Darwin's Backyard

by James T. Costa

  2. Honeycomb is best studied in chunks, so you can see how multiple cells relate to one another. The comb consists of two back-to-back layers of cells. Note that the cells are not perfectly horizontal, but angle upwards slightly.

  3. Hold the comb up to the light and note how each of the three facets of the pyramidal cell base is shared by one of the three cells opposite—in other words, how each cell on one side of the comb is centered on the intersection of three cells on the opposite side of the comb.

  4. Try to dissect out a single cell or a pair of cells by cutting away cells on either side of the one or two cells you selected. This won’t be easy, but isolating a cell or two will help you better discern cell shape: note the hexagonal prism shape along the long axis, and the three-sided rhombic pyramidal base. The shape of the base is important; each facet of the three-sided base serves as one of the facets for three cells opposite in the intact comb.

  Hexagonal prism structure of honeybee cells, each of which terminates in a three-sided pyramid at the base as seen at upper left and the photograph. Note that cells on each side of the comb align with one facet of each of three cells on the side opposite as shown at top center and right. Drawing and photograph by Leslie C. Costa.

  II. Exploring the Geometry of Bees’ Cells by Analogy

  Mathematician Joel Hass and colleagues may have experienced toil and trouble of Shakespearean proportions as they labored to provide a formal proof of the Double Bubble Conjecture, which held that two joined bubbles enclose volumes separated by the least possible surface area (see: en.wikipedia.org/wiki/Double_bubble_conjecture). The minimal area of the shared wall means the least amount of soap solution goes into it. When honeybees build a common wall between two adjacent cells, they too are conserving building material. The most economical state is a flat wall, hence the tendency of initially circular or cylindrical cells surrounded by six such cells to morph into a regular hexagon. The wall-sharing principle can be demonstrated with bubbles—a fun if messy way to explore bee cell geometry.

  A. Materials

  • Plastic drinking straw

  • Plastic plate or lid of a large margarine container

  • For an all-purpose bubble solution, mix together the following ingredients and let the solution sit at room temperature for a couple of hours:

  1. 2 cups (500 ml) water

  2. 2 cups (500 ml) Johnson’s® baby shampoo

  3. 2 teaspoons (10 ml) glycerine*

  *Many recipes call for the addition of small amounts of glycerine or corn starch for added strength, but don’t overdo it. Note, too, that hard (high mineral content) water makes bubble-making difficult. Try distilled water, available at many grocery stores, if you live in an area with hard water.

  B. Procedure

  1. Let’s practice making bubbles. Pour a small amount of bubble solution onto the plastic plate or a plastic lid from a margarine container.

  2. Gently blow into the straw as it touches the bubble solution on the plate to make bubbles. Remove the straw and continue to blow bubbles to form clustered bubbles.

  3. Observe how bubbles in a cluster intersect, in particular the angles that form between shared surfaces.

  4. Let’s do this more methodically: starting again with a clear bubble-free surface, form two bubbles roughly the same size and gently bring them together and attempt to link them. The resulting double bubble should look something like the left-hand bubble figure (A) below, sharing a common flat wall. The closer the two bubbles are in size, the flatter the wall they share—that is the minimal surface area.

  5. Next, attempt to merge a third bubble, about the same size as the first two, with the joined double bubble. The resulting triple bubble should look something like (B). The shared walls between the three bubbles will form a Y shape, and the angles between the bubble walls will be 120° (360° divided by 3). It may be easiest to measure them by photographing the triple bubble from the side and taking measurements of the angle of the shared walls from a printout of the photo. Why do the triple bubbles form an angle of 120° and not others? This is a result of each bubble exerting equal pressure such that the walls between them are rendered flat.

  6. Try to add more bubbles to a cluster on a flat surface. They will rearrange themselves to create multiple 120° angles in the shape of a Y. A network of 120° angles creates a pattern of hexagons, like bees’ cells—and for the same reason: this is the configuration that produces the most cells with the least amount of building material (in this case bubble solution) to form the walls.

  5

  A Grand Game of Chess

  It was early May of 1855, and Darwin was having a bad day. He had been at the Zoological Gardens in London, feeding a nice sampler of soaked millet, lettuce, cabbage, linseed, barley, and onion seeds to the goldfish. Things began promisingly enough: “they took them in mouth & kept them for some seconds,” he jotted in his notebook. But it quickly became clear that these fish were not going to be willing participants in his study; after half a minute or so the fish forcibly spat out the seeds, and he could not tempt them to nibble further. Glumly, he wrote a woebegone letter to his cousin William Darwin Fox: “I am rather low today about all my experiments . . . all nature is perverse & will not do as I wish it, & just at present I wish I had the old Barnacles to work at & nothing new.”1

  What led to this curious scene, with the respectable middle-aged naturalist trying to force-feed goldfish? As usual there was a method to the apparent madness. Darwin was convinced that plants and animals were far more mobile than many of his colleagues realized, and he was devising all sorts of experiments to prove his point. His seed-eating fish scheme was just one of many.2 He imagined that birds eating fish that in turn had just eaten seeds might turn out to be an unexpectedly common way that plants became dispersed far and wide. It seemed perfectly plausible that once a bird voided the remains of the fish with its (hopefully mostly undigested) seed cargo in some far-flung place—the distant shores of a remote island, or in a valley beyond a lofty mountain range—the bird would have performed a useful seed dispersal service: carried far and conveniently fertilized, to boot, the seeds just might germinate and establish a new population. The first step was to establish that fish would indeed eat seeds, but those pesky fish would have none of it.

