Darwin's Backyard

by James T. Costa

  The physiological division of labor was an industrial analogy from economics and manufacturing. In Chapter 3 of his Introduction, in a line that Darwin scored, Milne-Edwards explained how “in the creations of nature, as in the manufacture of men, it is mostly by the division of labor that perfectibility is obtained.” The key word there is “perfectibility.” In economics that might mean increased production efficiency, per capita productivity, or perhaps overall economic prosperity. In the analogy of Milne-Edwards it refers to increased complexity and improved functionality of the organism—as tissues and organs divide the “labor” of physiology, the organism as a whole prospers. Coelenterates like sea jellies, say, with a simple sac for a stomach that also functions for respiration are less efficient at feeding and breathing than animals with separate gut and respiratory organs, the implication being that by doing double-duty, tissues and organs by definition cannot specialize and so cannot achieve a high degree of efficiency in any one thing. That limits the adaptive possibilities for the organism. Similarly, having a single type of muscle is more limiting than the adaptive possibilities inherent in having multiple muscle types (e.g., skeletal, smooth, and cardiac), each specialized for a different function.

  Darwin transferred Milne-Edwards’s idea from the tissues and organs of organisms to different organisms in the economy of nature: in an analogy of an analogy he conceived of an ecological division of labor. It was a remarkable idea: just as the overall productivity of the organism as a whole increases with increasing physiological division of labor, so too, Darwin thought, can more species live and prosper together in any one area if they are specialized in slightly but sufficiently different ways. This hypothesis resonated with and helped crystallize ideas on natural selection, extinction, adaptation, and diversity swirling around in Darwin’s head. Before reading Milne-Edwards, Darwin had a general conception of diversification of species over time through isolation and selection. He studied variation with an eye to intermediate forms as evidence of a slow diversification; his barnacle studies were conducted with an eye to that, as well as to the evolution of separate sexes from a hermaphroditic ancestral state. After Milne-Edwards, Darwin saw diversification more in the context of division of labor, and his idea of an ecological division of labor was key. As he labored to finish up his barnacle work this idea was never far from his mind—not to mention his perennial back-burner researches into variation and domestic animals, cross-pollination, and other subjects.

  In November 1854 Darwin scribbled down a thought: “It is indispensable to show that in small & uniform areas there are many Families & genera. For otherwise we cannot show that there is a tendency to diverge (if it may be so expressed) in offspring of every class, & so to give diverging tree-like appearance to the natural genealogy of the organised world.”10 What he was saying was that if in a small area, homogeneous in its conditions (of soil characteristics, say, and temperature, rainfall, etc.), you were to find great diversity, then that would suggest the existence of a tendency or force (like natural selection) promoting diversification and coexistence. Otherwise, a small homogeneous area would be expected to support one or a few species. Darwin thinks that was likely the case ancestrally, but over time through competition and selection the more divergent offspring variants tended to persist while those too similar in their ecological needs were rendered extinct. It’s a dynamic that leads to more and more divergent varieties and species occupying that small and homogeneous area. He soon came to realize that this dynamic is not simply an interesting by-product of selection, but is, in fact, key to how selection drives the evolution of the diversity of life. But what evidence could he bring to bear on this idea?

  Darwin thought back to the monotonous heathlands of Hartfield, in Sussex, home of Emma’s sister Sarah Wedgwood and Charles’s sister Charlotte, and of Leith Hill, in Surrey, where his family had just visited his sister Caroline and her husband Joe Wedgwood. (Like Charles, Caroline had married a Wedgwood cousin—Emma’s brother Josiah III, who went by Joe.) Contrast the heathland monoculture with the abundant flora of fertile meadows; both are crowded with plants, but one cannot doubt, Darwin mused, that overall more plant (and thus animal) life is supported in the latter. Maybe this could be proved experimentally: “I think [the] amount of chemical change should if possible be taken as measure of life, viz amount of carbonic acid [carbon dioxide] expired or oxygen in plants.” Easier said than done, however—“Gas” Darwin’s chemistry days were a distant memory now.

