by Colin Tudge
Early in the nineteenth century the English anatomist Richard Owen provided one more, crucial, conceptual advance: one that answered Aristotle’s problem of how to distinguish important characters from less important. The important characters, said Owen, are ‘homologous’ – features which may have different functions in different creatures, but nonetheless clearly have a common origin. Thus the wings of birds, the front legs of horses and the arms of people serve very different functions, yet they all originate as forelimbs. This can be seen from common observation – and can be seen unequivocally when you look at the embryos. The wings of flies serve the same general purpose as the wings of birds, but are clearly very different. They arise as projections from the back, quite independently of the limbs. Bird wings and fly wings are merely ‘analogous’. The creatures with homologous, shared features should be grouped together (and birds, horses and people are all classed as ‘vertebrates’, with flies in the separate category of ‘insects’).
Cases like this are obvious. But when biologists are looking at unfamiliar structures in unfamiliar plants – and especially at fossils that are reduced to fragments – the crucial distinction between homologies and analogies can be very hard to make. Even in what may seem like clear-cut cases, the distinction may not be easy. Charles Darwin wondered whether flowers are homologous with the cones of conifers. They have a roughly similar structure (at least when compared with primitive flowers, such as magnolias) and they do the same job. The general consensus today is that they are not homologous. Conifers and flowering plants invented their sexual organs separately.
The trek from Aristotle to Linnaeus, with the additional insight of Owen, takes us halfway to modern taxonomy. But even by the time of Linnaeus, a sea-change was in the offing.
THE FINAL ROAD TO MODERNITY
Most European and American biologists until well into the nineteenth century took it for granted that life began on earth in the way described in Genesis. God created everything. He made each creature separately: the enormous diversity reflects the fertility of his mind. He placed each one in the environments to which it was best suited – shaggy bears in the north, smooth-haired bears in the tropics (Malaysia, South America), and so on. Each creature is ‘adapted’ to its environment – for if it were not, it could not live there, and the general phenomenon of adaptation was explained by God’s beneficence. He moulded creatures to thrive in whatever conditions he placed them in. Of course he did. He is benign.
But Genesis also implied that the world was created quickly – on the first day, on the second day, and so on. Furthermore, in the seventeenth century a zealous Irish bishop called Ussher added up the reported ages of all the patriarchs listed in the early books of the Old Testament, and concluded that the earth must have been created in 4004 BC, which made it less than 6,000 years old. Genesis also describes the Flood, in which Noah rescued a male and female of all the creatures. Present-day creatures are all descended from the couples that Noah took on to his ark. Clearly the creatures that lived before the Flood were the same as the ones that live now.
The general rationalism of the eighteenth century, the huge exercises in civil engineering that ate deep into the bedrock, and the new, growing, formal science of geology, nibbled away at the details offered in Genesis. It was clear by the end of the eighteenth century that the earth was much older than 6,000 years (even though the geologists who discovered this, such as Scotland’s James Hutton, typically remained as devout as ever). In the early nineteenth century formal collections of fossils, most spectacularly of dinosaurs and other ancient reptiles, showed beyond reasonable doubt that a huge range of creatures existed before the Flood, yet did not survive it – and also suggested that many of the creatures that surround us now, like elephants and oak trees, did not exist at the time of the dinosaurs. Clearly there had not been a once-for-all creation of plants and beasts that had remained unchanged ever since. Clearly the ones that were created first are long gone, replaced by others. Either there had been a series of separate creations (not recorded in Genesis) or the initial creatures had changed over time, to give rise to those of the present day. The idea that creatures might change over time was, and is, the idea of evolution.
Many people floated general notions of evolution in the eighteenth century. Even Linnaeus, it seems, though on the whole content with conventional theology, was veering towards it at the end of his life. Several formal descriptions and explanations were published in the late eighteenth and early nineteenth centuries, of which the best known is that of Lamarck. What was lacking, though, was a plausible mechanism: a way of explaining how and why there are so many different creatures on earth and how each one is adapted to its surroundings; and also how there could be change over time even though all creatures in general give rise to offspring who resemble themselves (‘like begets like’).
The biologists who finally provided the convincing account, and the plausible mechanism, were two Englishmen: Alfred Russel Wallace and Charles Darwin. Independently, they came up with the idea that Darwin called ‘natural selection’. Creatures do give birth to offspring that are like themselves – but the offspring (if sexually produced) are not identical with their parents. There is variation. Some variants, inevitably, will be better adapted to the prevailing conditions than others. But not all can survive, because all creatures are able to produce more offspring than the environment can support. The survivors, therefore, are the ones that are best adapted to the conditions. To the Victorians, the word ‘fit’ meant ‘apt’. So the ones that were best adapted were the ‘fittest’. In the 1860s Herbert Spencer, a philosopher who at that time was extremely famous, summarized the idea of natural selection as ‘survival of the fittest’, a phrase that Darwin later adopted.
