We rightly celebrate the English naturalist Charles Robert Darwin in this year, 2009, the 200th anniversary of his birth. He is the author of On the Origin of Species by Means of Natural Selection, published in 1859.
In that work, he argues not only for the fact of evolution - all organisms are the descendants, by a slow, natural process, from just one or a few simple forms - but also he proposes a mechanism of change. More organisms are born than can survive and reproduce; this leads to an ongoing “struggle for existence” (and, more importantly, struggle for reproduction).
Those organisms that do survive and reproduce tend on average to be different from those that do not. Hence, there is a continual sifting or (as Darwin called it) a natural selection. Given enough time, there will be overall change or evolution - what Darwin himself called “descent with modification”. (The last word of the Origin is “evolved”, but generally it was not until the 1860s that the word “evolution” came into general use.)
What is important about Darwin’s theory is that he showed that change is not random. The differences between organisms in the struggle for life tend to reflect their usefulness. In particular, successful organisms will develop features - adaptations - that help them to succeed. Eyes, teeth, noses, penises, vaginas, bark, leaves, roots, wings, fur, fins, scales - all of these serve the ends of survival and reproduction.
The Origin was a carefully constructed work. In his Autobiography, Darwin referred to it as “one long argument”. He had been trained at Cambridge University and came under the influence of one of the chief scientific methodologists of the day, the historian and philosopher of science William Whewell (pronounced Hule, to rhyme with Jewel). Whewell was interested in showing why it is appropriate to accept the wave theory of light, even though we have no direct experience of the light waves themselves. He argued that in science we proceed as in a court of law. You propose a hypothesis; you look for clues. The hypothesis explains the clues, and conversely the clues support the hypothesis. Light waves explain the classic findings, like the interference patterns in Young’s double slit experiment. Conversely, the experiment supports wave theory over rival theories like Newton’s particle theory.
Whewell never accepted evolution - story has it that, when the Origin appeared, he refused to let it be put on the library shelves of Trinity College, of which he was then the master (principal). But Darwin learnt from Whewell and in the Origin he realised that he had a case very similar to that of the wave theory. Just as no one sees waves, so no one sees the long process of evolution through selection. Hence, he uses just the kind of indirect, inferential argumentation that Whewell proposed and defended. (Technically it is known as a “consilience of inductions”.) Thus, having introduced natural selection, and shown its plausibility by analogy with the successes of animal and plant breeders, Darwin goes systematically through the world of animals and plants, looking for clues to evolution through natural selection, showing how he can explain them and how conversely they support his central hypothesis.
In the realm of instinct or animal behaviour, Darwin zooms in on the honey bee - well known to his contemporaries, many of whom would have had hives in their back gardens. He shows how the wax cells that the bees build for their young are exactly hexagonal (six sided), and how this is by far the strongest system and way ahead of other options in the economical use of the building material. He shows also that there is a range of efficiency in the bee world, from those that are not very good at the job to the domestic bee which is excellent. In other words, we have a superb adaptation, and evidence of its gradual development through natural selection, in turn supporting the evolutionary hypothesis. (Note that this is not a viciously circular argument, but the kind of feedback argument we use all of the time. The butler’s guilt explains the bloodstains, and conversely the bloodstains point to the butler’s guilt.)
Moving next to paleontology, to the fossil record, Darwin argues on the one side that the gaps that we find are to be expected, given the low chances of fossilisation, and on the other side - more positively - that the roughly progressive record from primitive forms to complex forms is what we would expect under evolution through selection. He makes much of the fact that the further we go back in the record, the more we find organisms that seem to possess general features, that is to say, features shared today by organisms in other respects very different. This is just what we would expect from a branching form of descent.
Geographical distributions of organisms, biogeography, was a favorite of Darwin. It was in his 20s, when he was serving as ship’s naturalist on board HMS Beagle, that he visited the Galapagos Archipelago in the Pacific and discovered the differences between organisms (the giant tortoises, and the birds, especially the mocking birds and finches) on very closely positioned islands. How could one explain this except through evolution? Analogously in the Origin, Darwin makes much of the fact that the Galapagos denizens, although different, are very much like the denizens of South America and not at all like the inhabitants of West Africa. Conversely, the organisms found on the Canary Islands in the Atlantic are like the African forms and different from those found in South America.
Systematics, the relationships between organisms, had been a topic of much interest since the 18th century, when the great Swedish taxonomist Carl Linnaeus had started the task of bringing order to chaos. Darwin takes this over lock, stock and barrel showing how the relationships reflect genealogy, supporting and being explained simultaneously. Anatomy is a triumph.
Since Aristotle people had realised that there are similarities, isomorphisms, between the parts of organisms that are very different. (In the 1840s, the anatomist Richard Owen labeled these “homologies”.) The forelimb of the horse, the arm and hand of the human, the wing of the bat, the flipper of the porpoise are all used for very different ends and yet the bones are alike, molded as it were for the specific purposes. How else does one explain this totally useless fact of nature except through the process of evolution?
It was not that Darwin wanted to deny the existence of a Creator - indeed at the time of writing the Origin he believed in a kind of deity who worked through law, an Unmoved Mover - but he did want to deny a Creator who did everything through inexplicable miracle.
Michael Ruse, the Lucyle T. Werkmeister Professor of Philosophy at Florida State University will be speaking at Sydney Ideas, the University of Sydney’s international public lecture series on Tuesday, February 17, 2009. On Monday, February 16, 2009, he will also be officially opening Accidental Encounters, the University of Sydney’s Macleay Museum Darwin exhibition. First published in the Sydney Morning Herald on February 12, 2009.