Darwin and seeing the "obvious"

Often it requires the genius of a conceptual thinker, which [Louis] Agassiz clearly was, to launch a new way of thinking about natural phenomena. Just how clear-and paradoxically, at the same time unnoticed-the evidence for glaciation was, even to naturalists who were accustomed to careful observation, is nicely illustrated by a passage from Charles Darwin's autobiography. In 1831, just a few years before the publication of Agassiz's ice age theory, Darwin was on a field trip in Wales with a prominent geologist [Adam Sedgwick!]. Anxious to find fossils, they scoured hill and valley, examining the rocks in great detail. But they completely missed the evidence for glaciation that surrounded them. Years later and by then fully aware that the Earth had experienced extensive glaciation, Darwin wrote about his earlier field excursion: "On this tour I had a striking instance of how easy it is to overlook phenomena, however conspicuous, before they have been observed by anyone... neither of us saw a trace of the wonderful glacial phenomena all around us; we did not notice the plainly scored rocks, the perched boulders, the lateral and terminal moraines."
The italics above are mine. They emphasize the common experience that is implicit in the phrase "point out the obvious." Some things are invisible until someone shows them to you; then they pop up everywhere. Darwin marveled that he and his geological colleague could have overlooked these glacial features that he late found so conspicuous that "a house burned down by fire did not tell its story more plainly than did this valley."

From Frozen Earth: The Once and Future Story of Ice Ages. by Doug Macdougall (University of California, 2004) pp. 15-16

What makes this especially interesting is that this is something Darwin understood completely from his own field. When he first visited the Galapagos Islands on his famous voyage of discovery, he did not realize the significance of what he saw, it was only on reflection after he returned that he began to formulate his Theory of Evolution through Natural Selection. David Quammen in his book The Song of the Dodo., discusses how Darwin did not even realize at the time of his visit the need to catalogue on what island he had collected a specimen on.

The most remarkable aspect of the natural history of the Galapagos, Darwin wrote in his Journal with the wisdom of hindsight, was that "the different islands to a considerable extent are inhabited by a different set of beings." What he meant by that murky phrasing was that the different islands supported distinct species and subspecies within certain lineages. "My attention was first called to this by the Vice-Governor, Mr. Lawson, declaring that the tortoises differed from the different islands, and that he could with certainty tell from which island any one was brought."....

...

Darwin would later regret that he had let the vice-governor's comment about shell differences [among the tortoises] slide past him. "I did not for sometime pay sufficient attention to this statement, and I had already partially mingled together the collections from the two islands." He hadn't guessed [yet] that islands so similar in physical conditions, and so close together, could be inhabited by distinct sets of creatures.

His methodological mistake, as I've mentioned earlier, didn't apply just to tortoises; he had also jumbled some of his bird specimens together without regard to their island of origin. "Unfortunately most of the specimens of the finch tribe were mingled together," he admitted in a later edition of the Journal. Bear in mind that even the first published edition of this Journal was written after he had arrived back in England; it was a literary composition for which his Beagle diary and field notebooks supplied raw material. During the actual field work, his perspective had been different. Collecting with zeal but without time or distance to reflect, he had lumped specimens together carelessly and ignored the suggestive patterns of archipelago speciation [which of course no one had recognized before]. Only later, back in England, with help from taxonomic specialists, did he notice enough to bemoan the mistake and imagine the implications of the data he hadn't quite gathered. Such retrospective insights were exactly what distinguished the Journal (especially in its revised editions) from the diary.

From The Song of the Dodo.. by David Quammen (Scribner, 2004) pp. 215-16

It is easy to regard Quammen as to hard on Darwin, but this issue aside, what this really shows is how even the most perceptive mind can miss the evidence if it does not have any idea to where it is pointing. When he was collecting his specimens it was very unlikely that Darwin could have seen the big picture that having a set of specimens and the leisure to examine them and consider what the collection meant, that he could have known what he was “supposed” to have been seeing.

Psst... over this way

Psst... over this way, originally uploaded by SwirlingStillness.

Curiously, at least one ice-age giant may return to North America even without the help of humans. Fourteen thousand years ago much of North America south of the latitude of New York was inhabited by a giant armadillo known as Dasypus bellus. At about twenty kilograms it was three times as heavy as the living armadillo, but otherwise the two were virtually identical. Beginning about 13,000 years ago the giant armadillo was driven out of what is now the United States. Human hunting was likely responsible, for the large armadillos probably hibernated in fissures among rocks, where they were easily spotted and dispatched. Some armadillos, however, survived in the dense rainforests and rugged terrain of Central America. These were smaller than those living further north, and they like many species, have shrunk in average body size over the past 13,000 years. This is because they were hunted assiduously by the Indians who considered them a delicious repast and, as we have seen, hunting pressure can lead to the selection of rapidly maturing dwarfs.

As the Indian population itself began to decline in the seventeenth century and hunting pressure eased, the selective pressure felt by the armadillos reversed. Small armadillos began a rapid advance north. Between 1905 and 1914, at the height of their northward charge, they advanced at a rate of more than twenty-five kilometres a year. Today's small armadillos have still not reached as far north as their giant ancestors did 13,000 years ago. This is because they are more likely to die during cold winters than large armadillos-they cannot accumulate as much fat and they lose heat more rapidly. As cold winters kill the smaller individuals, however, natural selection is acting upon the advance guard in the armadillo invasion and they are again increasing in size. If the trend continues, in a few centuries from now the giant Dasypus bellus may once again stalk the Appalachian forests and thrive as far north as New York.

