Darwin’s Tree of Life

The Grandeur of a View that Changed the World

“Ask the beasts and they will teach you,” we read in Job (12:7). My new book takes its title from that verse, placing the natural world as envisioned by Charles Darwin’s theory of evolution in conversation with Christian belief in a loving God who creates, redeems, and promises a blessed future for our world. When we ask the animals and plants about their origin and relationship with God, a picture emerges of how they are cherished by divine love prior to, and apart from, the emergence of humanity. The evolution of the human species introduces sin into the world, seen today in our destruction of habitats and the resulting extinction of species. In this context, listening to the beasts fosters a deep ecological ethic as humans aim to replace their domination over nature with mutual regard and responsible care in the community of creation. The goal of this dialogue is to discover how love of the natural world is an intrinsic part of believers’ passion for the living God—to practical and critical effect. In this essay, the first of a two-part series, I hope to make clear how Darwin’s work changed our understanding of nature and humankind’s place in creation. Then, in the second essay, to be published in the next issue, I will explore what the Darwinian revolution means for Christian theology.

Charles Darwin’s On the Origin of Species is surely one of the most cited and least understood books of all time. Contrary to what many presume, it does not recount the story of life’s beginnings on earth. Nor does it trace the sequence of life from its earliest forms to the higher mammals. In fact, it’s not a story of origins at all. Rather, the book is “one long argument,” as Darwin put it, crafted to show that there is such a story to begin with. Origin makes the case that over millions of years, species of plants and animals have evolved from original parents, diversifying, dying, and generating new forms of life, according to the workings of natural selection.

Darwin begins his presentation like a good teacher, appealing to what readers already know. Chapter 1 describes the familiar practice of breeding—choosing animals and plants with desirable traits and mating them in the hope of reproducing and strengthening those features—and focuses on pigeon breeding, a popular British hobby. Darwin himself kept a flock of up to ninety birds, and in a dazzling nine-page riff on their variations, he carefully compares the “wonderful differences” found among his many breeds. Virtually every anatomical detail differs, from their beaks and nostrils, to the shape and size of their eggs, to their manner of flight and the sound of their voices. Most breeders believed that each type of pigeon descended from its own particular ancestor, so that current stock mirrored a preexisting range of original wild stock. But Darwin argued that since crossbred pigeons produce fertile offspring, they all descend from the same aboriginal form, despite their distinctive looks. If a wide variety of pigeons could descend from one species as a result of domestic breeding, he asked, why couldn’t that be the case for the rest of the planet’s plants and animals?

Darwin was well aware that he was covering new ground. “The laws governing inheritance are quite unknown,” he wrote. Yet he reasoned that since we can breed animals to produce characteristics we desire, species must not be set in stone, but rather must be capable of departing from the parent type. Selective breeding reveals “the great effect produced by the accumulation in one direction, during successive generations, of differences absolutely inappreciable by the uneducated eye.”

This set the stage for the next step in his argument, which uses the human selection of domesticated species as an analogy for similar selective processes in nature—processes driven by a “principle”:

I have called this principle, by which each slight variation, if useful, is preserved, by the term of Natural Selection, in order to mark its relation to man’s power of selection. We have seen that man by selection can certainly produce great results, and can adapt organic beings to his own uses, through the accumulation of slight but useful variations.... But Natural Selection, as we shall hereafter see, is a power incessantly ready for action, and is as immeasurably superior to man’s feeble efforts, as the works of Nature are to those of Art.

Prevailing ideas held that each species was the product of an act of “special creation” and as such remained stable over time. In the eighteenth century, the Swedish naturalist Carl Linnaeus had devised a system of classifying plants and animals (with significant revisions, it is still used today) which assigned all natural things to one of three kingdoms: animal, vegetable, or mineral. He then divided kingdoms into classes, classes into orders (such as Lepidoptera for moths and butterflies), and orders into families. From these flow genera—such as Canis, for dog—which then split into species, such as Canis lupus, the wolf, and from there into subspecies (for animals) and varieties (for plants). For Linnaeus, classification revealed the plan of creation. As he put it, “God created, Linnaeus arranged.”

