because natural selection is constrained to work with what already exists, there is only so much that it can actually achieve. Natural selection starts by modifying species that have already evolved to a certain state, rather than by redesigning them from scratch. This is similar to asking a tailor to do some alterations to an old dress instead of asking the Versace fashion house to design a new one. Consequently, natural selection leaves quite a bit to be desired in terms of design. (Wouldn’t a visual field covering all 360 degrees or having four hands be nice? And were having nerves in the teeth or a prostate gland that totally surrounds the urethra really such great ideas?) So even if certain characteristics confer a fitness advantage, as long as there is no heritable variation that achieves this result, natural selection could never produce such characteristics. Imperfections are, in fact, natural selection’s unmistakable fingerprint.

You have probably noticed that Darwin’s theory of evolution is, by its very nature, not easily provable by direct evidence, since it typically operates on such long timescales that watching grass grow feels like a fast-paced action movie by comparison. Darwin himself wrote to the geologist Frederick Wollaston Hutton on April 20, 1861, “I am actually weary of telling people that I do not pretend to adduce evidence of one species turning into another, but I believe that this view is in the main correct, because so many phenomena can thus be grouped and explained.” Nevertheless, biologists, geologists, and paleontologists have amassed a huge body of circumstantial evidence for evolution, most of which is beyond the scope of this book, since it is not related directly to Darwin’s blunder. Let me only note the following fact: The fossil record reveals an unmistakable evolution from simple to complex life. Specifically, over the billions of years of geological time, the more ancient the geological layer in which a fossil is uncovered, the simpler the species.

It is important to mention briefly a few of the pieces of evidence supporting the idea of natural selection, since it was the notion that life could evolve and diversify without there being a goal to evolve toward that was the most deeply unsettling aspect of the theory to Darwin’s contemporaries. I have already mentioned one clue demonstrating the reality of natural selection: the resistance to drugs developed by various pathogens. The bacterium known as Staphylococcus aureus, for instance, is the most common cause for the types of infections known as staph infections, which affect no fewer than a half million patients in American hospitals each year. In the early 1940s, all the known strains of staph were susceptible to penicillin. Over the years, however, due to mutations producing resistance and through natural selection, most staph strains have become resistant to penicillin. In this case, the entire process of evolution has been compressed in time dramatically (due partly to the selective pressure exerted by humans), since the generations of bacteria are so short lived and the population is so enormous. Since 1961, a particular staph strain known as MRSA (an acronym for methicillin-resistant Staphylococcus aureus) has developed resistance not just to penicillin but also to methicillin, amoxicillin, oxacillin, and a whole host of other antibiotics. There is hardly a better manifestation of natural selection in action.

Another fascinating (although controversial) example of natural selection is the evolution of the peppered moth. Prior to the industrial revolution, the light colors of this moth (known among biologists as Biston betularia betularia morpha typica) provided ample camouflage against the background of its habitat: lichens and trees. The industrial revolution in England brought with it immense levels of pollution that destroyed many lichens and blackened many trees with soot. Consequently, the white-bodied moths were exposed suddenly to massive predation, which led to their near extinction. At the same time, the melanic, dark-colored variety of the moth (carbonaria) started to flourish around 1848, because of its much improved camouflage characteristics. As if to demonstrate the importance of “green” practices, the white-bodied moths started reappearing again once better environmental standards had been adopted. While some studies of the peppered moth and the phenomenon described above (“industrial melanism”) have been criticized by a number of creationists, even some of the critics agree that this is a clear case of natural selection, and they argue only that this does not provide proof of evolution, since the net result is merely of one type of moth morphing into another rather than into an entirely new species altogether.

Another common, more philosophical, objection to natural selection is that Darwin’s definition of it is circular, or tautological. Put in simple terms, the adverse judgment goes something like this: Natural selection means “survival of the fittest.” But how do you define the “fittest”? They are identified as those that survive best; hence, the definition is a tautology. This argument stems from a misunderstanding, and it is absolutely false. Darwin did not use “fitness” to refer to those who survive but to those who, when compared with other members of the species, could be expected to survive because they were better adapted to the environment. The interaction between a variable feature of an organism and the environment of that organism is crucial here. Since the organisms compete for limited resources, some survive and some don’t. Furthermore, for natural selection to operate, the adaptive characteristics need to be heritable, that is, capable of being genetically passed on.