In 1858 Darwin and Wallace presented their ideas in a joint paper, which was read on their behalf to the Linnean Society of London. The Linnean is a august society of biologists, still with its headquarters in Piccadilly, that was founded to commemorate Linnaeus. Darwin and Wallace’s paper was surely the most momentous ever presented to them – indeed it was one of the most momentous ever presented anywhere. But the Linnean’s president, in his annual report for 1858, dourly reported that nothing much of interest had happened that year. In 1859 Darwin (who had been thinking about the ideas for longer, and had a much broader scientific background) expounded the ideas more fully in On the Origin of Species by Means of Natural Selection, generally referred to as the Origin. The Origin changed the course of modern biology, and also changed all philosophy and theology. In it, Darwin speaks of ‘descent with modification’. Most other biologists preferred, and prefer, the term ‘evolution’.
In truth, Darwin made four outstanding contributions that are central to our theme. First, he established once and for all that evolution is a fact. Secondly, he provided the plausible mechanism – natural selection. Thirdly (a separate issue) he argued that species are not as the Platonists still conceived them to be – once-for-all creations that could not be changed. Over evolutionary time, he said, species could change into other species, and the lineages could branch so that any one species could give rise to many different types that would all then evolve along separate lines.
Finally, he proposed that all the creatures that have ever lived on earth are descended from the same common ancestor that lived millions of years in the past (although Darwin did not know how many millions). We share a common ancestor with robins and mushrooms and oak trees. This at a stroke answers the deepest problem: why there is order in nature. To be sure, we can say that God designed butterflies and bees along similar lines simply because he has a tidy mind. But we can also argue that butterflies and bees are similar because, in the deep past, they shared a common ancestor: the first ever insect. Deeper back in time, the first ever insect shared a common ancestor with the first ever shrimp – and insects and shrimps clearly have a lot in common. Even before that, the common ancestor of insects and shrimps shared a common ancestor with spiders. So
although insects are clearly very different from spiders, they, too still have quite a bit in common.
Since all creatures are literally related, they can all be represented on one great ‘family tree’; although a family tree drawn on such a scale is more properly called ‘phylogenetic’, from the term ‘phylogeny’, which refers to the evolutionary relationship between different groups of creatures (it comes from the Greek phylos, meaning ‘tribe’). This idea chimes beautifully with Linnaeus’s classification. Linnaeus’s kingdoms represent the great boughs of this all-embracing phylogenetic tree. The classes and orders are the thinner branches. The individual species are the twigs.
Some were offended by Darwin’s grand view of phylogeny. Some continue to argue that it is blasphemous, because it seems to contradict Genesis, which states that God created human beings separately from all other creatures, in his own image. Others are affronted by Darwin’s particular suggestion that human beings are most closely related to apes. The Creationist movement is still strong worldwide – not just in the United States. Some professional biologists are Creationist fundamentalists. In absolute contrast, many modern biologists and philosophers argue that since evolution by means of natural selection seems to offer a plausible alternative to the account in Genesis, then this means that religion in general is obsolete and God is dead.
In truth, neither of these extreme positions is valid. It makes no sense to reject evolutionary ideas; and it makes no sense to try to use those ideas to justify atheism. Leading churchmen of the late nineteenth century knew this (and Darwin is buried in Westminster Abbey). Many modern biologists who are steeped in evolutionary theory remain devout. Many take the wondrousness and subtlety of evolution as further proof that God is indeed marvellous, and demands reverence. Many indeed continue to argue in the spirit of the seventeenth century that the true purpose of science is to enhance appreciation of God’s works. For my part, I feel that Darwin’s is a glorious vision. I love the notion that we are literally related to all other creatures: that apes are our sisters, and mushrooms are our cousins, and oak trees and monkey puzzles are our distant aunts. Conservation, on such a view, becomes a family affair.
Conceptually, too, with Darwin’s great insight the task of taxonomy became easier. All the taxonomist has to do is identify creatures that share common ancestors. The way to do that is to identify shared characters that are homologous. In fact, Richard Owen remained wedded to the conventional theology of his day and never fully accepted the idea of evolution – and yet, ironically, his idea of homology provides a principal clue to evolutionary relationships. But in practice it can be very difficult to decide which of the characters that different creatures share are truly homologous – and even if the difficulties are overcome there is still one theoretical snag.
The snag is as follows – and again for simplicity I will use an example from animals rather than from plants; but the principle applies universally. Suppose you wanted to work out whether human beings were more closely related to horses, or to lizards. Suppose you decide to count the number of toes – a perfectly good ‘character’. Then you would conclude that the human and the lizard are closer – because both have five toes. The horse, with one toe, is the odd one out. Yet everything else about horses, human beings, and lizards, suggests that horses and people belong together (in the class of the mammals) and that lizards are the odd ones. This is the same kind of problem that Aristotle identified. Owen’s idea of homology is not all that helpful in this context. After all, the feet of lizards, horses and human beings are all homologous.