But Darwin’s emphasis on variation challenged the neatness of Linnaeus’s system. He stressed the importance of slight differences, intermediary forms, and weak variations that seemed to grow stronger over time. For instance, nearly two hundred British plants considered varieties by some botanists were deemed individual species by others. Part of the problem, Darwin argued, lay in the idea that species are immutable. A different idea—that of an evolving relationship among organisms—offered a very different approach to classification. Instead of being seen as a collection of separate entities, forms of life could be divided into groups that gradually emerged from other groups. Natural selection, Darwin suggested, works in an incremental and cumulative fashion, so that changes amplify over time and new species are born.

But now a question arose. In breeding domesticated species, humans select properties they want to perpetuate. Given that nature does not act in a similarly purposeful fashion, what exactly governs the selection?

To answer this question, in Chapter 3—titled “Struggle for Existence”—Darwin delved into the circumstances in which selection occurs. Here we see the influence of Thomas Robert Malthus’s 1798 work, An Essay on the Principle of Population, which held that the tendency of human populations to grow faster than their ability to produce food would inevitably create clashes over sustenance. Darwin found this theory useful for his own ideas about the natural world. In every species more individuals are born than can possibly survive. In fact, without some destruction of life, there would be no room left. In this circumstance, nature applies a system of checks and balances. Thus we see species eating but also serving as prey; spreading out but being driven back; fighting off or succumbing to disease; surviving or dying in severe weather events; waging battle or setting up novel forms of cooperation.

“I use the term Struggle for Existence in a large and metaphorical sense,” Darwin wrote, “including dependence of one being on another, and including (which is more important) not only the life of the individual, but success in leaving progeny.” From the outset he insisted that the idea of a struggle included not just competition with others, but also mutual dependence or cooperation. “Owing to this struggle for life,” he wrote, “any variation, however slight and from whatever cause proceeding, if it be in any degree profitable to an individual of any species, in its infinitely complex relations to other organic beings and to external nature, will tend to the preservation of that individual, and will generally be inherited by its offspring.”

It is worth recalling that to an audience steeped in natural theology’s worldview, nature was largely a harmonious place, operating according to God’s plan. Few paid heed to the real state of nature, in which survival is not assured. Darwin offered a corrective view:

We behold the face of nature bright with gladness, we often see superabundance of food; we do not see, or we forget, that the birds which are idly singing around us mostly live on insects or seeds, and are thus constantly destroying life; or we forget how largely these songsters, or their eggs, or their nestlings, are destroyed by birds and beasts of prey; we do not always bear in mind that though food may be now superabundant, it is not so at all seasons of each recurring year.

This is a deeply ecological vision of nature, entailing a network of intricate interdependencies expressed both in competition and cooperation. Over time, each being’s structure becomes related, in an essential yet often hidden manner, to that of all other organic beings with which it competes for food, or on which it preys, or from which it has to escape, or with which it cooperates. Think of the teeth and talons of the tiger, or the hook of the parasite that clings to the tiger’s fur. Every variation that gives its owner an advantage in the struggle for existence stands in close relationship with the land, climate, and other creatures. As these variations are inherited and accumulate, they slowly adapt each form to its environment.

Is this process haphazard? When we look at a beautiful riverbank, Darwin acknowledged, we might be tempted to impute its pleasing proportions, its numbers and kinds of species, to chance. He then rejected this idea: “How false a view this is!” The world’s beauty arises from the mutual interactions of species in the struggle for life. Natural selection acts not only on visible characteristics, as human breeders do, but “on every organ, every shade of constitutional difference, on the whole machinery of life,” singling out advantage or disadvantage in the struggle for life. And so organisms are ever more beautifully adapted to their situations. Leaf-eating insects are green, bark-feeders mottled grey, the alpine ptarmigan white in winter, and the black grouse the color of peaty earth. Under the pressure of selection, small advantageous differences—caused by chance mutations—steadily increase, resulting in breeds that ultimately differ in character from one another and from their common parent. Over whole geological periods and thousands of generations, the result is the origin of new species and interacting communities, connected by complex mutual relations.