  That winter and spring, 1855, Darwin’s preoccupation with diversity overlapped with his growing interest in the related problem of the geographical distribution of species and varieties. In fact, the two interests dovetailed nicely: it was all a matter of spatial scale. Diversification and diversity in small areas, where individuals experience the direct effects of competition and selection, ultimately bears on geographical distribution on the scale of continents. Relevant, too, is the question of how species become distributed as we see them on local, regional, and global scales. He began a series of experiments looking at seed dispersal: it was astounding to him that no one had ever considered doing so before. All through the severe winter of 1855, Darwin and the children diligently followed the progress of seeds they set floating in artificial saltwater, tipping in snow to simulate cold seawater, chalking up floaters versus sinkers, and testing germination after progressively longer periods of immersion. We’ll explore that rich line of experimental investigation fully in Chapter 5; for now just note that through that winter Darwin was dreaming of field experiments to come.

  A Taste for Botany

  Nineteen-year-old Catherine Thorley of Tarporley, Cheshire, joined the family as governess in 1848. She worked at Down House for nearly 10 years, through the dark days of the loss of Annie and the happy ones that saw the birth of Frank, Lenny, and Horace. And although she had her work cut out for her with Etty, then an adolescent with an attitude, all in all she was well loved by the family. She taught the children French, dancing, music, and etiquette, and also had a love of plants and a curiosity about her employer’s work. So, it’s perhaps no surprise that June 1855 found Miss Thorley gamely assisting Darwin in his latest hobbyhorse: a flowering plant survey of Great Pucklands Meadow and environs, the 13-acre field just over the hedgerow from Darwin’s sandwalk, owned by his friend and neighbor Sir John Lubbock. Darwin wrote to Hooker about it on June 5th:

  Miss Thorley & I are doing a little Botanical work (!) for our amusement, & it does amuse me very much, viz making a collection of all the plants, which grow in a field, which has been allowed to run waste for 15 years, but which before was cultivated from time immemorial; & we are also collecting all the plants in an adjoining & similar but cultivated field; just for the fun of seeing what plants have arrived or dyed out. Hereafter we shall want a bit of help in naming puzzlers.—How dreadfully difficult it is to name plants.11

  Before posting the letter he surprised himself with his first grass identification, excitedly appending a P.S.: “I have just made out my first Grass, hurrah! hurrah! I must confess that Fortune favours the bold, for as good luck would have it, it was the easy Anthoxanthum odoratum: nevertheless it is a great discovery; I never expected to make out a grass in all my life. So Hurrah. It has done my stomach surprising good.” Darwin may have exaggerated his inability to identify plants, but it’s true that A. odoratum, pleasantly scented sweet vernal grass common throughout Europe and Eurasia, is a rather easy one to identify. Others he shipped off to Hooker. Ten days later he sent his thanks: “You cannot imagine what amusement you have given me by naming those 3 grasses: I have just got paper to dry & collect all grasses.— If ever you catch quite a beginner, & want to give him a taste for Botany tell him to make perfect list of some little field or wood. Both Miss Thorley & I agree that it gives a really uncommon interest to the work, having a nice little definite world to work on, instead of the awful abyss & immensity of all British Plants.”12

  The survey continued over the course o
f the year, often with the kids helping out. Their approach to surveying was more casual than systematic, randomly searching for anything new rather than sweeping along transects or scrutinizing quadrats the way a modern botanist would do it. But it suited his purpose: he may have said the survey was done for his amusement, but there was more to it than that. He saw the field and wood as a “small uniform area”; how much diversity was found there? In the end a whopping 142 plant species falling into 108 genera and 32 orders were identified in Great Pucklands alone. One hundred fifty years later, in June 2005, three generations of Darwin’s descendants (ranging from 21 months to 78 years of age) gathered at Great Pucklands to help kick off a repeat of their ancestor’s survey along with botanists from London’s Natural History Museum.13 The NHM team surveyed in the summers of 2005–2007, taking samples in the last year for DNA barcoding. They found some new species and many of Darwin’s old friends (including his triumphantly identified sweet vernal grass)—overall nearly the same numbers of species and genera as Darwin and Miss Thorley had found, but rather fewer orders. Why? It largely reflects changes in classification: in Darwin’s day British flowering plants were classified into about 86 plant orders, while the latest classification recognizes 62 orders. That means some of Darwin and Miss Thorley’s plant orders have since been combined with others.

