The Blind Watchmaker (44 page)

COPERNICUS WRONG. FLAT EARTH THEORY VINDICATED.

But, to be fair, Gould’s remark was aimed not so much at the alleged ‘gradualism’ of the Darwinian synthesis as at another of its claims. This is the claim, which Eldredge and Gould dispute, that all evolution, even on the grandest geological timescale, is an extrapolation of events that take place within populations or species. They believe that there is a higher form of selection which they call ‘species selection’. I am deferring this topic to the next chapter. The next chapter is also the place to deal with another school of biologists who, on equally flimsy grounds, have in some cases been passed off as anti-Darwinian, the so-called ‘transformed cladists’. These belong within the general field of taxonomy, the science of classification.

This book is mainly about evolution as the solution of the complex ‘design’ problem; evolution as the true explanation for the phenomena that Paley thought proved the existence of a divine watchmaker. This is why I keep going on about eyes and echolocation. But there is another whole range of things that the theory of evolution explains. These are the phenomena of diversity: the pattern of different animal and plant types distributed around the world, and the distribution of characteristics among them. Although I am mainly concerned with eyes and other pieces of complex machinery, I mustn’t neglect this other aspect of evolution’s role in helping us to understand nature. So this chapter is about taxonomy.

Taxonomy is the science of classification. For some people it has an undeservedly dull reputation, a subconscious association with dusty museums and the smell of preserving fluid, almost as though it were being confused with taxidermy. In fact it is anything but dull. It is, for reasons that I do not fully understand, one of the most acrimoniously controversial fields in all of biology. It is of interest to philosophers and historians. It has an important role to play in any discussion of evolution. And from within the ranks of taxonomists have come some of the most outspoken of those modem biologists who pretend to be anti-Darwinian.

Although taxonomists mostly study animals or plants, all sorts of other things can be classified: rocks, warships, books in a library, stars, languages. Orderly classification is often represented as a measure of convenience, a practical necessity, and this is indeed a part of the truth. The books in a large library are nearly useless unless they are organized in some nonrandom way so that books on a particular subject can be found when you want them. The science, or it may be an art, of librarianship is an exercise in applied taxonomy. For the same kind of reason, biologists find their life made easier if they can pigeonhole animals and plants in agreed categories with names. But to say that this is the only reason for animal and plant taxonomy would be to miss most of the point. For evolutionary biologists there is something very special about the classification of living organisms, something that is not true of any other kind of taxonomy. It follows from the idea of evolution that there is one uniquely correct branching family tree of all living things, and we can base our taxonomy upon it. In addition to its uniqueness, this taxonomy has the singular property that I shall call
perfect nesting
. What this means, and why it is so important, is a major theme of this chapter.

Let us use the library as an example of nonbiological taxonomy. There is no single, unique, correct solution to the problem of how the books in a library or a bookshop should be classified. One librarian might divide his collection up into the following major categories: science, history, literature, other arts, foreign works,
etc.
Each of these major departments of the library would be subdivided. The science wing of the library might have subdivisions into biology, geology, chemistry, physics, and so on. The books in the biology section of the science wing might be subdivided into shelves devoted to physiology, anatomy, biochemistry, entomology, and so on. Finally, within each shelf, the books might be housed in alphabetical order. Other major wings of the library, the history wing, the literature wing, the foreignlanguage wing, and so on, would be subdivided in similar ways. The library is, therefore, hierarchically divided in a way that makes it possible for a reader to home in on the book that he wants. Hierarchical classification is convenient because it enables the borrower to find his way around the collection of books quickly. It is for the same kind of reason that the words in dictionaries are arranged in alphabetical order.

But there is no unique hierarchy by which the books in a library must be arranged. A different librarian might choose to organize the same collection of books in a different, but still hierarchical, way. He might not, for instance, have a separate foreignlanguage wing, but might prefer to house books, regardless of language, in their appropriate subject areas: German biology books in the biology section, German history books in the history section, and so on. A third librarian might adopt the radical policy of housing all books, on whatever subject, in chronological order of publication, relying on card indexes (or computer equivalents) to find books on desired topics.

