The Accidental Species: Misunderstandings of Human Evolution (6 page)

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.
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Dutch men and women tend to be taller, on average—1.87 and 1.69 meters respectively,
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whereas urban men and women of the east African nation of Malawi tend to be shorter, 1.67 and 1.55 meters.
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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).
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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, then 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.
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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, mortality 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.
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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.