From The Eternal Frontier. by Tim Flannery (William Heinemann: London, 2001) pp. 347-348

(special thanks to Stuart on flickr for this great photo, and the post title, you can see other great photos by him here)

Tobacco

In order to prevent their leaves being eaten by insects, the shrubs and herbs that make up the genus Nicotiana, to which the tobacco plant belongs, produce an extraordinary cocktail of chemicals. On average 10 percent of the plants' metabolic effort is spent producing just the alkaloids that go into the mix. The flow of chemicals to the glands of some American species is so copious that it literally drowns the insects attempting to make a meal of it. Such devotion of its metabolic efforts indicates the plant faces formidable enemies in nature, and tobacco eating insects are a resilient lot. Indeed, the newly hatched larvae of the tobacco grub recoil at their first bite of a tobacco leaf, but soon reconcile themselves to their toxic food and will thereafter take no other.

South America is the principal home of the tobacco genus [properly the genus Nicotiana], but some members are flung as far and wide as southern Africa and Australia, suggesting that the lineage may be a venerable one which evolved at a time when these landmasses were joined [Gondwanaland]. The North American species probably travelled north with the glyptodonts and sloths after the formation of the Panamanian land bridge. People, like tobacco grubs, acquired a taste for the toxic chemicals, and it was from among these immigrant species that North American Indians first selected plants that offered a good smoke. By AD 500 the Maya were already in the habit, and by the seventeenth century smoking had spread through vast areas of the continent.

Curiously, the tobacco enjoyed by Sir Walter Raleigh and first grown in Virginia in 1612 is not the species cultivated for smoking today. Raleigh relished the aroma of Nicotiana paniculata [This seems to be a very salt tolerant species it appears that makes sense in the Tidewater], which, although it is no longer smoked, has not vanished entirely as a crop-it is still grown in Eurasia as a source of insecticides. [Interestingly enough this does not even seem to be the same species that John Rolfe replaced when he reformed tobacco planting in Virginia, as Nicotiana rustica is usually the plant credited with being too strong. N. rustica has some reputation of being a hallucinigen and being used in Native American mgic and religion, it is fact a Chilean Mapuche import, and it makes sense that it might have introdued by the settlers, just as its successor was introduced from Barbados.] The plant that fills the fields of the American South today is Nicotiana tabacum, which appears to be a hybrid species with Argentinian and Bolivian ancestry.

From The Eternal Frontier. by Tim Flannery (William Heinemann: London, 2001) pp. 277-8

I have always read that Indian tobacco as smoken by Medicine men and shamans was not the same stuff we smoke today, this seems to shed a little light on the subject. The question the becomes, where do I find some Nicotiana rustica to try smokin?

Ecological Release: Grizzlies and North American Indians

How I wondered, did such large animals diversify so quickly? The answer lies not only in the richness of the continent but in the 'ecological release' experienced by the bears in their new homes. With few competitors and a huge variety of resources they quickly diversified and adapted to local conditions. Indians of course do not represent incipient new subspecies as grizzlies do, but for millennia before 1492 they were adapting through cultural change to local conditions even more rapidly than grizzlies were through natural selection. As a result both Indians and grizzlies have developed exceptional diversity in a very short time.

Whenever a species arrives in a new habitat, a series of evolutionary forces come into playthat have a dramatic effect upon it. The nature of a founding population has a considerable effect on the process. Such populations are never truly representative of the population from which the are drawn, and this leads to a founder effect. It may have been, for example, that there were no expert bow and arrow makers among the first people to cross into North America and this craft my have been lost to the New World through this founder effect. [Surely this is correct, but even if one subscribes to a Clovis first theory of the population of North America, the oldest evidence of the bow anywhere that I know of, from the Mesolithic Natufian complex in the Middle East, and is from at least a millenium after the the latest the Clovis people were in the Americas.]

If the new homeland that a species invades is essentially an open field, with few competitors but lots of food, then the species goes through a period of ecological release. For creatures such as birds this can result in a population that is more varied (in beak shape, leg length or size) than the parent population. This happens because in the absence of competition almost all variants can make a living [and find mates] of some sort. This phase of the evolutionary process is typically brief, occurring over a few decades or centuries. Then as the open field is filled, individuals in the variable population begin to be selected for various traits and begin to adapt to local conditions. Long beaks and long legs may be favored in one environment and short beaks and short legs, or some combination of both, in another.

Thus begins the long and final phase of the adaptive process that makes species. Known as evolution by natural selection, the process can be thought of as a great centrifuge, throwing apart the geographically separated portions of a once similar people or species, creating diversity out of uniformity. The richer and more diverse the environment is, the faster the centrifuge can be thought of as turning. The centrifugal force is felt most strongly after the closing of the frontier, and for organisms like large mammals its results via natural selection are evident only after thousands or tens of thousands of years. For humans, however, who adapt through learned cultures, the process can happen swiftly.

From The Eternal Frontier. by Tim Flannery (William Heinemann: London, 2001) pp. 237-238