by Henry Gee
2: All about Evolution
The word “evolution” is probably one of the most abused words in any argument about science. To some, it is a rallying cry to rationality. To others, it’s a term of abuse, the term “evolutionist” hardly less derogatory than “abortionist.” There can be few other words that get so much mileage while remaining so poorly understood. “When I use a word,” said Humpty Dumpty in Lewis Carroll’s Through the Looking-Glass, “it means just what I choose it to mean—neither more nor less.” Matters are made worse by the fact that the meaning of the word has changed over time, and remains ambiguous to this day.
When inventing the wheel, it is best to ensure that it is round before deciding what color to paint it. So, before we can get a handle on the word “evolution” in all its protean and subtle variety, one must first understand how it works, on the most basic nuts-and-bolts level. This is why Darwin started The Origin of Species by outlining such a mechanism—and not mentioning the word “evolution” at all. Darwin had very good reasons for not using the word in his masterpiece, as I shall explain a bit further on. Until then one might do a lot worse than follow his example.
Like many people these days, we in the Gee household keep chickens in our backyard. The hens are of several different breeds. We started with bantams, small birds whose function is more ornamental than anything else. They don’t lay many eggs, perhaps ninety per bird per year. They are, however, long-lived. At the time of writing, one of our first hens, a Pekin bantam, is four years old and still going strong. Our next two hens, Polish bantams, are almost as old, and in rude and squawking health. We also have several standard-sized hens, which lay more and bigger eggs.
But the prizes for productivity go to those in the flock that started their careers in intensive egg-production facilities. A battery hen can lay as many as three hundred eggs per year, but at a cost—the hens don’t live long. When a battery hen stops laying regularly, she dies of old age. Battery hens have been bred that way, to invest as much energy as possible into producing eggs, at a cost to their own bodily maintenance. Our first four battery rescues died of old age within two years, and we are now on our second quartet.
All the battery hens have russet feathers and red combs. They look just like the Rhode Island Reds my mother kept when I was a boy. As every backyard farmer knows, Rhode Islands are just about the best hens to keep if you like lots of eggs. These battery birds plainly have Rhode Island in their heritage, but they’ve been turbocharged to ramp up egg production at the cost of virtually everything else. In other words, they have been selected. If farmers depend for their livelihood on selling as many eggs as possible, they will breed future stock from the most productive egg-layers, and make the rest of the hens into cat food. They’d continually breed from the best layers in each generation, until, many generations down the line, they’d have created a new breed of hen that routinely lays many more eggs than any hen in the original flock.
This idea—the “artificial” selection by stockmen intent on breeding hens that lay more eggs, sheep with fleecier fleece, bulls with beefier beef, and so on—is intuitive, makes sense to anybody—and was how Darwin started the Origin.
What Darwin did next was a master stroke. Once he’d established artificial selection as an obvious and unarguable phenomenon, Darwin used it as an analogy for what goes on in the natural world. In nature the role of farmers is played by the environment. Creatures won’t be “artificially” selected by farmers for this trait or that, but “naturally” selected by the ever-changing environmental conditions in which they live. If the climate turns cold, those elephants that happen to have more body hair will be more likely to survive than those that are less hirsute—long enough to breed and pass on their hairiness to their offspring, while the baldies devote their energies to keeping warm rather than reproducing. If the climate continues cold, the bald elephants will eventually be replaced by woolly mammoths.
The beautiful thing about natural selection is its simplicity. All it requires to work are four things, three of which are readily apparent with eyes to see. They are heritable variation, the ever-changing environment, superabundance of offspring, and the passage of long periods of time.
Let’s look first at heritable variation. This means that any group of creatures will differ in their appearance or constitutions from one another, and that this variation is inherited from their parents. Unless they are identical siblings, the children in a family will inherit different traits from their parents, to different degrees. Some will be taller, some shorter, some darker, some fairer. For example, if you gathered every adult male (or adult female) in your town and measured them, you’d find that they’d vary greatly in height. You’d have to group men and women separately, as height is in part related to gender—on average, the men in any given population are taller than women from the same population. You’d find that most people would be middling in height, somewhere between 1.5 and 1.9 meters tall. People much shorter or taller than this are relatively rare. Any population is varied, but variation tends to cluster around a “mean” or “average” value. Calculating an average value is easy: add all the heights together, and divide what you get by the number of people you’ve measured.
The more people you measure, the better, because your result will be a better approximation of reality. If you can’t measure everyone in your neighborhood, say, you should still try to measure as large a sample as possible. If you can’t do that, you should try to ensure that the people you measure are picked at random. For example, if you measured the heights of the first three people you met, and they happened to be a coven of very small witches, or from a team of very tall basketball players, you shouldn’t be surprised that your sample is unrepresentative of people in your neighborhood in general.