These three library plans are quite different from each other, but they would probably all work adequately and would be thought acceptable by many readers, though not, incidentally, by the choleric, elderly London clubman whom I once heard on the radio berating his club’s committee for employing a librarian. The library had got along for a hundred years without organization, and he didn’t see why it needed organizing now. The interviewer mildly asked him how he thought the books ought to be arranged. ‘Tallest on the left, shortest on the right!’, he roared without hesitation. Popular bookshops classify their books into major sections that reflect popular demand. Instead of science, history, literature, geography, and so on, their major departments are gardening, cookery, ‘TV titles’, the occult, and I once saw a shelf prominently labelled ‘RELIGION AND UFOs’.

So, there is no
correct
solution to the problem of how to classify books. Librarians can have sensible disagreements with one another about classification policy, but the criteria by which arguments are won or lost will not include the ‘truth’ or ‘correctness’ of one classification system relative to another. Rather, the criteria that are bandied about in argument will be ‘convenience for library users’, ‘speed of finding books’, and so on. In this sense the taxonomy of books in a library can be said to be arbitrary. This doesn’t imply that it is unimportant to devise a good classification system; far from it. What it does mean is that there is no single classification system which, in a world of perfect information, would be universally agreed as the only correct classification. The taxonomy of living creatures on the other hand, as we shall see, does have this strong property that the taxonomy of books lacks; at least it does if we take up an evolutionary standpoint.

It is, of course, possible to devise any number of systems for classifying living creatures, but I shall show that all but one of these are just as arbitrary as any librarian’s taxonomy. If it is simply convenience that is required, a museum keeper might classify his specimens according to size and keeping conditions: large stuffed specimens; small dried specimens pinned on cork boards in trays; pickled ones in bottles; microscopic ones on slides, and so on. Such groupings of convenience are common in zoos. In the London Zoo rhinos are housed in the ‘Elephant House’ for no better reason than that they need the same kind of stoutly fortified cages as elephants. An applied biologist might classify animals into harmful (subdivided into medical pests, agricultural pests and directly dangerous biters or stingers), beneficial (subdivided in similar ways) and neutral. A nutritionist might classify animals according to the food value of their meat to humans, again with elaborate subdivision of categories. My grandmother once embroidered a cloth book about animals for children, which classified them by their feet. Anthropologists have documented numerous elaborate systems of animal taxonomy used by tribes around the world.

But of all the systems of classification that could be dreamed up, there is one unique system, unique in the sense that words like ‘correct’ and ‘incorrect’, ‘true’ and ‘false’ can be applied to it with perfect agreement given perfect information. That unique system is the system based on evolutionary relationships. To avoid confusion I shall give this system the name that biologists give to its strictest form: cladistic taxonomy.

In cladistic taxonomy, the ultimate criterion for grouping organisms together is closeness of cousinship or, in other words, relative recency of common ancestry. Birds, for instance, are distinguished from nonbirds by the fact that they are all descended from a common ancestor, which is not an ancestor of any nonbird. Mammals are all descended from a common ancestor, which is not an ancestor of any non-mammal. Birds and mammals have a more remote common ancestor, which they share with lots of other animals like snakes and lizards and tuataras. The animals descended from this common ancestor are all called amniotes. So, birds and mammals are amniotes. ‘Reptiles’ is not a true taxonomic term, according to cladists, because it is defined by exception: all amniotes except birds and mammals. In other words, the most recent common ancestor of all ‘reptiles’ (snakes, turtles, etc.) is also ancestral to some non-‘reptiles’, namely birds and mammals.

Within mammals, rats and mice share a recent common ancestor with each other; leopards and lions share a recent common ancestor with each other; so do chimpanzees and humans with each other. Closely related animals are animals that share a recent common ancestor. More distantly related animals share an earlier common ancestor. Very distantly related animals, like people and slugs, share a very early common ancestor. Organisms can never be totally unrelated to one another, since it is all but certain that life as we know it originated only once on earth.

