Nilsson and Pelger's purpose was not only to show that there is a smooth trajectory of improvement from a flat non-eye to a good fish {163}
Figure 5.14 Nilsson and Pelger's
theoretical evolutionary series leading
to a ‘fish’ eye. The number of steps
between stages assumes, arbitrarily,
that each step represents a 1 per cent
change in magnitude of something.
See text for translation from these
arbitrary units into numbers of
generations of evolution. {164}
eye. They were also able to use their model to estimate the time it would take to evolve an eye from nothing. The total number of steps that their model took was 1,829 if each step achieved a I per cent change in the magnitude of something. But there is nothing magic about I per cent. The same total quantity of change would have taken 363,992 steps of 0.005 per cent. Nilsson and Pelger had to re-express the total quantity of change in non-arbitrary, realistic units, and that means units of genetic change. In order to do this, it was necessary to make some assumptions. For example they had to make an assumption about the intensity of selection. They assumed that for every 101 animals possessing an improved eye who survived, 100 animals without the improvement survived. As you can see, this is a low intensity of selection as common sense might judge it — you are almost as well off without the improvement as with it. They deliberately chose a low, conservative or ‘pessimistic’ figure because they were bending over backwards to bias their estimate of rate of evolution towards being, if anything, too slow. They also had to make two other assumptions: of ‘heritability’ and of ‘coefficient of variation. The coefficient of variation is a measure of how much variation there is in the population. Natural selection needs variation to work upon and Nilsson and Pelger again deliberately chose a pessimistically low value. Heritability is a measure of how much of the variation, out of a given population's available variation, is inherited. If the heritability is low it means that most of the variation in the population is environmentally caused, and natural selection, for all that it may ‘choose’ individuals to live or die, will have little impact on evolution. If heritability is high, selection has a large impact on future generations because individual survival really translates into gene survival. Heritabilities frequently turn out to be more than 50 per cent, so Nilsson and Pelger s decision to settle on 50 per cent was a pessimistic assumption. Finally, they made the pessimistic assumption that different parts of the eye could not change simultaneously in one generation.
'Pessimistic’ in all these cases means that the estimate that we finally come up with for how long it takes an eye to evolve is likely to be on the long side. The reason we call an over-estimate pessimistic {165} rather than optimistic is this. A sceptic about the power of evolution, such as Emma Darwin, is naturally drawn to the view that an organ as notoriously complicated and many-parted as an eye, if it can evolve at all, will take an immense time to evolve. Nilsson and Pelger's final estimate was actually astoundingly short. At the end of the calculation, it turned out that it would take only about 364,000 generations to evolve a good fish eye with a lens. It would have been even shorter if they had made more optimistic (and this probably means more realistic) assumptions.
How long is 364,000 generations in years? That depends on the generation time, of course. The animals we are talking about would be small marine animals like worms, molluscs and small fish. For them, a generation typically takes one year or less. So Nilsson and Pelger's conclusion is that the evolution of the lens eye could have been accomplished in less than half a million years. And that is a very very short time indeed, by geological standards. It is so short that, in the strata of the ancient eras we are talking about, it would be indistinguishable from instantaneous. The plaint that there hasn't been enough time for the eye to evolve turns out to be not just wrong but dramatically, decisively, ignominiously wrong.
Of course there are some other details of a full-fledged eye that Nilsson and Pelger have not yet dealt with and which might (though they don't think so) take rather longer to evolve. There is the preliminary evolution of the light-sensitive cells — what I have been calling photocells — which they regarded as having been accomplished before the start of their model evolution system. There are other, advanced features of modern eyes such as the apparatuses for changing the focus of an eye, for changing the size of the pupil or y~stop’, and for moving the eye. There are also all the systems in the brain that are needed for processing the information from the eye. Moving the eye is important, not just for the obvious reason but, more indispensably, to hold the gaze still while the body moves. Birds do this by using the neck muscles to keep the whole head still, notwithstanding substantial movements of the rest of the body. Advanced systems for doing this involve quite sophisticated brain mechanisms. But it is easy to see that rudimentary, imperfect adjustments would be better than nothing, {166} so there is no difficulty in piecing together an ancestral series following a smooth path up Mount Improbable.