When you see reports of preference in the press, such as peoples’ voting intentions, or whether their cats prefer ex-battery chicken of one brand over another, you should look out for the small print saying that the evidence comes from a poll of, say, 1,000 people chosen at random. It’s important to get lots of people, and to pick them by chance. This chance element is vitally important. There’s the probably apocryphal story of a market researcher who found that ninety-nine of a hundred people asked ate porridge for breakfast: it turned out that the people asked all came from the McPherson page of the Inverness telephone directory. This, without meaning any offense to residents of the fine city of Inverness who happen to be called McPherson, is probably not a representative sample of people as a whole.
From this it is clear that variation acts at different levels. As people vary in height even in your neighborhood, so do people from different places. Different populations have different average heights. The average American man is 1.76 meters tall, whereas the average American woman is 1.62 meters tall.1 Dutch men and women tend to be taller, on average—1.87 and 1.69 meters respectively,2 whereas urban men and women of the east African nation of Malawi tend to be shorter, 1.67 and 1.55 meters.3 This means that although men tend to be taller than women in general, the average Dutch woman will be taller than the average Malawian man. Because people tend to marry within their locality or ethnic group, the figures for average height differ from place to place.
Although people vary in all sorts of ways, and even though traits might be influenced by other things, such as nutrition and the environment, it’s plain that height tends to run in families—that is, variation is inherited. Tall parents tend to have tall children. My own daughters are among the tallest in their year groups—but I am relatively tall for an Englishman (1.83 meters, against the average of 1.75), and my wife is very much taller than the average Englishwoman (1.8 against 1.6 meters).4 She also comes from a family of tall women, who tended to marry guardsmen—not just tall, but proverbially tall. Hmm. The tallness strong within them it is.
From all this it’s clear that people (and other animals) vary, and that this variation can be passed on through the generations. If this weren’t true, the
n farmers wouldn’t be able to breed prime egg-laying hens by selecting the best layers in each generation as brood stock. Such variation is entirely obvious to anybody, yet in Darwin’s day nobody knew how variation was maintained. In his time it was generally assumed that the traits of parents got merged among the offspring—but if this were the case, all the variation would quickly get mixed together (like mixing paint of lots of different colors to get brown), and everyone would tend to look the same. But this doesn’t happen. Offspring are always varied. Even if the human population were well mixed, such that every person on Earth were obliged to choose their partner through a worldwide dating service, and did so for generations, their children would still vary in height, skin tone, eye color, and a host of other traits. The answer came long after Darwin, with the discovery of genetics, in which it is shown that traits are the expressions of atoms of inheritance called genes, which combine and recombine with one another to create variation, but remain individual and distinct. Some traits are influenced by single genes. Others, such as height, are influenced by many thousands.
The second factor that contributes to natural selection is the variability of the environment in which organisms live. I mentioned the case of mammoths above. If the climate turns cold, hairier elephants will have a better chance of surviving to reproductive age than elephants that are less hairy. Because hairiness will be to some extent inherited, the tendency toward hairiness will spread, so that, over time, the population of elephants will become hairier, on average.
You’ll of course have appreciated that the environment is very much more complicated than this cartoon explanation implies. The term “environment” means any circumstance, however small, that affects the chances of a creature surviving long enough to pass its traits on to the next generation. The environment doesn’t just mean the climate, or even the weather, but also the relationships that a creature has with other creatures, whether of different species or its own. The environment is therefore not one single thing, but uncountably many, each one changing minute by minute. A creature will have to be able to gather enough resources to grow, all the while trying not to be eaten by other creatures. Once mature, a creature will have to find a mate, and produce offspring, whose interests might differ from its own. All such factors constitute the environment.
Not surprisingly, some parts of the environment actually act in opposition to one another. Perhaps the best-known example is the case of sickle-cell anemia. This is an inherited disorder in which a person’s red blood cells fold up like squashed footballs and become very stiff. This makes them poor at carrying oxygen round the body. The malformed cells are also prone to clogging up blood vessels, causing all kinds of potentially life-threatening complications, including increased incidence of infection, damage to internal organs, thrombosis, and stroke. Sickle-cell anemia is a very serious disease indeed, and children with the disease stand much less chance of living long enough to reproduce than children without it. As a result, sickle-cell anemia is rare in most populations—people die of it before they can grow up to have children themselves.
The inheritance of sickling is well understood: it results from a defect in a single gene that codes for part of the molecule of hemoglobin, the protein in red blood cells that carries oxygen in the blood. Most genes are carried in two versions or “alleles,” one inherited from the father, the other from the mother. A child can carry two normal alleles, one normal allele alongside one sickling allele, or two sickling alleles. Only that child whose unhappy lot it is to carry two sickling alleles will suffer full-blown anemia. People with two normal alleles will, of course, not get the disease. People with one normal and one sickling allele will be normal, because the normal allele will produce more than enough normal hemoglobin to get by, and they are likely to suffer only if they happen to find themselves up a mountain where oxygen is scarce and hemoglobin has to work overtime.