One further idea is needed to sort this out, and this was pinned down formally in the 1950s by a German biologist (in fact an entomologist) called Willi Hennig. He distinguished between homologous characters that are ‘primitive’, and those that are ‘derived’. ‘Primitive’ characters are those that are inherited from the very earliest ancestor of all the creatures in question. Thus lizards, horses and human beings are all distant descendants of some ancient amphibian that lived about 350 million years ago – and that ancestor had five toes. For all the descendants of that amphibian ancestor, the default position is also to have five toes. But some of those descendants have lost at least some of the toes – as birds have done and so (quite separately) have horses. Horses have lost four of the five fingers – all except the middle one. So too have asses and zebras. The point is that horses, asses and zebras all inherited their one-toed-ness from the same ancestor, the first ever one-toed equine, who lived somewhat more than 5 million years ago. Although human beings have many ‘derived’ features – including enormous brains – we happen to have retained the five-toed limbs of the first amphibian ancestor – the ‘primitive’ feature. So have lizards. But the fact that lizards and people have such characters in common does not show any special relationship. However, our big brains and our forward-looking eyes are ‘derived’ features, which were not present in that ancient amphibian ancestor, or indeed among the first ancestral mammals. They arose only among primates. They are among the characters that show our special, close relationship to chimpanzees.
By the same token, we can see that oaks, chestnuts and beeches all belong together (in the family Fagaceae) because all enclose their seeds within very similar casings (the cup of the acorn, the shell of the beech and chestnut). This casing is a derived feature, that shows their affinity. All three also, of course, have green leaves. But the leaves are primitive features, also found in magnolias and eucalypts, or indeed in pines and araucarias. The mere presence of leaves tells us nothing about the relationships of oaks, chestnuts and beeches – beyond the fact that all three are plants.
Hennig provided a whole list of rules for deciding whether shared homologous features are primitive or derived, and his general approach is known as ‘cladistics’, from the word ‘clade’, meaning all the descendants of a common ancestor. Cladistics has become the taxonomic orthodoxy only in past few decades. Much of the traditional classification in conventional textbooks does not incorporate Hennig’s ideas. Traditional taxonomists sometimes (quite often in fact) treated primitive and derived features together, and so created many groupings that look convincing – since all the creatures in the various groups do have plenty of characters in common – but in fact, if you look closely, are no more convincing than a classification would be that placed humans and lizards together, and excluded horses. In the chapters that describe the various groups of trees, you will find many instances of reclassification. This is partly because botanists are now revisiting old territory, and distinguishing more clearly than was often done in the past between the shared, homologous characters that are derived, which denote true, close relationships; and characters that are merely primitive.
Thus taxonomy has advanced conceptually over the past few decades – and it has advanced, too, in technique. From earliest times taxonomists looked at the obvious, ‘gross’ anatomy of creatures. From the seventeenth century onwards they could refine their observations with the help of microscopes – which also helped to reveal the insights provided by embroys. From the 1930s they could look even closer, with electron-microscopy, and home in on ‘microanatomy’. The fossil record has grown wonderfully, too, these past few decades. Some recently discovered fossils, recovered by modern techniques, offer the same microanatomical detail as living tissue. Brilliant. Then, of course, there are DNA studies – exploring and comparing the detailed chemical structure of genes.
But all these approaches have their drawbacks. All are subject to the trap that has beset all taxonomists since Aristotle: divergence and convergence. That is, creatures that are very closely related may adapt rapidly to different circumstances and finish up looking very different; and creatures that are not at all related may adapt to similar circumstances, and finish up looking much the same. Thus it transpires (when you look closely) that the family of oaks, beeches and chestnuts (Fagaceae) is closely related to that of cucumbers, melons, squashes and marrows (Cucurbitaceae): a fine case of diver
gence. On the other hand, as we have seen, many tropical rainforest trees have leaves that look very similar even though they may not be closely related, simply because all are adapted to dryness on the one hand and downpours on the other: a striking example of convergence. Fossils can be wonderfully instructive – but although some fossils show fine detail, most are to some extent fragmented and the fossil record as a whole is, as the palaeontologists say, ‘spotty’. Only one in many millions of extinct creatures gets to be fossilized and then discovered, and whole vast groups must have gone missing. Thus everything we know suggests that flowering plants and conifers share a common ancestor but it is very hard to find truly convincing links between the two within the fossil record, vast though it has become.
This, too, is why DNA studies do not provide the royal road to truth that was hoped for. Genes, too, may diverge or converge, just as anatomical features do, and so they can deceive. Even more to the point, different genes in the same organism may tell different stories. Thus studies in the 1980s suggested that the genes of red seaweeds are very different indeed from those of green plants, and that the two groups should be placed in different kingdoms that were miles apart on the grand Darwinian phylogenetic tree. But later studies in the 1990s, which looked at a different set of genes within red seaweeds and green plants, suggested that the two are very closely related – so closely that the two groups are, as taxonomists put the matter, ‘sisters’. The later studies are probably more accurate than the earlier ones – but it is always hard to be sure. Judgement and experience play as much part in modern taxonomy as they always did in the past, and there will always be disagreements about who is really related to whom. In taxonomy, as in science as a whole, there are no royal roads to truth. Some of the continuing debate is reflected in Chapters 4 to 8.