 

TWO PRINCIPLES AMPLIFY the outcome of natural selection, acting like its right and left hands: divergence and extinction. Divergence is premised on the idea that more life can flourish in a given area if it is occupied by organisms that draw from the same resources in different ways. If a region is filled to capacity with a species of carnivorous quadruped, for example, its numerous descendants will thrive only if they diverge to feed on new kinds of prey, climb trees, take to the water, or become less carnivorous. As selected and favored forms increase in number, meanwhile, filling niches and consuming resources, so will the less favored forms decrease and become rare. Ancestor species and transitional forms in particular are likely to diminish in number as their better-adapted progeny multiply. Many once-thriving species are now extinct while better-adapted ones have taken their place. Extinction is inevitable, Darwin insisted, “for the number of places in the polity of nature is not indefinitely great.” Divergence drives lineages apart, and extinction erases evidence of the transition.

In face of the dominant paradigm, Darwin’s theory of evolution argued that the true basis of classification is genealogical. A community of descent is the hidden bond that ties together all living beings. Darwin’s theory reveals the inner affinity of all organic beings to one another—an affinity he fashioned, finally, into the metaphor of the great tree of life. Picture a spreading evolutionary tree that links nature and history into an indivisible whole, spanning the ages. The outer layer of budding twigs and green leaves represents the multitudes of species alive today. Branching out from the trunk are the major limbs, representing previous periods of growth of the main groups of organisms; these were themselves once budding twigs when the tree was small. They fork into smaller limbs that divide into lesser and lesser branches, signifying the splitting into multiple descendant species. Lower down are dead branches, standing for extinct ancestral forms. Only two or three twigs that flourished when the tree was a mere bush may have survived as long shoots. Many a limb has decayed and dropped off, representing whole families of organisms known to us only from the fossil record. Over millions of generations, all living beings are connected in this great tree:

As buds give rise by growth to fresh buds, and these, if vigorous, branch out and overtop on all sides many a feebler branch, so by generation I believe it has been with the great Tree of Life, which fills with its dead and broken branches the crust of the earth, and covers the surface with its ever branching and beautiful ramifications.

This is an audacious account of the origin of species. In it, all organic beings, living and dead, are related to one another, historically and biologically; all take their place in a single narrative of creative struggle, divergence, thriving, death, extinction, and further breakthrough. Since natural selection can act only by the preservation and accumulation of infinitesimally small modifications, Darwin envisioned it as a slow, intermittent process. Beneficial variations, at first barely appreciable, steadily increase, and new breeds emerge that trend away in character both from one another and from their common parent. The process may be slow, but it is powerful. Now expand this pattern of repeated forking and branching, adaptation and extinction, to every creature alive at the same time, all interacting under the pressure of selection in the struggle for existence. It boggles the mind.

Researchers today estimate that a complete inventory of all species that have ever lived would number in the billions. Most are altogether absent from the fossil record—and most of those that were recorded have become extinct. What does it mean that death and extinction are so much a part of the story of life? Grounded on the belief that each new variety, and ultimately each new species, is produced by having some advantage in the struggle for life, Darwin’s theory holds that the extinction of less-favored forms almost always follows. When a new, slightly improved variety of short-horn cattle was raised in England, it first supplanted the older varieties in the same neighborhood; eventually it took the place of other breeds in other countries. So too with nature, where the appearance of new forms and the disappearance of old forms are bound together. Time and again Origin emphasizes that extinction is integral to the process of evolution.

