Darwin's Backyard

by James T. Costa

  Into every 3 gallons (11.4 liters) of fresh water, dissolve:

  • 10½ ounces (298 grams) pure (not iodized) table salt

  • 1½ ounces (43 grams) magnesium chloride

  • 1 ounce (28 grams) Epsom salts

  • ½ ounce (14 grams) plaster of Paris

  Chemical and mineral mixtures such as these give freshwater all the qualities of seawater, making it suitable for marine life. If you were setting up a saltwater aquarium to support fish, we would have to pay attention to salinity and quality of the freshwater used. Since we’re simply floating seeds in our saltwater, there is no need to worry about that. By the way, even if you live close to the ocean it is still preferable to use artificial saltwater, as ocean water is full of algae and other microorganisms that will die and decay, fouling the water.

  B. Other materials

  • Notebook and pencil

  • Seeds of at least six species (e.g., assorted vegetable and wildflower seed packets)

  • 500 ml (17 fl. oz.) beakers or flasks (or even mason jars), one for each plant species. The bottom of a 1- or 2-liter (1- to 2-quart) beverage bottle will work too.

  • Pipette or turkey baster

  • Spoon

  • Forceps or tweezers

  • Plastic wrap

  • Labeling tape and marker

  • Potting soil

  • For seed planting: Petri dishes or similar chambers, or one or more germination flats (ideally, partitioned into individual planting units) or paper cups, with potting soil

  C. Procedure

  1. Prepare the saltwater in an aquarium tank, carboy, or large flask, depending on volume, and keep at room temperature. Measure 300 ml (10 fl. oz.) saltwater for each beaker or flask. (If the vessel does not have a 300-ml (10 fl. oz.) measurement mark, make one with a marker or tape as a reference for maintaining the water level.) Choose a location out of direct sunlight where the flasks won’t be disturbed. Place the beakers or flasks there and, once positioned, using fingers or forceps carefully place (not drop) 10 seeds of a single species onto the water surface in one vessel, labeling it with the date, time, and plant species. Repeat for each of the remaining seed species, one species per beaker. (Don’t worry if your seeds sink: this situation is addressed below.) Loosely cover the mouth of each vessel with plastic wrap; some water will evaporate, and a pipette or baster can be used to replenish evaporated water. To do this, carefully dispense water along the inner wall of the vessel so as to minimize agitating the water surface. (If you have a large number of participants doing this experiment it may be desirable to set up duplicates of the experiment at several different stations, comparing the results of each group later.)

  2. Record the status of the seeds daily, logging for each vessel the number of seeds still floating and the number that have sunk. This can be continued indefinitely, as Darwin did by removing seeds at intervals to see how long they would remain viable after prolonged exposure to saltwater, but it will be more practical for most people to run the experiment for a set period of time—1, 2, or 3 weeks, or whatever you prefer. Larger groups with replicates of each seed species might harvest one replicate per species after, say, 1 week of exposure, a second replicate after 2 weeks, etc. Alternatively, all replicates can be floated for the same period of time in order to obtain descriptive statistics on seed performance among replicates.

  3. At the conclusion of the time-span determined for the experiment, record the number of floating versus sunken seeds for each species. First retrieve the floating seeds from each vessel using the spoon, taking care not to sink any seeds. Then retrieve the sunken seeds, making sure to keep each species separate and the still-floating and sunken seeds of each species separate. Rinse seeds in fresh water and plant the seeds using one of the two following methods:

  Paper toweling method. Use separate dishes for floaters and sinkers of each species. Place one piece of paper towel on the bottom of the dish, saturate with water, and place seeds. Saturate the second piece of paper toweling and place over the bottom piece, covering the seeds. Place the lid on the dish to seal in moisture. Ensure that all dishes are labeled (species, floater versus sinker, date).

  Soil method. Carefully plant each set in adjacent units of the germination flat. (If the flat does not have individual planting units, use string to grid off the flat.) Plant floating and sunken seeds of each species separately. Repeat as necessary, water, and cover the flats or cups with plastic wrap.

