Evolution means change in a gene pool. Change in a gene pool means that some genes become more numerous, others less. Genes that used to be common become rare, or disappear altogether. Genes that used to be rare become common. And the result is that the shape, or size, or colour, or behaviour of typical members of the species changes: it evolves, because of changes in the numbers of genes in the gene pool. That is what evolution is.
Why should the numbers of different genes change as the generations go by? Well, you might say it would be surprising if they didn’t, given such immensities of time. Think of the way language changes over the centuries. Words like ‘thee’ and ‘thou’, ‘zounds’ and ‘avast’, phrases like ‘stap me vitals’, have now more or less dropped out of English. On the other hand, the phrase ‘I was like’ (meaning ‘I said’), which would have been incomprehensible as recently as 20 years ago, is now commonplace. So is ‘cool’ as a term of approval.
So far in this chapter, I haven’t needed to go much further than the idea that gene pools in separate populations can drift apart, like languages. But actually, in the case of species, there is much more to it than drifting. This ‘much more’ is natural selection, the supremely important process that was Charles Darwin’s greatest discovery. Even without natural selection, we’d expect gene pools that happen to be separated to drift apart. But they’d drift in a rather aimless fashion. Natural selection nudges evolution in a purposeful direction: namely, the direction of survival. The genes that survive in a gene pool are the genes that are good at surviving. And what makes a gene good at surviving? It helps other genes to build bodies that are good at surviving and reproducing: bodies that survive long enough to pass on the genes that helped them to survive.
Exactly how they do it varies from species to species. Genes survive in bird or bat bodies by helping to build wings. Genes survive in mole bodies by helping to build stout, spade-like hands. Genes survive in lion bodies by helping to build fast-running legs, and sharp claws and teeth. Genes survive in antelope bodies by helping to build fast-running legs, and sharp hearing and eyesight. Genes survive in leaf-insect bodies by making the insects all but indistinguishable from leaves. However different the details, in all species the name of the game is gene survival in gene pools. Next time you see an animal – any animal – or any plant, look at it and say to yourself: what I am looking at is an elaborate machine for passing on the genes that made it. I’m looking at a survival machine for genes.
Next time you look in the mirror, just think: that is what you are too.
4
WHAT ARE THINGS
MADE OF?
IN VICTORIAN TIMES, a favourite book for children was Edward Lear’s Book of Nonsense. As well as the poems about the Owl and the Pussycat (which you may know because it is still famous), The Jumblies and The Pobble Who Has No Toes, I love the Recipes at the end of the book. The one for Crumboblious Cutlets begins like this: ‘Procure some strips of beef, and having cut them into the smallest possible slices, proceed to cut them still smaller, eight or perhaps nine times.’
What do you get if you keep on cutting stuff into smaller and smaller pieces?
Suppose you take a piece of anything and cut it in half, using the thinnest and sharpest razor blade you can find.
Then you cut that in half, then cut that half in half, and so on, over and over again.
Do the pieces eventually get so small that they can’t get any smaller? How thin is the edge of a razor blade? How small is the sharp end of a needle?
What are the smallest bits that things are made of?
The ancient civilizations of Greece, China and India all seem to have arrived at the same idea that everything is made from four ‘elements’: air, water, fire and earth. But one ancient Greek, Democritus, came a bit closer to the truth. Democritus thought that, if you cut anything up into sufficiently small pieces, you would eventually reach a piece so small that it couldn’t be cut any further. The Greek for ‘cut’ is tomos, and if you stick an ‘a’ in front of a Greek word it means ‘not’ or ‘you can’t’. So ‘a-tomic’ means something too small to be cut any smaller, and that is where our word ‘atom’ comes from. An atom of gold is the smallest possible bit of gold. Even if it were possible to cut it any smaller, it would cease to be gold. An atom of iron is the smallest possible bit of iron. And so on.