  Fortunately, not all of Darwin’s dispersal investigations went awry. In the years leading up to the publication of the Origin he carried out an astonishing range of experiments and related studies bearing on the dispersal and movement of plants and animals. Explaining the geographical distribution of species was key to Darwin’s evolutionary ideas, after all, and he puzzled over every aspect from modes of seed and snail dispersal to the ebb and flow of plant and animal populations as climate cycles over the eons. Ensconced in his study at Down House he nonetheless had an exhilarating global perspective—it was a “grand game of chess,” he enthused, “with the world for a board.”3

  A Grand Subject

  The question of geographical distribution was of the highest importance in Darwin’s day. Naturalists knew that species distribution was not random—there were obvious differences among species on continents and even more in regions, but just what gave rise to that pattern? Beginning in the late eighteenth and early nineteenth centuries, naturalists had first wondered if the earth could be divided up into distinct centers of creation, judging by the richness of their species. Whatever the source of species, whether by special creation or some mysterious natural process, tropical regions and remote islands clearly seemed to be special—islands more than continents, because while the absolute number of species may be greater on continents, islands often have far greater endemism (unique species) and greater diversity at higher taxonomic levels like genus, family, and even order.

  A related concern was the question of single versus multiple centers of creation; that is, did all species originate from one geographic locale from which they dispersed the world over, or could species arise in different areas, whether simulta
neously or over time? Some early biogeographers trying to reconcile the puzzle of species distribution with their commitment to scripture (the Noachian flood in particular) favored the single-location idea: the Swedish naturalist Linnaeus, for example, suggested that Noah’s ark came to rest on the peak of a towering mountain. When the waters receded and the area was completely exposed, there would be a range of climatic zones available for the Noachian menagerie, from alpine tundra and boreal forest through grasslands and lowland forest. Never mind the difficulties of the frigid conditions on the summit to begin with—that’s only the tip of the iceberg of difficulties in Linnaeus’ scheme. The important point is that all the species were imagined to have migrated worldwide from a single area.

  This and similar models for a single source and subsequent spread of all species raised as many questions as it answered, of course, like how multitudes of land species subsequently colonized remote oceanic islands, why related groups of species tended to be found in adjacent geographical areas, and, later, how to explain why the species of a given continent are related to the now extinct, fossil species of that same continent? Most naturalists rejected religion-based scenarios—even those otherwise committed to a divine origin for species. The alternative was multiple centers of creation, and here there are important distinctions to be made. Different groups of species might arise in different parts of the world, resulting in the obvious regionalism we see: marsupials largely confined to Australia, say, bears and cats primarily in northern hemisphere areas, and remote islands having their own sets of species found nowhere else. Charles Lyell preferred the idea of multiple centers of origin to the extent that it neatly explained island endemics. But could the same species arise in more than one geographic locale, maybe even at different times? Explaining regional differences in species is one side of the coin; the other is regional similarity. Widespread groups pose their own puzzle; how to explain the occurrence of the very same species on mountaintops in Europe and the Appalachians of eastern North America, for example, or continental Europe and islands like Madeira, far out to sea?

  In the late eighteenth and early nineteenth centuries, some naturalists became explorers precisely to obtain ever more detailed information about the distribution of species. Some, like the Prussian polymath Alexander von Humboldt and his botanist friend Aimé Bonpland, paid special attention to the distribution of plants of the American tropics and tried to quantify their similarities and differences along altitudinal and latitudinal gradients. Humboldt was trying to gain a glimpse into the plan of the Creator by uncovering the underlying pattern of species distributions.

  As a young undergraduate at Cambridge in the late 1820s under the tutelage of John Stevens Henslow, Darwin was exposed to a great many pressing philosophical questions, but perhaps none was so pressing as the “centers of creation” issue and its relationship to the mystery of species origins. His vicar-naturalist professors impressed upon him the conviction that a detailed comparative study of geographical distribution held the solution. Perhaps this is why so many naturalists considered travel a rite of passage of sorts, as it was the only way to obtain firsthand knowledge of different species and make the careful observations and collections required to discern patterns. This was what led Humboldt abroad to the New World, and his rapturous writings in turn inspired a generation of eager young naturalists. Darwin was no exception.

  Alexander von Humboldt’s profile of the Andean volcano Chimborazo, first published in his 1807 Essai sur la géographie des plantes. Pioneering the cross-sectional representation of altitudinal zonation, this map shows the transition of plant communities from the lowland tropics through the frozen arctic zone at the summit. Humboldt inspired Darwin’s determination to travel abroad.