  4. When all the seeds are planted, expose dishes, germination flats, or cups to indirect sunlight (not direct sunlight) or a growth lamp. Monitor daily and record numbers of seeds of each category (species, floating versus sunken, etc.) germinating.

  Record, for each species and each saltwater exposure time (1 week, 2 weeks, etc.):

  (a)Number and percentage of floating seeds germinated versus non-germinated

  (b)Number and percentage of sunken seeds germinated versus non-germinated

  5. At the conclusion of this experiment tabulate the data, noting percentage of seeds sinking and remaining afloat for the experimental period, and the percentage of each category germinating. The results can be related back to Darwin’s original puzzle: How do species colonize remote islands? Two sets of conclusions can be drawn from our Darwin-inspired experiment, relating to flotation and viability in saltwater. First, the seeds of some species do remain afloat for an extended period. What was the duration of your experiment, and how far might they be carried by currents in that time? In a letter to Hooker, Darwin pointed out that “many sea-current go a mile an hour: even in a week they might be transported 168 miles: the Gulf-stream is said to go 50 & 60 miles a day.” Indeed, this is likely an underestimate for some currents: with a maximum estimated surface velocity of 5.6 mph, the Gulf Stream could carry floating seeds over 130 miles per day! Try your hand at calculating this.

  6. The sunken seeds might seem less informative for Darwin’s purposes. However, they too provide insights, in showing to what extent seeds might remain viable after saltwater exposure whether floating or sinking. What proportion of your sunken seeds germinated as compared to the floating ones?

  II. Avian Airlift: Darwin’s Duck-Feet Experiment

  Struck by the occurrence of freshwater snails on many remote islands, Darwin looked into transport by sea and by air: floating or rafting, or airlifted by birds. Experiments showed that freshwater snails could not long survive in saltwater, so he decided that they were given a lift instead. Can waterfowl carry aquatic snails? Snails or other creatures might climb aboard the feet of ducks and geese, especially when ducks sleep, with their feet dangling in the water. To test this Darwin turned to his “snailery”—an aquarium with freshwater snails of all ages—in which he dangled dried duck’s feet. He soon found that several young snails climbed aboard. How long could they hang on? As he later wrote in the Origin: “These just hatched molluscs, though aquatic in their nature, survived on the duck’s feet, in damp air, from twelve to twenty hours; and in this length of time a duck or heron might fly at least six or seven hundred miles, and would be sure to alight on a pool or rivulet, if blown across the sea to an oceanic island or to any other distant point.” Here’s a duck-foot experiment that would do Darwin proud. We’ll see how much aquatic life might colonize model duck feet that we construct.

  A. Materials

  • 3/16 in. diameter wood dowels, cut into 6 in. (~15 cm) lengths

  • Carpet tacks

  • Small fishing tackle weights, assorted sizes

  • Ping-Pong balls or corks

  • One or more water-durable materials (e.g., denim, Velcro, canvas), cut into shape of duck foot using accompanying sketch

  • Shallow pan

  • Squirt bottle, pipet, or turkey baster

  • Magnifying glass or dissecting microscope

  • Petri dishes

  • Small knife, nail, or wood screw to carefully bore hole in the Ping-Pong ball or cork

  • Silicon or rubb
er cement

  B. Procedure

  Making your duck foot model:

  1. Using the sketch pictured here as a template or one of your own making, trace the duck-foot outline onto fabric or other materials used to simulate the webbed portion of a duck’s foot.

  2. Using the knife or nail, carefully bore a hole large enough to insert the dowel in opposite sides of the Ping-Pong ball. Insert the dowel through the ball, with approximately 1 in. (2 cm) protruding on one end of the ball (“top”).

  3. Secure the dowel in the ball, and seal against water leakage, by applying a dab of silicon or rubber cement around the dowel where it contacts the ball. If using a cork, bore a hole on one end about ¼ in. (0.6 cm) deep and firmly insert one end of the dowel.

