Читать книгу Quantum Evolution: Life in the Multiverse - Johnjoe McFadden - Страница 29

A favourite argument of anti-evolutionists is that complex structures, such as the mammalian eye, could not possibly have evolved by random mutations. To make this point, a metaphorical monkey is often recruited to bang away at a typewriter, typing in random characters. The question is then asked: how long would it take our simian typist to type out the whole of Hamlet? The answer can be fairly easily calculated. He would have to type out about 25³⁰⁰⁰⁰ words (24 letters plus the space key raised to the power of the number of words in the text) to have a reasonable chance of typing in the correct text. If the monkey were a fairly proficient typist, say, one hundred words a minute, it would take him 25³⁰⁰⁰⁰, divided by one hundred, so approximately 10⁴⁰⁰⁰⁰ minutes to hit the keyboard the requisite number of times. The number of minutes since the Big Bang are however a mere 10²¹, a number vastly smaller. If we had a cosmic army of monkeys, one for every single electron in the universe and they had all been typing merrily away ever since the Big Bang, they would not have had sufficient time to achieve a tiny fraction of this feat.

However, the odds are radically transformed if we move from an entirely random selection of keys to adding one extra ingredient, selection. Imagine that we start with a small army of monkeys (a few hundred) and allow each to hit the keyboard once, selecting for breeding only those that correctly typed the first letter of Hamlet, W. Now, suppose that the ability to type that one letter is inherited, so that the progeny of these W monkeys invariably type the letter W with their first bang on the keyboard. Their next attempt at literary creativity – the next letter – would again be random, but once more we select for breeding only those that type H, the play’s next letter. (Once again, saying that the ability to type the second character is inherited in the same manner as the first.) Continuing this breeding policy for just nine generations would breed monkeys that would competently type the first line: ‘WHO’S THERE’ (we will allow ourselves to add in the punctuation). If we continued with our breeding programme, allowing about ten years for each generation, then it would take a mere 300,000 years for us to breed a line of Shakespearean monkeys, able to type the entire text of Hamlet!

The odds are so much better because we have introduced selection into the random process. The only difference between this and Darwinian evolution is that the selection we have introduced is artificial – we are doing the selection. In nature, it is the environment that does the selection: natural selection. Perhaps regrettably, the ability to type lines of Shakespeare is unlikely to impress many female monkeys and would not cut much mustard in monkey society. A tropical forest environment is unlikely to favour a line of literary monkeys. The ability to see is, however, vital. A monkey equipped with sharper eyesight might be more successful at finding fruit with which to tempt prospective mates. It may more readily spot the attack of a rival. The monkey eye is thereby subject to Darwinian natural selection and it is this that shrinks the odds of developing the eye’s complex structure from essentially zero to something achievable within geological lengths of time.

The key to the feasibility of this evolutionary scenario is the existence of a selective advantage for each and every step from simple to complex. This point is crucial to Darwinian evolution. The eye could only evolve if all the precedents to its modern form were viable and each had an advantage over its predecessor. Creationists claim that this is the weak point of the argument, for before an eye can reach the complex structure of its modern form, it must evolve through a thousand intermediate stages. But what use is half or a tenth of an eye? Surely an eye is only useful when all its parts are present and functioning?

This is a surprising claim, since Darwin himself used the eye to illustrate how the evolution of complex structures was indeed feasible by natural selection. In his essay, Organs of extreme complication and perfection’, Darwin pointed out that far from half an eye or a tenth of an eye being of no value, there are many living animals with half an eye or a tenth of an eye which manage very nicely with their supposedly imperfect vision. Many microbes, including photosynthetic bacteria, possess the most rudimentary vision. These bacteria are able to swim towards bright light where their photosynthetic skills are most effectively deployed. They even have colour vision since they are able to concentrate where in the spectrum their chlorophyll absorbs the most light. Mutants can be isolated that lack this phototactic ability, demonstrating that the gene for some kind of photoreceptor is encoded in their DNA and thereby subject to mutation and natural selection. Whether bacterial vision represents a tenth or even one-hundredth of an eye is a matter of opinion but it certainly gives the bacteria a selective advantage over blind mutants. It is even possible that light sensitivity did not originally evolve with a role in vision at all. Many primitive organisms have light-sensitive proteins that are used to set their circadian (the biological rhythms that track night and day) clocks. It may be that this clock-setting function of primitive eyes arose well before their value for seeing was harnessed.

From its origins as a single photoreceptor protein in a bacterial cell wall, the next step towards the eye may be the patch of light-sensitive cells found on the body surface of some starfish, jellyfish, leeches and worms. These animals are unable to form an image but can respond to different levels of light and darkness, which may allow them to locate the brighter, and more productive, shallow waters and rock-pools. More complex light receptors are found in limpets, clams and flatworms in which the photosensitive cells form a shallow cup used to detect the direction of light. The obvious next step was to add some kind of focusing mechanism to form a simple image. Some molluscs achieve this by the pinhole camera principle – light is forced to travel through a narrow aperture that focuses the image onto a cup of light-sensitive cells, which we may now call the retina. Vertebrates, insects and octopuses instead incorporate a transparent lens that allows more light into the eye, yet focuses the image. Finally, a variable aperture pupil might be added to control the amount of light allowed in; thus we have the mammalian eye.

The key to this evolutionary scenario is its gradualism. There are however a group of eminent palaeontologists who challenge it. Stephen Jay Gould and Niles Eldridge point out that the fossil record does not actually record gradual changes in species. Instead most species, including most horses, appear abruptly in the fossil record, change very little over their entire history and then disappear just as unceremoniously. This pattern is well known to palaeontologists who have usually attributed it to the imperfection of the fossil record: the missing links between one species and another have all died without the decency to leave their remains as fossils. Yet recent exhaustive studies of well-preserved species, such as marine snails, tend to support the view that, generally, evolution seems to hop and jump, rather than crawl.

Gould and Eldridge claim that the punctuated pattern of change is a real phenomenon which reflects two rates of evolution. The first, stasis, is exemplified by living fossils like crocodiles that have changed very little or not at all for millions of years. The second pattern of evolution occurs more sporadically and is characterized by geologically instantaneous speciation events (sometimes called macroevolution) in which one or several new species are generated. The pattern of long periods of stasis interspersed with rapid spurts of evolutionary innovation, they term ‘punctuated equilibrium’. Evolution that goes at two different speeds clearly needs some kind of gearing mechanism to change from one to another. Gould and Eldridge suggest that when evolution makes a jump, natural selection may be acting at a higher level to select whole species or groups of species, rather than individuals. However, many other evolutionary biologists, such as Richard Dawkins, take great exception to the view that natural selection acts on any unit higher than that of an individual (or even a single gene).

Whether evolution proceeds by tiny steps or big leaps, by examining the fossil record we can trace the evolution of man and animals back to the emergence of the first animals in the Cambrian explosion five hundred and fifty million years ago. Rocks earlier than the Cambrian explosion have very few fossils – which are nearly all microbes. Unfortunately, microbial fossils are not very distinctive. They give few clues about the evolutionary changes that led to the emergence of animals. To go deeper into the history of life we need to dig into DNA, rather than rocks.