In order to focus rays that are coming from a very distant target, you need a weaker lens than to focus rays that are coming from a close target. To focus sharply both far and near is a luxury one can live without, but in nature every little boost to the chances of survival counts and as a matter of fact different sorts of animals display a variety of mechanisms for changing the focus of the lens. We mammals do it by means of muscles that pull on the lens and change its shape a little. So do birds and most reptiles. Chameleons, snakes, fishes and frogs do it in the same way a camera does, by pulling the lens a little way forwards or backwards. Animals with smaller eyes don't bother. Their eyes are like a Box Brownie: approximately, though not brilliantly, in focus at all distances. As we get older our eyes sadly become more Box Brownie-like and we often need bifocal glasses to see both near and far.
It is not at all difficult to imagine the gradual evolution of mechanisms for changing focus. When experimenting with the polythene bag filled with water, I quickly noticed that the sharpness of focus could be made better (or worse) by poking the bag with my fingers. Without being consciously aware of the shape of the bag, without even looking at the bag but concentrating on the quality of the image being projected, I simply poked and squashed the bag at random until the focus got better. Any muscle in the vicinity of a lump of vitreous mass could, as a by-product of contracting for some other purpose, incidentally improve the focus of the lens. This opens up a broad highway for gentle improvement all the way up the slopes of Mount Improbable, which could culminate in either the mammal or the chameleon method of changing the focus.
Changing the aperture — the size of the hole through which light is admitted — may be slightly more difficult, but not much. The reason for wanting to do this is the same as in a camera. For any given sensitivity of film/photocells, it is possible to have too much light (dazzle) as well as too little. Moreover, the narrower the hole, the better the depth of focus — the range of distances that are simultaneously in focus. A sophisticated camera, or eye, has a built-in light {167} meter which automatically stops down the hole when the sun comes out, and opens up the hole when the sun goes in. The pupil of a human eye is a pretty sophisticated piece of automation technology, something that a Japanese micro-engineer could be proud of.
But, once again, it isn't difficult to see how this advanced mechanism might have got its start on the lower slopes of Mount Improbable. We think of the pupil as circular, but it doesn't have to be. Any shape would do. Sheep and cattle have a long, horizontal, lozenge-shaped pupil. So do octopuses and some snakes, but other snakes have a vertical slit. Cats have a pupil which varies from a circle to a narrow, vertical slit (Figure 5.15):
Does Minnaloushe know that his pupils
Will pass from change to change,
And that from round to crescent,
From crescent to round they range?
Minnaloushe creeps through the grass
&
nbsp; Alone, important and wise,
And lifts to the changing moon
His changing eyes.
W.B.Yeats
Even expensive cameras often have pupils which are crude polygons rather than perfect circles. All that matters is that the quantity of light entering the eye should be controlled. When you realize this, the early evolution of the variable pupil ceases to be a problem. There are lots of gentle paths to be followed up the lower slopes of Mount Improbable. The iris diaphragm is no more an impenetrable evolutionary barrier than is the anal sphincter. Perhaps the most important quantity that needs to be improved is the speed of responsiveness of the pupil. Once you have nerves at all, speeding them up is an easy glide up the slopes of the mountain. Human pupils respond fast, as you can quickly verify by shining a torch in your eye while looking at your pupil in a mirror. (You see the effect most dramatically if you shine the torch in one eye while looking at the pupil in the other: for the two are ganged together.) {168}
Figure 5.15 Various pupils including that of a camera. The exact shape of a pupil doesn't matter, which is why it is allowed to be so variable: (a) reticulated python; (b) human; (c) cat; (d) long-nosed tree snake; (e) camera. {169}
As we've seen, the Nilsson and Pelger model developed a graded index lens, which is different from most man-made lenses but like those of fishes, squids and other underwater camera eyes. The lens arises by condensation of a zone of locally high-refractive index within previously uniform transparent jelly.
