Darwin is one of the most widely revered and enduringly controversial figures in the history of science. Both are exceptional feats for such a mild-mannered gentleman. Much of the controversy surrounding Darwin concerns the presumptive truthfulness of his evolutionary theory. Darwin's ideas about evolution were so ground-breaking that, more than one hundred and fifty years after the publication of On the Origin of Species (1859), many people still refuse to accept Darwin's basic precepts.
Shortly after graduating from Cambridge, Darwin received an invitation to ship out on the HMS Beagle. Darwin's journey on the Beagle stands out as one of the world's most important scientific events. The journey was history-making not only because modern biology owes its existence to Darwin's circumnavigation, but, more specifically, because of the crucial change in Darwin's thinking that the voyage inspired. Darwin embarked on the HMS Beagle as a conventional creationist, but he finished it as a radicalized evolutionist.
An important stimulant to Darwin's evolving scientific ideas, and one of the few texts that Darwin carried aboard the Beagle, was Charles Lyell's newly published Principles of Geology (1832). Darwin studied Lyell's Principles carefully. Its dramatic departure from creationism lay in Lyell's theory of uniformitarianism. Lyell argued that creation had not taken place at a singular moment in the all-too-recent past. Rather, creation was an ongoing process. The wind and rain that erode the earth generally do so at the level of dust motes. If afforded sufficient time, the relentless forces of accretion and erosion could build peaks that touched the clouds and then, particle by particle, reduce the ruggedest range of mountains to a chain of low, rolling hills.
If such a thing were true, then uniformitarian change could only be accomplished over extraordinary expanses of time. How many years would it take for the buffeting winds and seasonal rains to carve the Grand Canyon? The answer: Eons upon eons, and many magnitudes more years than could easily fit within the creationists' young earth paradigm.
As the journey progressed, Darwin finally arrived at the Galapagos Islands. Though he had witnessed many wonders during his travels, the bizarre menageries that he encountered in the Galapagos exceeded anything that he had yet imagined. From giant tortoises to endless varieties of land crabs and snails, Darwin marveled at the seeming adaptability and (dare he think it?) mutability of the species that he observed. Perhaps as he gazed upon the spectacle of marine iguanas bobbing in the surf, Darwin gave thought to a new and unsettling idea. Just as tiny and slow-paced geological changes had the net result, over the long haul, of introducing extraordinary alterations to the earth's geology, might not the same be true for living organisms? In other words, could the tiniest physiological changes accumulate sufficiently across time to bring about the transmutation of species?
Darwin was both fascinated and disturbed by the implications of this idea. Though other early scientists, including Darwin's grandfather, Erasmus Darwin (1796), had toyed with the idea that life had evolved through random natural processes, no one had been able to explain how that could happen. However, it was in the Galapagos that Darwin finally witnessed the diversity of life forms that would help him reveal the basic mechanisms of evolution.
In particular, while touring the Galapagos, Darwin noted a phenomenon that tends to arise with regularity among island species. After migrating to islands, species often undergo remarkable alterations, e.g., small animals grow, large animals shrink, dietary and habitat preferences shift, etc. Precisely why such a phenomenon should be so prevalent in diffuse island ecologies remained a mystery until Darwin studied the many varieties of finches on the Galapagos. Galapagos finches exhibit such a wide variety of shapes and sizes that Darwin initially misclassified the birds as entirely different species. Indeed, it was only after consulting with John Gould, an expert in bird physiology, that Darwin realized that the finches were in fact much more closely related. As a result, Darwin experienced a revelation.
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Darwin postulated that island migrants encounter unique population pressures. That is, when migrant species initially arrive on the shores of hospitable islands their populations tend to explode. However, success quickly becomes a migrant's worst enemy because rapid population growth tends to exhaust available resources. By the way, Thomas Malthus (2003) also had a substantial impact on Darwin's evolutionary thinking. Quite simply, Malthus argued that population tends to grow geometrically whereas food resources can only expand arithmetically. As a result, if unchecked, population growth among any successful species (i.e., lilies on a pond, humans in Manhattan, finches in the Galapagos, etc.), will, within the space of only a few generations, rapidly exhaust available food resources. Under such circumstances, species that depend upon the seemingly limitless bounty of their local environs will soon discover that their luck has run out.
Thus, Darwin speculated that, like many island success stories, a singular species of finch had migrated from South America to the Galapagos. Finding itself in an environment that was largely devoid of predators and competitors, the finches flourished. However, like so many of their migratory counterparts, the finches soon encountered a problem. Finch populations expanded to the point that food became scarce and, thus, competition for the islands' limited resources became increasingly intense. In turn, the combined pressures of overpopulation, scarcity of resources, and the resultant competition for survival triggered a process that transformed the fortunes of finches on the Galapagos. It was this insight that enabled Darwin to crack the mystery of natural selection.
In brief, Darwin argued that there are a number of crucial biological dynamics that energize the evolutionary process:
Variation : whether it's dogs, grass, or fruit flies, organisms tend to vary from one individual to the next
Overpopulation : from oak trees to salmon, parents tend to produce more progeny than can survive to maturity
Struggle for survival : the overproduction of progeny tends to inspire a high-stakes competition to secure limited resources
Survival of the fittest : individuals with advantageous genetic traits enjoy an edge in the competition for scarce resources
Evolution through natural selection : winners of bio-ecological competitions survive and pass advantageous genetic traits to their offspring--which, in turn, brings about the gradual transmutation of species
With the above in mind, Darwin asserted that the finches which had survived on the Galapagos were those that had developed some sort of competitive advantage over their counterparts. As a result, multiple subspecies emerged among Galapagos finches, each of which was genetically equipped to take advantage of resources that were distinct from those preferred by their former messmate. Thus, Galapagos finches may have arrived on the islands as a single species, however, due to survival pressures, the finches evolved into a wide array of subspecies with distinct body types, divergent diets and unique survival strategies. Furthermore, Darwin argued that the speciation process that had transmuted Galapagos finches was essentially the same for every other species. Therefore, Darwin concluded that every living creature owed its existence to a process of evolutionary transmutation. Instead of an all-powerful god intentionally creating life in its existing form, Darwin's new theory explained how life could evolve randomly through a long, slow natural process.
And so began the culture wars that have raged until this very day.
Darwin, Charles. On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life . 1st ed. London: John Murray, 1859.
Darwin, Erasmus. Zoonomia: The Laws of Organic Life . London: J. Johnson, 1796.
Lyell, Charles. P rinciples of Geology: Being an Attempt to Explain the Former Changes of the Earth's Surface, By References to Causes Now in Operation . Volume One. London: William Clowes, 1832.
Malthus, Thomas. An Essay On The Principle Of Population (1798 1st edition, plus excerpts 1803 2nd edition). New York: W. W. Norton, 2003.