  Darwin would have found these results interesting indeed—but perhaps unsurprising. Those 140-something plants are perpetually vying to expand, yet they are kept in mutual check generation after generation. In Darwin’s thinking, what we are seeing is a showdown in slow motion. The reason we see rich diversity in the meadow, the reason that highly varied species manage to live closely together for a long period of time in rather small and uniform areas, is that competition and selection have over the eons led to that very diversification and promoted their coexistence—each of the plants is adapted to ever-so-slightly different conditions, just different enough on a microgeographical scale that for all intents and purposes they compete little though they grow side by side. If you think about it, this is an ecological vision: the community itself emerges, evolves even, as constituent species become coadapted. Coadaptation does not have to mean adapted for direct interaction like pollinator and flower. Instead, it is a simple coexistence, with the species partitioning resources in such a way that they co-adapt to living closely together.

  The one thing missing from Darwin and Miss Thorley’s botanical survey was the temporal dimension. Such assemblages of species in a given locale change over long periods of time, with species coming and going, some persisting long periods and others winking out quickly. These ideas began to resonate more deeply with his interest in both dispersal (on a global as well as a local scale) and seed viability—again, taken up in Chapter 5. Darwin’s musings on dispersal and seed viability in connection with his interest in divergence led him to discover the seed bank concept: the idea that the soil of a given area is chock full of seeds deposited there by birds, wind, or water, or dropped by plants that once occupied the spot. In large part the stock of seeds in the ground represent ghosts of plant communities past. Often they are very much alive, even after decades or centuries. Think of lotus seeds found in ancient Egyptian tombs germinating after eons. Maybe the earth is saturated with seeds similarly entombed, and that if the seeds become exposed to the right conditions, they would germinate and flourish anew.

  In another ecological vision Darwin realized that as assemblages develop, they alter the very space they occupy. The concept of ecological succession was articulated by the American plant ecologist Frederic Clements a half century after Darwin and Miss Thorley botanized in Great Pucklands Meadow, but we can see early glimmers of this concept in Darwin’s experiments that began in the summer of 1855. That July, for example, he noticed that where he had pulled up some thorny shrubs the previous spring, he now saw weedy yellow-flowered charlock (Sinapis arvensis), a mustard, coming up. Charlock often gets into cultivated fields, thriving in open conditions, but cannot survive being shaded out by overtopping shrubs and trees. He thought it was curious that they should appear. Maybe the plant is a fast colonist of favorable sites, or maybe it had been there for many years, dating to when that area was under cultivation. He did an experiment to find out, and recorded it in his notes as follows:

  July 21. dug 3 places about 2 by nearly 3 ft square: cut off turf of weeds, & dug up ground one spit deep. [a “spit” was the length of a spade blade]

  Aug 1st numerous seedlings came up. some of them crucifers from taste

  Aug 19th. Young Plants now recognizable, but marked by sticks to make quite sure: (a few rather younger) six in one place, 10 in another, 5 in the third = 21 young plants.—Grass fields all round, thick hedges—no charlock even at end at present, from there in clover 120 yd. distant.14