We now know that there are about 100 different kinds of atoms, of which only about 90 occur in nature. The few others have been concocted by scientists in the lab, but only in tiny quantities.
Pure substances that consist of one kind of atom only are called elements (same word as was once used for earth, air, fire and water, but with a very different meaning). Examples of elements are hydrogen, oxygen, iron, chlorine, copper, sodium, gold, carbon, mercury and nitrogen. Some elements, such as molybdenum, are rare on Earth (which is why you may not have heard of molybdenum) but commoner elsewhere in the universe (if you wonder how we know this, wait for Chapter 8).
Metals such as iron, lead, copper, zinc, tin and mercury are elements. So are gases such as oxygen, hydrogen, nitrogen and neon. But most of the substances that we see around us are not elements but compounds. A compound is what you get when two or more different atoms join together in a particular way. You’ve probably heard water referred to as ‘H2O’. This is its chemical formula, and means it is a compound of one oxygen atom joined to two hydrogen atoms. A group of atoms joined together to make a compound is called a molecule. Some molecules are very simple: a molecule of water, for example, has just those three atoms. Other molecules, especially those in living bodies, have hundreds of atoms, all joined together in a very particular way. Indeed, it is the way they are joined together, as well as the type and number of atoms, that makes any particular molecule one compound and not another.
You can also use the word ‘molecule’ to describe what you get when two or more of the same kind of atom join together. A molecule of oxygen, the gas we need in order to breathe, consists of two oxygen atoms joined together. Sometimes three oxygen atoms join together to form a different kind of molecule called ozone. The number of atoms in a molecule really makes a difference, even if the atoms are all the same.
Ozone is harmful to breathe, but we benefit from a layer of it in the Earth’s upper atmosphere, which protects us from the most damaging of the sun’s rays. One of the reasons Australians have to be especially careful when sunbathing is that there is a ‘hole’ in the ozone layer in the far south.
Crystals – atoms on parade
A diamond crystal is a huge molecule, of no fixed size, consisting of millions of atoms of the element carbon stuck together, all lined up in a very particular way. They are so regularly spaced inside the crystal, you could think of them as being like soldiers on parade, except that they are parading in three dimensions, like a shoal of fish. But the number of ‘fish’ in the shoal – the number of carbon atoms in even the smallest diamond crystal – is gigantic, more than all the fish (plus all the people) in the world. And ‘stuck together’ is a misleading way to describe them if it makes you think of the atoms as solid lumps of carbon closely packed with no space in between. In fact, as we shall see, most ‘solid’ matter consists of empty space. That will take some explaining! I’ll come back to it.
All crystals are built up in the same ‘soldiers-on-parade’ way, with atoms regularly spaced in a fixed pattern that gives the whole crystal its shape. Indeed, that is what we mean by a crystal. Some ‘soldiers’ are capable of ‘parading’ in more than one way, producing very different crystals. Carbon atoms, if they parade in one way, make the legendarily hard diamond crystals. But if they adopt a different formation they make crystals of graphite, so soft it is used as a lubricant.
We think of crystals as beautiful transparent objects, and we even describe other things like pure water as ‘crystal clear’. But actually, most solid stuff is made of crystals, and most solid stuff is not transparent. A lump of iron is made of lots of tiny crystals packed toget
her, each crystal consisting of millions of iron atoms, spaced out ‘on parade’ like the carbon atoms in a diamond crystal. Lead, aluminium, gold, copper – all are made of crystals of their different kinds of atoms. So are rocks, like granite or sandstone – but they are often mixtures of lots of different kinds of tiny crystals all packed together.