  4. Use a thumbtack to affix the duck “foot” itself to the base of the longer end of the dowel (“leg”). (Note: multiple models can be made, each with different material for the foot; the efficacy of the different materials in adhering organisms can then be compared.)

  5. The aim is to float the model duck foot in water, with the leg and foot dangling down into the water column. Since the dowel leg itself is buoyant, it will likely be necessary to attach a small fishing weight to the shaft of the carpet tack affixing the foot. Test by placing your duck foot model into water (aquarium or filled sink) sufficiently deep to allow the leg to bob upright, foot down. Attach additional tackle weights as necessary to the base to get the model to float upright, but being careful not to add so much weight that the model sinks altogether. When you have the weight in balance, your duck foot is ready for deployment! A fishing line or light string can be attached to the top of the model using a thumbtack.

  Fishing with model duck feet, in hopes of catching hitchhiking aquatic organisms. Photograph by Leslie C. Costa.

  Deploying the model:

  1. Like a fishing drop line, your duck foot can be deployed into a pond or lake by carefully tossing it into the water and tying off the line so it doesn’t float away.

  2. Permit your duck foot model to float in the pond for at least 2 or 3 days (duration is open-ended). When you recover your model, have a pan or sealable bag ready to receive the model immediately after lifting it out of the water. Don’t collect extra water, just remove the foot from the water directly into the pan or bag. At home, carefully rinse the webbed “foot” over the pan using a squirt bottle filled with fresh water.

  3. Use a magnifying glass or dissecting microscope to inspect the foot for any evidence of plant or animal life. Similarly, samples of the pan water can be transferred using a pipet or baster to a petri dish or similar object for observation under the dissecting microscope. It may be difficult to distinguish some small organisms like protists and algae from debris, but careful observation will reveal their structure, and show protozoa continually moving around. Larger organisms—insect larvae, minute snails, etc.—should be obvious. Record the organisms observed, and attempt to count or estimate their numbers.

  4. This experiment has a flexible run-time, from days to weeks or longer. With a number of copies the models can be retrieved and analyzed a few at a time—at, say, 1-week intervals over the course of 1–3 months. The number and diversity of organisms adhering to the feet can be tabulated for individual “ducks” and for the group as a whole. Those deploying multiple duck feet can graph the results over time, and/or graph the yield from different foot materials. Do some materials do a better job of attracting colonists? Does the type or abundance of organisms vary among materials, with different materials being richer in different groups?

  See also:

  J. T. Costa, “Sailing the backyard Beagle: Darwin-Inspired Voyages of Discovery in Backyard and Schoolyard,” in Darwin-Inspired Learning, ed. M. J. Reiss, C. J. Boulter, and D. L Sanders (Rotterdam: Sense Publishers, 2015), 131–146.

  “Biogeography” at the Darwin Correspondence Project: www.darwinproject.ac.uk/learning/universities/getting-know-darwins-science/biogeography.

  6

  The Sex Lives of Plants

  In the late 1830s Darwin was preoccupied with sex, a preoccupation with a curious parallel between his work and his personal life. The fall of 1838 saw him discover the mechanism of species change, and a month later propose to his cousin Emma Wedgwood (the culmination of a period of deliberation complete with a list of pros and cons of marriage which happily concluded “Marry–Marry–Marry Q.E.D.”). They were married just two months later, in January 1839, and moved to Upper Gower Street in Bloomsbury, London, a little place with such gaudy decor that the couple dubbed it “Macaw Cottage.” By the following April Emma realized she was expecting a child—William Erasmus was born later that same year to the Darwins’ great joy. All through this period Darwin was also actively exploring the principles of “generation,” as reproduction was called, in relation to the variation, crossing, fertility, and vigor of individuals in nature and under domestication, a line of inquiry that began more or less with his conversion to the heretical notion of transmutation in the spring of 1837.