  Note his simple but effective technique of marking the seedlings as they come up with sticks—we’ll see him put that to good use again. The 21 young charlock plants that sprung up in his little test plots (with none in sight in nearby fields) pointed to the seeds being there the whole time, just awaiting exposure under the right conditions. He was beginning to realize that the earth was thick with the propagules of flowering plants, mosses, ferns, fungi, just waiting. It reinforced his growing “ecological” sense of an interconnectedness that extends in both space and time. Interconnectedness at varying geographical scales, with diverse assemblages of species coexisting, an entangled bank with continued competition as well as mutual interdependencies; and varying temporal scales, with the makeup and structure of these species assemblages of the entangled bank dynamically changing, morphing according to the vagaries of inexorable changes in climate, geology, and selection. The geographical and temporal sense of ecological interconnectedness resonated deeply with Darwin’s overarching revelatory insight back in 1837, on how species are related in space and time.

  Now he was beginning to see how diversity could emerge from an area even the size of the palm of one’s hand. That August he made a telling comment in his notes:

  When many individuals crowded together some will die, so will forms. creations cause ‹death› extinction—like birth of young causes death of old.—All classification follows from more distinct forms being supported on same area.15

  Two insights from this brief note are worth pointing out. First, this statement reveals that Darwin saw that the process of close competition forces divergence (which involves the extinction of intermediate forms) and gives rise to the classification system used by naturalists. “All classification follows” because the dynamic process of diversification, along with its simultaneous extinction and gap-creation, explains the nested hierarchy we see in taxonomic classification. Several strands of thought were gelling here for Darwin: selection, diversification, geographical distribution, extinction, classification. It was a tremendously exciting synthesis.

  Earlier in this note, Darwin restates his hypothesis that far more “forms” can be supported in an area when they are diverse, and adds a note to himself to give examples. Later he inserted which case studies to use to support his thesis: “Trifolium at Lands end. Larch wood. Coral [island]. Hooker facts.” There is so much behind even that one line: the first example comes from an 1847 paper on the plants of remote Land’s End, at the tip of the Penwith Peninsula of Cornwall, in which C. A. Johns reported finding nine species of legumes in a very small space—an area no bigger than his hat. Darwin copied out Johns’ comment that “Had the rim been a little wider, I might have included Genista tinctoria & Lotus corniculatus.” These too are legumes, so Johns found nearly a dozen legume species coexisting in an area a foot across or less. The second case, “Larch wood” refers to the larch woodland at one end of Great Pucklands Meadow, which was included in Darwin’s botanical study with Miss Thorley. “Coral island” refers to data he collected on the Cocos (Keeling) Islands, atolls in the Indian Ocean that Darwin visited on the Beagle. The plants he collected there, described a few years later by Henslow, were
now called into service to support his idea of small and uniform areas supporting great taxonomic diversity. As for “Hooker facts,” Darwin meant the information furnished by his friend Joseph Hooker, in answer to Darwin’s incessant inquiries for information.

  Think Globally, Act Locally

  Darwin may have been inspired by his little palm-sized testing plots to transfer that approach to his diversity studies. The following March, of 1856, he fenced off a 3 × 4-foot plot of lawn in his backyard and set orders for it not to be mown. It was another “small and uniform” area, an unremarkable patch that like the rest of the lawn had been mown regularly probably since the house was built. It ended up being the Great Pucklands Meadow in miniature. “I found the species 20 in number, & as these belonged [to] 18 genera & these to 8 orders & they were clearly much diversified,” he reported in Natural Selection.16 The results of his Lawn Plot Experiment, as it came to be called, were duly reported in the Origin as well. Thinking big, he extrapolated the results of Great Pucklands and his lawn plot to entire regions and continents—his own version of the environmental activists’ mantra to “think globally, act locally.” “We see on a great scale, the same general law in the natural distribution of organic beings,” he declared, pointing to data gleaned from the literature on diversity found co-existing in heaths, mountaintops, salt- and freshwater marshes, and lakes and rivers. He began to see that Johns’s observation at Land’s End was something of an exception: where Johns found nearly a dozen species of the same family in a small area, usually Darwin was finding cases of multiple families and orders. The preponderance of cases led him to believe that greater diversity in uniform areas was the general rule.