Sand is crystalline, too. In fact, many sand grains are just little bits of rock, ground down by water and wind. The same is true of mud, with the addition of water or other liquids. Often, sand grains and mud grains get packed together again to make new rocks, called ‘sedimentary’ rocks because they are hardened sediments of sand and mud. (A ‘sediment’ is the bits of solid stuff that settle in the bottom of a liquid, for example in a river or lake or sea.) The sand in sandstone is mostly made of quartz and feldspar, two common crystals in the Earth’s crust. Limestone is different. Like chalk it is calcium carbonate, and it comes from ground-down coral skeletons and sea shells, including the shells of tiny single-celled creatures called forams. If you see a very white beach, the sand is most likely calcium carbonate from the same shelly source.
Sometimes crystals are made entirely of the same kind of atoms ‘on parade’ – all of the same element. Diamond, gold, copper and iron are examples. But other crystals are made of two different kinds of atoms, again on parade in strict order: alternating, for example. Salt (common salt, table salt) is not an element but a compound of two elements, sodium and chlorine. In a crystal of salt, the sodium and chlorine atoms parade together alternately. Actually, in this case they are called not atoms but ‘ions’, but I’m not going to go into why that is. Every sodium ion has six chlorines for neighbours, at right angles to each other: in front, behind, to left, to right, above, and below. And every chlorine ion is surrounded by sodiums, in just the same way. The whole arrangement is composed of squares, and this is why salt crystals, if you look at them carefully with a strong lens, are cubic – the three-dimensional form of a square – or at least have squared-off edges. Lots of other crystals are made of more than one kind of atom ‘on parade’, and many of them are found in rocks, sand and soil.
Solid, liquid, gas – how molecules move
Crystals are solid, but not everything is solid. We also have liquids and gases. In a gas, the molecules don’t stick together as they do in a crystal, but rush freely about within whatever space is available, travelling in straight lines like billiard balls (but in three dimensions, not two as on a flat table). They rush about until they hit something, such as another molecule or the walls of a container, in which case they bounce off, again like billiard balls. Gases can be compressed, which shows there is a lot of space between the atoms and molecules. When you compress a gas, it feels ‘springy’. Put your finger over the end of a bicycle pump and feel the springiness as you push the plunger in. If you keep your finger there, when you let the plunger go it shoots back out. The springiness that you are feeling is called ‘pressure’. The pressure is the effect of all the millions of molecules of air (a mixture of nitrogen and oxygen and a few other gases) in the pump bombarding the plunger (and everything else, but the plunger is the only part that can move in response). At high pressure the bombardment happens at a higher rate. This will happen if the same number of gas molecules are confined in a smaller volume (for instance, when you push the plunger of a bicycle pump). Or it will happen if you raise the temperature, which makes the gas molecules charge about faster.
A liquid is like a gas in that its molecules move around or ‘flow’ (that’s why both are called ‘fluids’, while solids aren’t). But the molecules in a liquid are much closer to each other than the molecules in a gas. If you put a gas into a sealed tank, it fills every nook and cranny of the tank up to the top. The volume of gas rapidly expands to fill the whole tank. A liquid also fills every nook and cranny, but only up to a certain level. A given amount of liquid, unlike the same amount of gas, keeps a fixed volume, and gravity pulls it downwards, so it fills only as much as it needs of the tank, from the bottom upwards. That’s because the molecules of a liquid stay close to each other. But, unlike those of a solid, they do slide around over each other, which is why a liquid behaves as a fluid.
A solid doesn’t even try to fill the tank – it just retains its shape. That’s because the molecules of a solid don’t slide around over each other like those of a liquid, but stay in (roughly) the same positions relative to their neighbours. ‘Roughly’ because even in a solid the molecules do sort of jiggle about (faster at higher temperatures): they just don’t move far enough from their position in the crystal ‘parade’ to affect its shape.
Sometimes a liquid is ‘viscous’, like treacle. A viscous liquid flows, but so slowly that, although a very viscous liquid eventually fills the bottom part of the tank, it takes a long time to do so. Some liquids are so viscous – flow so slowly – that they might as well be solid. Substances of this kind behave like solids, even though they’re not made of crystals.