  That spring it didn’t take Darwin long to realize that agricultural improvement held the key to the process of species change. After all, animal and plant breeders are in the business of creating and improving novel varieties or breeds, and he had a sense that this was related in some way to the formation of species—varieties were, he was convinced, “incipient species” and the difference between variety and species was one of degree and not kind. He immersed himself in the agricultural literature: the writings of William Marshall, John Wilkinson, John Sebright, William Youatt, Robert Bakewell, and others—a “Who’s Who” of leading breeders largely responsible for the first great agricultural revolution. Without knowing anything about the basis of heredity, these men built upon the animal and plant breeding techniques that humans had been practicing more or less unconsciously for millennia, elucidating these into principles for agricultural improvement on an unprecedented scale.

  Darwin took their lessons to heart, as we saw in Chapter 2. He filled his notebooks with entries bearing on crossing, hybridization, hermaphrodism (having both male and female reproductive organs), interfertility (full breeding capability), and sterility, trying to discern some signal in the noise. The breeders used terms like “picking,” “choosing,” and even “selecting” in describing the tricks of the trade, including Sir John Saunders Sebright, a giant of agricultural improvement who developed new breeds of cattle, sheep, and poultry. In his 1809 pamphlet The Art of Improving the Breeds of Domestic Animals, Sebright declared that “the alteration which may be made in any breed of animals by selection, can hardly be conceived by those who have not paid some attention to this subject; they attribute every improvement to a cross, when it is merely the effect of judicious selection.”1 In response, Darwin jotted in the spring of 1838 how the “Whole art of making varieties may be inferred.”2 When Darwin discovered nature’s version of Sebright’s “judicious selection” a few months later, he likely thought up the term “natural” selection with Sebright in mind.

  Bound up with questions about the origin of species was the origin of the variation that selection acted upon. Environment played a role, he thought, but so too might the reproductive process itself, a line of inquiry that predated his discovery of natural selection. If so, sexually reproducing species should vary more than asexual species, something his investigations seemed to confirm. A few months later, he was convinced of the ultimate benefit, and maybe even the long-term necessity, of sexually reproducing.

  He wrote experts for their opinion of this idea. Their replies were not always heartening. The venerable Rev. William Herbert (1778–1847) was a classical scholar, linguist, and clergyman as well as a noted experimenter in plant hybridization. Darwin wrote him wondering about Herbert’s declaration that certain plants do not cross: “I ask because I have been led to believe, that . . . every individual occasionally, though perhaps very rarely, after long intervals is fecundated by [another individual].”3 Herbert’s reply was equivocal
, and he pointed out that if outcrossing was essential, hybridization would be rampant, something we do not see in nature. Darwin second-guessed himself. He wrote in his notebook: “The weakest part of my theory is the absolute necessity, that every organic being should cross with another.”4 Budding and other asexual forms of reproduction meant low variation, so that these species would not be able to change and adapt very quickly. And self-fertilization meant inbreeding, which also led to low variation and a suite of ills ranging from deformity to sterility. Of the two, inbreeding was perhaps the greater problem for species in the long term.

  Darwin well knew that the breeders cautioned against close or long-continued inbreeding, which at the time was termed breeding “in-and-in.” Plant and animal improvement was (and is) an art as much as a science, finding the right balance between breeding like with like to preserve the desirable characteristics that stem from good combinations of genes and their variants (alleles), and not disrupt these by the inevitable necessity of intercrossing—like a team that plays especially well together but easily loses their mojo when a newcomer joins the group. In the long term, organisms and teams might benefit from at least periodic injections of new blood, but initially that comes at the price of disrupting group dynamics. (Today we understand the genetic basis of the problems stemming from inbreeding: recessive alleles with negative effects are expressed when two copies are inherited, and the incidence of this increases dramatically with inbreeding.) In Darwin’s day there was only the empirical observation that inbreeding usually led to reduced fertility and vitality, and that outbreeding led to health and vigor. Darwin was sure that the different outcomes of inbreeding and outbreeding had a bearing on the formation of varieties and species. It’s also likely that he felt unease in the knowledge that his own marriage was consanguineous, since he and Emma were first cousins.