by Colin Tudge
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 an 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 longer than Wallace had 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 of Species. 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. Second, he provided the plausible mechanism: natural selection. Third (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 people 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, 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 marvelous, 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 uncles and 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 decided 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 toes—all except the middle one. So too have asses and zebras. The point is that horses, asses, and zebras all inherited their one-toedness from the same ancestor, the first ever one-toed equine, who lived somewhat more than five 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, one 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 the 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 seem convincing—since all the creatures in the various groups do have plenty of characters in common—but, in fact, if you look closely, are actually 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 onward they could refine their observations with the help of microscopes—which also helped reveal the insights provided by embryos. 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 end up looking very different; and creatures that are not at all related may adapt to similar circumstances and end 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, and squashes (Cucurbitaceae): a fine case of divergence. On the other hand, as we have seen, many tropical rain-forest 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 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 were 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 were very closely related—so closely that the two groups were, 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. Judgment 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 5 through 10.
All these new approaches have caused taxonomists to modify Linnaeus’s original classification more than somewhat. In particular, modern taxonomists have greatly increased the number of kingdoms. Early-twentieth-century biologists decided that all single-celled creatures that are not green are “protozoa,” and put them in with the animals; and all single-celled organisms that are green they called “single-celled algae,” and put them in with the plants. Fungi and similar creatures such as slime molds were also rammed in with the plants. So were the brown seaweeds (wracks) and red seaweeds. Now it is clear that there is huge variation within the protozoa and the single-celled “algae,” so that they are now divided among about a half a dozen or a dozen different kingdoms (depending on who is doing the dividing); and some of those newly defined kingdoms contain both “protozoa” and “algae.” Fungi, the various groups of slime molds, and red and brown seaweeds now each have their own kingdoms; plants and animals are just two kingdoms among many, albeit by far the most conspicuous. Broadly speaking, all the kingdoms seem to divide into two great blocks, one including the plants (and red and green seaweeds and others) and the other containing the animals and fungi (and a lot of smaller types). In the early twentieth century, too, no one knew quite what to do with bacteria, although a few brave souls put them in a kingdom of their own. Now they are found to be so different that they are given their own “domain” (though, in truth, this is divided into two domains). The kingdoms of the plants, animals, fungi, seaweeds, and so on together form a third domain.
So now, the ranks that Linnaeus first described (species, genus, order, class, kingdom) have been increased to eight. The rankings now run: species, genus, family, order, class, phylum, kingdom, and domain (though botanists commonly substitute the term “division” for what the zoologists call class and/or phylum). Often these basic eight ranks are further subdivided or bunched together, as in “subfamily” or “superorder.” (But this can be overdone!)
All this may seem cumbersome, but it is all extremely useful. The family names make it easy to keep track—which is the most basic purpose of classification. Thus the 600 or so living species of conifer are now divided into eight families—and although 600 is too many for nonprofessionals, eight is straightforward; and if you can place conifers in their families, tell a pine from a swamp cypress, that is a lot better than nothing. The 300,000 or so species of flowering plant divide into 400 or so families—still too many for comfort; but the 400 or so are further grouped within about forty-nine orders, of which about thirty or so contain significant trees. It isn’t hard to get your head around this number (especially if you focus, for starters, on the top dozen or so and then work outward). Thus with just a small sense of modern taxonomy the whole bewildering world of trees, all 60,000 or so of them, begins to become tractable.
Then, too, the modern phylogenetic tree that includes all living creatures is, in effect, a graphic summary of their evolutionary history. If you know what group a creature belongs to, then you also know who its ancestors were, and who it is related to. We also know where at least some of the groups originated. Some began in the Southern Hemisphere—even way down in Antarctica, which once was forested. Some started life in Asia, and then spread west across Europe, and were blown as seeds across the Atlantic, and into America; or spread east across the Pacific, again into America. Some began in South America and tracked through the whole world. Most of this happened millions of years ago—long before human beings came on the scene (and stirred the pot even more). It is a wonderful thing to contemplate a living tree, or a fossil one, or any other creature. It is even more moving when we add the fourth dimension, of time, and see in our mind’s eye how the ancestor of the tree that grows in the field next door first saw the light in some remote corner of the globe millions or hundreds of millions of years in the past, and floated on its respective bit of continent as the continent itself circumnavigated the globe, and skirted around the glaciers of the ice age, and perhaps sweated it ou
t in some primeval, long-gone swamp, with alligators around its feet and the world’s first hawks and kingfishers scouting from its branches.
This is why I have been so keen to root this book in phylogeny—in modern taxonomy. It is an aide-mémoire to be sure, but more than that, it reflects evolution; and evolution reminds us of the glorious past of all living creatures. Without it, as the Russian-American geneticist Theodosius Dobzhansky said, biology makes no sense at all. The next chapter looks at a few of the historical details: how, in practice, modern trees are thought to have come into being.
3
How Trees Became
A TREE IS A BIG PLANT with a stick up the middle—and a big plant with a stick up the middle is not an easy thing to be. Darwin spoke of evolution as “descent with modification,” and it took a lot of descending—several billion generations—and a great deal of modification to get from the world’s first life to the world’s first plants to the creatures we recognize as oaks and monkey puzzles and eucalypts.
This chapter runs rapidly through the key events. It isn’t meant to be philosophical, but a point of philosophy continues to absorb me nonetheless. For when all Western thought was dominated by theology, change over time (like all aspects of nature) was seen to be part of God’s plan. Late-nineteenth-century and some twentieth-century theologians and scientists who were unhappy with Darwin’s particular idea that human beings descended from apes consoled themselves with the thought that those early apes were destined to become us; that they were mere prototypes, and prototypes are inevitably crude. In the same way, descriptions of evolution sometimes imply that the first land plants, say, somehow knew that their descendants would be vines and roses, redwoods and oaks, and saw themselves as a rehearsal. Again, the notion was that the course of evolution had been prescribed—or, as the Muslims say, “It is written.”