Solid, liquid and gas are the names we give to the three common ‘phases’ of matter. Many substances are capable of being all three, at different temperatures. On Earth, methane is a gas (it’s often called ‘marsh gas’, because it bubbles up from marshes, and sometimes it catches fire and we see it lit up as eerie ‘will o’ the wisps’). But on a large, very cold moon of the planet Saturn called Titan there are lakes of liquid methane. If a planet were colder still, it might have ‘rocks’ of frozen methane. We think of mercury as a liquid, but that just means it’s liquid at ordinary temperatures on Earth. Mercury is a solid metal if you leave it outside in the Arctic winter. Iron is a liquid if you heat it to a high enough temperature. Indeed, around the deep centre of the Earth is a sea of liquid iron mixed with liquid nickel. For all I know, there may be very hot planets with oceans of liquid iron at the surface, and perhaps strange creatures swimming in them, although I doubt that. By our standards, the freezing point of iron is rather hot, so at the surface of the Earth we usually encounter it as ‘iron – cold iron’ (Google it. It’s from the poet Rudyard Kipling), and the freezing point of mercury is rather cold, so we usually encounter it as ‘quicksilver’. At the other end of the temperature scale, both mercury and iron become gases if you heat them enough.
Inside the atom
When we were imagining cutting matter into the smallest possible pieces at the beginning of this chapter, we stopped at the atom. An atom of lead is the smallest object that still deserves to be called lead. But can you really not cut an atom any further? And would an atom of lead actually look like a tiny little chip of lead? No, it wouldn’t look like a tiny piece of lead. It wouldn’t look like anything. That’s because an atom is too small to be seen, even with a powerful microscope. And yes, you can cut an atom into even smaller pieces – but what you then get is no longer the same element, for reasons we shall soon see. What is more, this is very difficult to do, and it releases an alarming quantity of energy. That is why, for some people, the phrase ‘splitting the atom’ has such an ominous ring to it. It was first done by the great New Zealand scientist Ernest Rutherford in 1919.
Although we can’t see an atom, and although we can’t split it without turning it into something else, that doesn’t mean we can’t work out what it is like inside. As I explained in Chapter 1, when scientists can’t see something directly, they propose a ‘model’ of what it might be like, and then they test that model. A scientific model is a way of thinking about how things might be. So a model of the atom is a kind of mental picture of what the inside of an atom might be like. A scientific model can seem like a flight of fancy, but it is not just a flight of fancy. Scientists don’t stop at proposing a model: they then go on to test it. They say, ‘If this model that I am imagining were true, we would expect to see such-and-such in the real world.’ They predict what you’ll find if you do a particular experiment and make certain measurements. A successful model is one whose predictions come out right, especially if they survive the test of experiment. And if the prediction
s come out right, we hope it means that the model probably represents the truth, or at least a part of the truth.
Sometimes the predictions don’t come out right, and so scientists go back and adjust the model, or think up a new one, and then go on to test that. Either way, this process of proposing a model and then testing it – what we call the ‘scientific method’ – has a much better chance of getting at the way things really are than even the most imaginative and beautiful myth invented to explain what people didn’t – and often, at the time, couldn’t – understand.
An early model of the atom was the so called ‘currant bun’ model proposed by the great English physicist J. J. Thomson at the end of the nineteenth century. I won’t describe it because it was replaced by the more successful Rutherford model, first proposed by the same Ernest Rutherford who split the atom, who came from New Zealand to England to work as Thomson’s pupil and who succeeded Thomson as Cambridge’s Professor of Physics. The Rutherford model, later refined in turn by Rutherford’s pupil, the celebrated Danish physicist Niels Bohr, treats the atom as a tiny, miniaturized solar system. There is a nucleus in the middle of the atom, which contains the bulk of its material. And there are tiny particles called electrons whizzing around the nucleus in ‘orbit’ (though ‘orbit’ may be misleading if you think of it as just like a planet orbiting the sun, because an electron is not a little round thing in a definite place).