True cladistic taxonomy is strictly hierarchical, an expression which I shall use to mean that it can be represented as a tree whose branches always diverge and never converge again. In my view (some schools of taxonomists, that we shall discuss later, would disagree), it is strictly hierarchical
not
because hierarchical classification is convenient, like a librarian’s classification, nor because everything in the world falls naturally into a hierarchical pattern, but simply because the pattern of evolutionary descent is hierarchical. Once the tree of life has branched beyond a certain minimum distance (basically the bounds of the species), the branches never ever come together again (there may be very rare exceptions, as in the origin of the eukaryotic cell mentioned in Chapter 7). Birds and mammals are descended from a common ancestor, but they are now separate branches of the evolutionary tree, and they will never come together again: there will never be a hybrid between a bird and a mammal. A group of organisms that has this property of all being descended from a common ancestor, which is not an ancestor of any non-member of the group, is called a
elude
, after the Greek for a tree branch.

Another way of representing this idea of strict hierarchy is in terms of ‘perfect nesting’. We write the names of any set of animals on a large sheet of paper and draw rings round related sets. For example, rat and mouse would be united in a small ring indicating that they are close cousins, with a recent common ancestor. Guinea-pig and capybara would be united with each other in another small ring. The rat
mouse ring and the guinea-pig
capybara ring would, in turn, be united with each other (and beavers and porcupines and squirrels and lots of other animals) in a larger ring labelled with its own name, rodents. Inner rings are said to be ‘nested’ inside larger, outer rings. Somewhere else on the paper, lion and tiger would be united with one another in a small ring. This ring would be included with others in a ring labelled cats. Cats, dogs, weasels, bears,
etc.
would all be united, in a series of rings within rings, in a single large ring labelled carnivores. The rodent ring and the carnivore ring would then take part in a more global series of rings within rings in a very large ring labelled mammals.

The important thing about this system of rings within rings is that it is
perfectly nested
. Never, not on a single solitary occasion, will the rings that we draw intersect each other. Take any two overlapping rings, and it will always be true to say that one lies wholly inside the other. The area enclosed by the inner one is always totally enclosed by the outer one: there are never any partial overlaps. This property of perfect taxonomic nesting is not exhibited by books, languages, soil types, or schools of thought in philosophy. If a librarian draws a ring round the biology books and another ring round the theology books, he will find that the two rings overlap. In the zone of overlap are books with titles like ‘Biology and Christian Belief.

On the face of it, we might expect the classification of languages to exhibit the property of perfect nesting. Languages, as we saw in Chapter 8, evolve in a rather animal-like way. Languages that have recently diverged from a common ancestor, like Swedish, Norwegian and Danish, are much more similar to each other than they are to languages that diverged longer ago, like Icelandic. But languages don’t only diverge, they also merge. Modern English is a hybrid between Germanic and Romance languages that had diverged much earlier, and English would therefore not fit neatly in any hierarchical nesting diagram. The rings that enclosed English would be found to intersect, to overlap partially. Biological classificatory rings never intersect in this way, because biological evolution above the species level is always divergent.

Returning to the library example, no librarian can entirely avoid the problem of intermediates or overlaps. It is no use housing the biology and theology sections next door to each other and putting intermediate books in the corridor between them; for what then do we do with books that are intermediate between biology and chemistry, between physics and theology, history and theology, history and biology? I think I am right in saying that the problem of intermediates is inescapably, inherently a part of all taxonomic systems other than that which springs from evolutionary biology. Speaking personally, it is a problem that gives me almost physical discomfort when I am attempting the modest filing tasks that arise in my professional life: shelving my own books, and reprints of scientific papers that colleagues (with the kindest of intentions) send me; filing administrative papers; old letters, and so on. Whatever categories one adopts for a filing system, there are always awkward items that don’t fit, and the uncomfortable indecision leads me, I am sorry to say, to leave odd papers out on the table, sometimes for years at a time until it is safe to throw them away. Often one has unsatisfactory recourse to a ‘ miscellaneous’ category, a category which, once initiated, has a menacing tendency to grow. I sometimes wonder whether librarians, and keepers of all museums except biological museums, are particularly prone to ulcers.