Wednesday, May 27, 2009

From the Origin of Species to the origin of infection

This article has just appeared in a special Darwin 200 issue of Microbiology Today, where you can obtain a PDF of this piece and much more besides. But they have given me permission to post it here too, where it benefits from hypertext links for the interested reader.

From the Origin of Species to the origin of infection
2009 marks the bicentenary of the birth of Charles Darwin and the 150th anniversary of the publication of On the Origin of Species by Means of Natural Selection. The impact of Darwin’s theory of evolution by natural selection is felt throughout biology and even beyond it — in disciplines as diverse as computer science and cosmology. Darwin’s theory is widely touted as ‘the best idea anyone ever had’ and arguably ranks as the most influential change in human thought in modern times (although as microbiologists, we may wish to claim that the germ theory of infection is of more practical significance).

What about Darwin and microbiology? Antibiotic resistance is widely cited as a tangible example of Darwinian evolution. But Darwin himself lived through the birth of our discipline, so it is not surprising to learn that there are links between Darwin and the founding fathers of microbiology.

Darwin and natural selection

Charles Darwin was born and schooled in Shrewsbury, an English market town close to the Welsh border. He tried, and ducked out of, a medical education in Edinburgh, then studied at Cambridge with a view to joining the clergy. But his reputation as a naturalist earned him a place on a round- the-world trip on HMS Beagle, which primed him for his revolutionary ideas on evolution. He started to formulate his thoughts on evolution shortly after his return from the Beagle voyage, recording a riot of ideas, sometimes earthy or even vulgar, in a series of notebooks. For inspiration on his ideas of the struggle for existence and natural selection, he drew on the work of Robert Malthus, who had suggested that human populations always eventually out-run the means to sustain them. Darwin outlined his theory in a ‘pencil sketch’ of 1842 and an essay of 1844, but, preoccupied with other work, delayed publication until the late 1850s, when he was spurred into action by the rival work of Alfred Russel Wallace.

In On the Origin of Species by Means of Natural Selection, first published in 1859, Darwin eloquently (and presciently, given its subsequent influence on antibiotic resistance) emphasizes the remarkable power of natural selection:
‘We have seen that man by selection can certainly produce great results, and can adapt organic beings to his own uses, through the accumulation of slight but useful variations, given to him by the hand of Nature. But Natural Selection, as we shall hereafter see, is a power incessantly ready for action, and is as immeasurably superior to man’s feeble efforts, as the works of Nature are to those of Art.’ [link]
Later, in Variation Under Domestication [note: not The Descent of Man as stated in PDF], despite his ignorance of the nature of inheritance, Darwin points out that the variation that is a prerequisite of natural selection originates independently of the selection itself.

In the decades after his death, while Darwin’s ideas of evolutionary change and common ancestry were widely accepted, his principal mechanism for change, natural selection, was not. However, in the mid-20th century, Darwin’s intellectual legacy was reconciled with Mendelian genetics in what is often called ‘The Modern Synthesis’. As part of this reconciliation, bacteria were brought into the broader evolutionary genetic framework, principally through the experiments on the genetics of phage susceptibility published by Salvador Luria and Max Delbrück in 1941. In their famous fluctuation test, Luria and Delbrück confirmed Darwin’s hunch that variation precedes selection, rather than arising in response to it, and thrust bacteria centre stage as biological entities with fully-fledged genetics. A few years later, in 1945, Milislav Demerec repeated the fluctuation test on penicillin resistance in Staphylococcus aureus, showing for the first time the awesome power of natural selection to curtail our biochemical victories over micro-organisms.

Darwin and Pasteur

Shortly after Darwin published The Origin, in Paris, Louis Pasteur performed a series of experiments that demolished the theory of spontaneous generation. Darwin was well aware of the on-going controversy. In 1860, in a letter to his friend Lyell, he refers to the work of Pasteur’s rival Pouchet:
‘I have seen something about the infusorial experiments in Paris: Quatrefage objected to their accuracy. Some old experiments were several years ago tried in Germany with astonishing precautions (air all passed through sulphuric acid & caustic potash) and infusoria never appeared.’
A few years later in 1863, he wrote to the English botanist George Bentham:
‘I am very glad that you are going to allude to Pasteur; I was struck with infinite admiration at his work.’
In disproving spontaneous generation, Pasteur might be seen as undercutting Darwin’s theory of evolution by removing a mechanism for the generation of the first life forms. However, Darwin was sharp enough to realize that the conditions under which life first originated were likely to be quite different from those around today. In 1871, he wrote to his botanist friend Joseph Hooker:
‘It is often said that all the conditions for the first production of a living organism are now present, which could ever have been present. But if (and oh! what a big if!) we could conceive in some warm little pond, with all sorts of ammonia and phosphoric salts, light, heat, electricity, etc., present, that a proteine [sic] compound was chemically formed ready to undergo still more complex changes, at the present day such matter would be instantly devoured or absorbed, which would not have been the case before living creatures were formed.’
As far as I am aware, Pasteur never made any direct reference to Darwin, although he opened his 1864 address to the Sorbonne on spontaneous generation with these expressive lines:
‘Great problems are in question today, keeping every thinking man in suspense: the unity or multiplicity of human races, the creation of man 1,000 years or 1,000 centuries ago; the fixity of species, or the slow and progressive transformation of one species into another…’
Half a century later, an essay in Science magazine concluded:
‘Darwin, master of the organic world sleeps near Newton, master of the inorganic, in the great [Westminster] Abbey, among the most famous of his race. Pasteur rests alone in the chapel of his laboratory … Both rest forever among the immortals. the last half of the nineteenth century may well be called their age “the Age of Darwin and Pasteur”.’

Darwin, Cohn and the origin of infection

Darwin maintained an extensive network of correspondents. Among them was the German Jewish botanist and bacteriologist, Ferdinand Cohn (1828–1898), who is widely recognized as the father of bacterial taxonomy. Cohn was the first to classify bacteria according to their microscopic appearance and the first to describe sporulation in Bacillus. He was instrumental in publishing Robert Koch’s work on Bacillus anthracis.

From 1874 until 1882, Darwin and Cohn maintained a lively correspondence, principally on botany. On 26 September 1876 Darwin writes to invite Cohn and his wife to visit him at Down House. And then, the very next day, Darwin writes to his son Frank, saying that he hopes they will not come! Apparently the subsequent visit was a success. However, for microbiologists, one particular exchange of letters stands out. In January 1878, Cohn writes to Darwin, discussing Koch’s recent discovery of the anthrax bacillus. Darwin’s response is a triumphant celebration of the birth of medical microbiology:
‘I thank you sincerely for your most kind letter and I return your wishes for the New Year with all my heart. Your letter has interested me greatly. Dr Sanderson showed me some admirable photographs on glass by Dr Koch of the Organisms which cause Splenic Fever. But your letter and the valuable work which you have given me make the case much clearer to me. I well remember saying to myself between 20 and 30 years ago, that if ever the origin of any infectious disease could be proved, it would be the greatest triumph to Science; and now I rejoice to have seen the triumph.’ 
Further reading

Darwin, C. (1859). On The Origin of Species By Means of Natural Selection. London: John Murray.

Pallen, M. (2009). The Rough Guide to Evolution. London: Rough Guides.

Creager, A.N.H. (2007). Adaptation or selection? Old issues and new stakes in the postwar debates over bacterial drug resistance. Stud Hist Phil Biol Biomed Sci 38, 159–190.

Sedgwick, W.T. (1923). Darwin and Pasteur: an essay in comparative biography. Science 52, 286. – The complete works of Charles Darwin online. – Darwin Correspondence Project

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