Not all lenses evolved by condensing out from a gelatinous mass. Figure 5.16 shows two insect eyes that form their lenses in quite different ways. These are both so-called simple eyes, not to be confused with the compound eyes which we'll come to in a moment. In the first of these simple eyes, from a sawfly larva, the lens forms as a thickening of the cornea — the outer transparent layer. In the second one, from a mayfly, the cornea is not thickened and the lens develops as a mass of colourless, transparent cells. Both these two methods of lens development lend themselves to the same kind of Mount Improbable climb as we've already undertaken for the vitreous mass eye of the worm. Lenses, like eyes themselves, seem to have evolved many times independently. Mount Improbable has many peaks and hillocks.
Retinas, too, betray their manifold origins by their variable forms. With one exception, all the eyes I have so far illustrated have had their photocells in front of the nerves connecting them to the brain. This is the obvious way to do it, but it is not universal. The flatworm in Figure 5.4a keeps its photocells apparently on the wrong side of their connecting nerves. So does our own vertebrate eye. The photocells point backwards, away from the light. This is not as silly as it sounds. Since they are very tiny and transparent, it doesn't much matter which way they point: most photons will go straight through and then run the gauntlet of pigment-laden baffles waiting to catch them. The only sense in which it even means much to say that vertebrate photocells point backwards is that the ‘wires’ (nerves) connecting them to the brain depart in the wrong direction, towards the light rather than towards the brain. They then run over the front surface of the retina towards one particular place, the so-called ‘blind spot’. This is where they dive through the retina into the optic nerve, which is why the retina is blind at this spot. Although we are all technically blind at the spot, we scarcely know it because the brain is so clever at reconstituting the missing bit. We only notice the blind spot if the {170}
Figure 5.16 Two different ways for insect lenses to develop: (a) sawfly larva; (b) mayfly.
image of some small discrete object, which we have independent evidence exists, moves on to it: it then appears to go out like a light, apparently replaced by the general background colour of the area.
I've said that it makes little difference if the retina is back-to-front. A case could be made that, absolutely all other things being equal, it {171} might have been better if our retinas were the right way round. It is perhaps a good example of the fact that Mount Improbable has more than one peak, with deep valleys between. Once a good eye has started to evolve with its retina back-to-front, the only way to ascend is to improve the present design of eye. Changing to a radically different design involves going downhill, not just a little way but down a deep chasm, and that is not allowed by natural selection. The vertebrate retina faces the way it does because of the way it develops in the embryo, and this certainly goes back to its ancient ancestors. The eyes of many invertebrates develop in different ways, and their retinas are consequently the ‘right’ way round.
Setting aside the interesting fact of their pointing backwards, vertebrate retinas scale some of the loftiest peaks on the mountain. The human retina has about 166 million photocells, divided into various kinds. The basic division is into rods (specialized for low-precision, non-colour vision at relatively low light levels) and cones (specialized for high-precision colour vision in bright light). As you read these words you are using only cones. If Juliet had seen Halley's Comet, it would have been her rods that were responsible. The cones are concentrated in a small central area, the fovea (you are reading with your foveas) where there are no rods. This is why, if you want to see a really dim object like Halley's Comet, you must point your eyes not directly at it but slightly away, so that its meagre light is off the fovea. Numbers of photocells, and differentiation of photocells into more than one type, present no special problems from the point of view of climbing Mount Improbable. Both kinds of improvement obviously constitute smooth gradients up the mountain.