Now, you’d think that because of the sickling allele’s effects on the chances of a young person’s reaching adulthood, natural selection would have expunged it pretty smartly from the population. But there’s a catch. It so happens that people with the sickle-cell trait are more resistant to malaria than those without. Malaria is debilitating enough in adults, but in children it can be lethal. It is caused by a microscopic parasite that hides out in red blood cells for part of its life cycle. Fewer red blood cells mean a less friendly place for malaria. People with sickle-cell anemia will be very ill anyway, but in the lottery of life, serious illness is often preferable to immediate death. People who have one sickling allele and one normal allele will be very much less ill, but much more resistant to malaria than those with normal alleles.
In parts of the world where malaria is endemic, such as sub-Saharan Africa, a child with sickle-cell anemia, or even a “carrier” with one copy of the sickling allele covered by a normal copy, will be better able to resist malaria and survive than a child with two copies of the normal allele, who is more likely to die from malaria than from sickle-cell anemia. This difference is crucial, for it alters the balance of survival in favor of the child who has sickle-cell anemia over the child who has not—and has allowed the otherwise entirely unwelcome sickle-cell trait to persist. In places haunted by the specter of malaria, carrying a gene for a debilitating disease is actually an advantage—it is the lesser of two evils.
Sickle-cell anemia demonstrates that natural selection is not some agent that drives creatures ever closer to the perfection imagined by advertising copywriters. Far from striving for bigger, better, more complex, or more enlightened, it does precisely and only what it needs to do to get a creature from egg to adulthood—and no more. This can mean carrying a trait for a dreadful disease that happens to offer protection from something worse. And because the environment is complicated, subtle, and ever changing, it is always a mistake to reduce natural selection to a simple mechanism that creates trends or tendencies that can be easily identified as such, and whose causes can easily be worked out.
The third factor that contributes to natural selection is superabundance of offspring. This means that creatures tend to produce many more offspring than can possibly survive. And by “many more,” I mean vastly more. Anyone who thinks evolution is all about elegance and orderly perfection in nature would be shocked by its profligacy and waste.5 Next to our chicken run is a pond, which I dug specifically to encourage the arrival of frogs, which would feast on garden pests such as slugs. Each spring the pond bubbles with hot frog-on-frog action, after which the water seethes with thousands of tadpoles—only one or two of which will survive long enough to reach sexual maturity. In the fall, our apple tree is groaning under the weight of fruit, but few or none of its seeds will ever germinate. Every woman produces hundreds of eggs throughout her lifetime, but only a few will be fertilized and come to term; every man produces millions of sperm, but relatively few children.
In ages past, people used to have large families, expecting that many (or most) of their offspring would die of something or another before they reached adulthood. Demons hovered around every crib and outside every nursery. I mentioned malaria, but even today millions of people, most of them children, die from dysentery, diarrhea, tuberculosis, cholera, or the effects of malnutrition. Darwin’s daughter Annie died from scarlet fever, which is now relatively rare. When I was a child, less than half a century ago, children even in Britain were severely disabled by or even died from diseases such as measles, mumps, rubella, pertussis (whooping cough), diphtheria, and poliomyelitis. Smallpox was a vanishing threat, but had not at that time been entirely eradicated. There is a reason that many of these dread diseases are associated with childhood—people who contract them as children might not survive to adulthood.
Thanks to improvements in public health and, notably, the success of vaccination, most of these diseases now figure only in period dramas, despite the best efforts of a deluded few anti-vaccination campaigners to turn fiction back into documentary. In the developed world nowadays, mortalit
y among children is less likely to result from infectious disease than from accidents or relatively rare birth defects.
Inherited diseases (as opposed to infectious ones) result from the fact that in a process as complicated and delicate as the development of a creature from an egg, mistakes are often made. The process is so complicated that it’s a wonder any of us actually gets born, and it could be that genetic variation itself exists as a hedge against error. By this, I meant that a certain amount of sloppiness is tolerated in the system, creating variation, and those variations that cause lethal or severe inherited disease are the price we all pay for being born at all.6
In the meantime—and it sounds desperately cruel—natural selection is likely to favor an earlier death (rather than a later one) from a debilitating disease so that harmful traits are less likely to be passed on (unless they provide an advantage, as in the case of sickle-cell anemia) and, more immediately, so that parents can get on with devoting limited resources to producing healthier offspring instead. In a world in which the threat of disease or mishap is always present, superabundance is a way of beating the odds, of maximizing your chances of your progeny surviving long enough to reproduce. The gambler at the roulette table who places all his chips on a single outcome will almost certainly lose. The gambler who puts a chip on every possible outcome is bound to win something. The second gambler will have lost an awful lot of chips but can stay in the game, whereas the first will have lost all of them and has no choice but to leave the casino.
These three things—heritable variation, the changing environment, and superabundance of offspring—are neither particularly special nor inherently mysterious. The fourth factor is time, and that’s a little more tricky.
Darwin saw natural selection not as an agency in itself, but the ongoing result of the interaction of several factors. Creatures tend to produce offspring that vary, and this variation is heritable. They also tend to produce more of them than can possibly survive. Nature will select those few offspring that are most suited to living in the prevailing environment, in much the same way that a stockman will select those animals most suited to his ends. Given enough time, the creatures will change, their adaptations tracking changes in the environment.