In contrast, the further back in time we go, the closer species approach one another in structure and function. By tracking the history of life back into deep time, Origin demonstrates the strength of the argument that all forms of life unfurl through the eons as one grand natural system, linked by generation. All the great facts of geology and paleontology plainly reveal the theory of natural selection to be a better explanation of the history of life than special creation’s common view of the immutability of species. If species are immutable, fossils would not necessarily show gradations of structure over time and in close proximity, as they do. If species arose via special creation, then the same species should be able to reappear again and again to occupy similar niches throughout history, but they do not.

 

THE AGE OF EUROPEAN exploration helped naturalists accumulate knowledge of the earth’s flora and fauna. By the time Origin was published, the study of biogeography was an emergent science. The new worlds were stocked with strikingly unfamiliar plants and animals. Leading naturalists of the day interpreted the growing data as more evidence of the Creator’s design. As discoveries increased, they speculated on the number and locale of important “centers of creation” around the globe. Yet Darwin argued that natural selection offers a more plausible explanation of the distribution of life forms over the planet: species originate in one place from a common parent, then migrate and diverge, unless stopped by an impassable barrier. Over vast geological eras accompanied by large climate and land changes, similar species that now inhabit the most distant quarters of the world originally proceeded from the same source population.

Today’s best-known example, which Darwin did not use, is the different species of finches whose beaks evolved on the various Galápagos Islands. A multi-year study by Peter and Rosemary Grant documented how finch beaks differ from short, narrow, and shallow to long, wide, and deep, the differences correlating with the birds’ ability to harvest different types of seeds. In 1977, a severe drought visited the Galapagos Islands. Food supplies diminished so dramatically that finches hatched no eggs that year, and just 15 percent of them survived to see the next rain. The survivors had longer, wider, deeper beaks, allowing them to break open the tougher seeds. When they finally mated, their offspring had similar beaks. Six years later, rain water was plentiful, grass grew over the islands, and softer seeds abounded, allowing finches with shorter, shallower beaks to feed well and produce offspring. As the biologist David Reznick observes in his guide to Origin, this study shows that there is no best bird. There are only birds best suited to the conditions they inhabit.

That the theory of natural selection provided a better explanation than did special creation for the reality of life around us was supremely difficult for people of Darwin’s time to accept. “Nothing at first can appear more difficult than to believe that the more complex organs and instincts should have been perfected not by means superior to…human reason,” Darwin admitted, “but by the accumulation of innumerable slight variations, each good for the individual possessor.” This is partly why many of the eminent naturalists and geologists of his time rejected natural selection. Yet Darwin was adamant about the worth of his theory. He wasn’t demeaning creation. To the contrary, he confessed,  “When I view all beings not as special creations, but as the lineal descendants of some few beings which lived long before the first bed of the Silurian system was deposited, they seem to me to become ennobled.”

Even if Darwin could not convince his contemporaries, he held out hope that a new generation would embrace his theory. Once people saw that organisms have a history, he thought—when we finally understood that all living beings are related—the light would dawn, and there would follow a thrilling revolution in natural history. His theory itself is a bearer of hope. The final paragraph of On the Origin of Species invites us to ponder the multifarious reality of life and “to reflect that these elaborately constructed forms, so different from each other, and dependent on each other in so complex a manner, have all been produced by laws acting around us.” Darwin then offers this ringing conclusion:

There is grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.

And with that he brings his groundbreaking work to a close—not on a note of God-forsaking cynicism, not with a shrug, but with a tribute to cosmic beauty and wonder. It is a sentence whose eloquence has not been eroded by the tides of time.

This article was adapted from Ask the Beasts: Darwin and the God of Love, which will be published by Bloomsbury in March. Funding for this essay has been provided by a grant from the Henry Luce Foundation. 

Published in the January 24, 2014 issue: 

Elizabeth A. Johnson, CSJ, Distinguished Professor of Theology at Fordham University, is the author of Quest for the Living God, She Who Is, and several other books.

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