Big retinas see better than small retinas. You can fit more photocells in, and you can see more detail. But, as always, there are costs. Remember the surrealist snail of Figure 5.1. But there is a way in which a small animal can, in effect, enjoy a larger retina than it pays for. Professor Michael Land of Sussex University, who has an enviable track record for exotic discoveries in the world of eyes and from whom I have learned much of what I know about eyes, found a wonderful {172} example in jumping spiders*. No spiders have compound eyes: jumping spiders have taken the camera eye up to a remarkable peak of economy (Figure 5.17). What Land discovered was an extraordinary retina. Instead of being a wide sheet on which a full image can be projected, it is a long, vertical strip, not wide enough to accommodate a decent image. But the spider makes up for the narrowness of its retina by an ingenious makeshift. It moves its retina systematically about, ‘scanning’ the area where an image might be projected. Its effective retina is, therefore, much larger than its actual retina — rather on the same principle as the bolas spider with its whirling single thread approximates the catchment area of a proper web. If the jumping spiders retina finds an interesting object, such as a moving fly or another jumping spider, it concentrates its scanning movements in the precise area of the target. This gives it the dynamic equivalent of a fovea. Using this clever trick, jumping spiders have carried the lens eye to a respectable little peak in their local area of Mount Improbable.
I introduced the lens as an excellent remedy for the shortcomings of the pinhole eye. It isn't the only one. A curved mirror constitutes a different principle from a lens, but it is a good alternative solution to the same problem of gathering a large amount of light from each point on an object, and focusing it to a single point on an image. For some purposes a curved mirror is actually a more economical solution to the problem than a lens, and the biggest optical telescopes in the world are all reflectors (Figure 5.18a). A minor problem with a reflecting telescope is that the image is formed in front of the mirror, actually in the pathway of the incoming rays. Reflecting telescopes usually have a small mirror to reflect the focused image sideways into an eyepiece or a camera. The small mirror doesn't get in the way, not {173}
Figure 5.17 Jumping spider. {174}
Figure 5.18 Curved mirror solutions to the problem of forming images: (a) reflecting telescope; (b) Gigantocypris, a large planktonic crustacean painted by Sir Alister Hardy; (c) scallop eyes peeping through gap in shell; (d) cross-section of scallop eye; (e) Ca
rtesian oval. {175}
enough to spoil the image, anyway. No focused image of the little mirror is seen: it merely causes a small reduction in the total amount of light hitting the big mirror at the back of the telescope.
The curved mirror, then, is a theoretically workable physical solution to an important problem. Are there any examples of curved mirror eyes in the animal kingdom? The earliest suggestion along these lines was made by my old Oxford Professor, Sir Alister Hardy, commenting on his painting of a remarkable deep-sea crustacean called Gigantocypris (Figure 5.18b). Astronomers capture what few photons arrive from distant stars with huge curved mirrors in observatories like Mount Wilson and Palomar. It is tempting to think that Gigantocypris is doing the same thing with the few photons that penetrate the deep oceans, but recent investigations by Michael Land rule out any resemblance in detail. It is at the moment not clear how Gigantocypris sees.
There is another kind of animal, however, that definitely uses a bona fide curved mirror to form an image, albeit it has a lens to help. Once again, it was discovered by that King Midas of animal eye research, Michael Land. The animal is the scallop.
The photograph in Figure 5.18c is an enlargement of a small piece (two shell-corrugations in width) of the gap of one of these bivalves. Between the shell and the tentacles is a row of dozens of little eyes. Each eye forms an image, using a curved mirror which lies well behind the retina. It is this mirror that causes each eye to glow like a tiny blue or green pearl. In section, the eye looks like Figure 5.18d. As I mentioned, there is a lens as well as a mirror, and I'll come back to this. The retina is the whole greyish area lying between the lens and the curved mirror. The part of the retina which sees the sharp image projected by the mirror is the portion tightly abutting the back of the lens. That image is upside-down and it is formed by rays reflected backwards by the mirror.