  The peculiarities of geographical distribution and its meaning were very much on Darwin’s mind throughout his voyage on HMS Beagle. Recall from Chapter 1, for example, how in the final year of the voyage Darwin mused on this very topic amid the strange fauna of Australia: “I had been lying on a sunny bank & was reflecting on the strange character of the Animals of this country as compared to the rest of the world,” he wrote in his diary in January 1836. “An unbeliever in everything beyond his own reason, might exclaim ‘Surely two distinct Creators must have been [at] work.” But the ant lion he spied, a species distinct from those he knew back home but clearly related, got him thinking about commonalities rather than differences around the globe. “Would any two workmen ever hit on so beautiful, so simple and yet so artificial a contrivance? It cannot be thought so. The one hand has surely worked throughout the universe.”4

  Darwin’s “one hand”—a single Creator—does not necessarily entail that each species arose at a single location, but this is very likely what he believed. Certainly Lyell, whose watershed work Principles of Geology Darwin was reading on the Beagle voyage, thought so. In volume II of Principles Lyell suggested that “each species may have had its origin in a single pair . . . and species may have been created in succession at such times and in such places as to enable them to multiply and endure for an appointed period, and occupy an appointed space on the globe.”5 Darwin received this eagerly awaited second volume of the Principles in November 1832, when the Beagle stopped at Monte Video in South America. Having enthusiastically embraced Lyell’s arguments of the first volume for what would later be called a uniformitarian view of earth evolution—understanding the earth in terms of geological forces now in operation—Darwin was now intrigued by the long discussion of species in Lyell’s second volume. It may seem odd that a geological work would devote so much space to subjects like species variability and hybridization, habitats and biogeography, migration and dispersal, but it only underscores the central tenet of naturalists like Lyell and others that earth and the life upon it must be studied together to gain insight into the ultimate philosophical and theological question of origins: origins of earth, life, species, us. It was in this spirit that Lyell thoroughly reviewed contemporary understanding of geographical distribution and theories of the “original introduction of species”—all utterly compelling to the young Darwin. One later observer, the geologist John Wesley Judd, even suggested that Darwin’s reading the second Principles volume was the key event putting him on the path to solving the mystery of species origins. If the Principles did not spark Darwin’s interest in the origin of species, it certainly crystallized the issues for him and provided a research agenda.

  The second Principles volume probably also sparked Darwin’s interest in dispersal. The single-origin idea for species requires high dispersal ability. How else do species come to occupy often vast and sometimes even separated ranges, especially when the same mainland species is found on distant islands? In characteristic form, Lyell thoroughly reviewed the means of plant and animal movement on the globe by wind, water, and other agencies, from animal transport (within and without: seeds in stomachs and those adhering to fur) to accidental transport by rafting on floating vegetation or debris, whirlwinds, and icebergs. This all seemed very plausible to Darwin, though as we shall see next it didn’t deter other naturalists from postulating virtually any mechanism but simple dispersal to explain these patterns. There ensued a genteel debate, one that pitted Darwin against his sage mentor Lyell for a time. Few students today would scarcely believe that questions of the geographical distribution of species were once heady stuff. To Darwin, it was “that grand subject, that almost keystone of the laws of creation.”6

  Atlantis Rising

  Lyell’s vision of earth’s history was one of fluctuating levels of land and sea, and an equally fluctuating climate. The continents were thought to be incapable of moving, but there seemed ample evidence to show that the level of the land could change dramatically, if gradually, over vast stretches of geological time. The land is in a state of continual flux, and the geographical ranges of species ebb and flow in response. As Lyell expressed it in the Principles,

  Every flood and landslip, every wave which a hurricane or ear
thquake throws upon the shore, every shower of volcanic dust and ashes which buries a country far and wide to the depth of many feet, every advance of the sand-flood, every conversion of salt-water into fresh when rivers alter their main channel of discharge, every permanent variation in the rise or fall of tides in an estuary—these and countless other causes displace in the course of a few centuries certain plants and animals from stations which they had previously occupied.7

  It was clear to Lyell that God endowed the “numerous contrivances” of plant and animal dispersal to harmonize with the fluctuations of the inanimate world, fluctuations that he also ordained. Otherwise, we would not find species in the most unlikely, far-flung, and often environmentally extreme locales where we find them today. In his reference to harmonic balance of the animate and inanimate worlds, Lyell was reinforcing the idea that earth and the life upon it are intimately bound together.

  Lyell also saw dispersal, whether by chance or design, as augmented by shifting landscapes and climates. Corridors of movement open and close, volcanic islands like stepping stones across the seas form and eventually erode back into the depths, mountain chains erupt on land, forming barriers for some species and ridgelines of migration for others. The sea encroaches on the land and then withdraws, and the climate cycles through glacial and warm periods. Lyell’s vision of the earth was one of great areas of balanced uplift and subsidence: as mountains are raised on the west coast of South America, for example, the east coast slowly subsides into the sea. It was a grand vision, one that profoundly influenced Darwin and many other naturalists. Yet some were not convinced that species were quite so adept at getting themselves dispersed far and wide. They thought that most species needed help, at least when it comes to getting across the wide ocean basins. Island stepping stones might seem to fit the bill, but there arose—no pun intended—a school of thought that took that idea to an extreme, maintaining that now-sunken continental areas of great extent were once found in the ocean basins.