History of Ecological Sciences, Part 40: Darwin's Evolutionary Ecology

Abstract

Click here for all previous articles in the History of the Ecological Sciences series by F. N. Egerton Charles Darwin (1809–1882) was the greatest biological scientist and a major contributor to ecological sciences (Vorzimmer 1965, Acot 1983 Dajoz 1984:46–50, 58–83). Natural history before Darwin had many ingredients of ecology, but was weak in theory. The balance of nature, including Linnaeus' version, economy of nature (Egerton 2007b:81–84), was the main example, and it was never developed as a precise theory (Egerton 1973, Kricher 2009). The evolutionary ideas of Erasmus Darwin and Lamarck had ecological relevance (Egerton 2008, 2010a) but were not developed into an elaborate theory like Charles Darwin's theory of evolution by natural selection. What do I mean by evolutionary ecology? A coyote might eat different prey in different parts of its geographic range, so that is not much of an evolved relationship. However, other relationships have evolved. Augustin-Pyramus de Candolle, one of the leading botanists during the first three decades of the 1800s, was uninterested in studies on floral mechanisms that seemed to guide specific species of insects into pollinating a specific species of plants, because he did not believe that one biological species was modified to meet the needs of another species. He was aware of Lamarck's evolutionary speculations about species striving to change, but he was among the majority of botanists and zoologists who did not find Lamarck's teachings convincing. Darwin's first book after publishing the Origin of Species was on the mechanisms among orchid species that guide particular insect species to pollinate that orchid species—caused by natural selection, not by Lamarckian striving—one example of evolutionary ecology. Another example is a harmless animal species evolving by natural selection to mimic a dangerous species as a protection from predators. Darwin's Origin unleashed this line of thought before ecology became an organized science, and later ecologists readily adopted this intellectual tool (Kolasa 2011:28, 39). Darwin's Journal of Researches 1839 made substantial contributions to ecology (Egerton 2010b), and he was as well equipped after his voyage to advance understanding of the economy of nature as to advance evolutionary biology. After the Beagle publications, Darwin continued being an observational naturalist, but he also became an experimentalist. Darwin followed in the footsteps of three role models: Gilbert White, an observer, Humboldt, an observer–correlationist–experimenter, and Lyell, an observer–theoretician. Darwin commonly investigated several different subjects in a year, and even when our scope is limited to ecological subjects, a strictly chronological presentation is impractical. His post-Beagle books (Freeman 1965) usually were preceded by articles in periodicals on the subject. Fortunately, these articles are mostly republished in The Collected Papers of Charles Darwin (two volumes, 1977), and more completely in Darwin's Shorter Publications, 1829–1883 (Darwin 2009). All of Darwin's books are available on the Internet 〈Darwin–online.org.uk〉, and are also republished (Darwin 1986–1990). His Correspondence has been published by Cambridge University Press since 1985 (18 volumes extend to 1870) and is also online 〈darwinproject.ac.uk〉. The present discussion is organized in the chronological order in which he published relevant books, since the Beagle volumes covered in part 37 (Egerton 2010b). Darwin was one of the world's greatest correspondents, and many of his correspondents were happy to send useful information to him. His publications are also available in a collected set (1986–1990) and at Darwin online. After leaving the Galapagos Islands, Darwin had wondered in 1836 whether mockingbirds from different islands were varieties or species (Egerton 2010b:412–414). In March 1837, ornithologist John Gould convinced Darwin that his finch and mockingbird specimens from the Galapagos Islands were different species, and that realization made Darwin an evolutionist (Egerton 2010b:416). Darwin then began keeping notebooks in which he recorded his readings and thoughts on transmutation. His eureka moment came on 28 September 1838 (Darwin 1987:375), when he read Thomas Robert Malthus' Essay on the Principle of Population (Edition 6, 1826). Why did he read a book that would seem to be peripheral to his quest? He had recently finished his Journal of Researches 1839, which he had modeled on Humboldt's Personal Narrative of Travels, and Humboldt had praised Malthus' Essay (Egerton 1970:331–332). Using information in his notebooks, Darwin wrote two early drafts of his theory, 1842 and 1844 (1909). Those notebooks and drafts provide insights into the literature he read and the progress in his thinking, 1837–1844 (Limoges 1970, Manier 1978, Kohn 1980, 1985, Ospovat 1981, Hodge 2003). Anonymous publication of Robert Chambers' Vestiges of the Natural History of Creation 1844, followed by unfavorable reviews from naturalists, inhibited Darwin from publishing his theory at that time. Instead, he wrote two monographs on living barnacles and two on fossils (1851–1854) that “brought about a new way of thinking about morphological comparisons” (Ghiselin 1969:109), and one cirripedologist (Crisp 1983:73–74) even suggested that these monographs could be considered Darwin's greatest works, even though Darwin had misunderstood aspects of female anatomy! The interest he developed in invertebrates at Edinburgh continued throughout the voyage of the Beagle, and he had wanted to include a volume on invertebrates to Zoology of the Voyage of H. M. S. Beagle, but had not managed to do so (Love 2002:266–269). His specific interest in barnacles had been piqued in January 1835 when he discovered in the Chonos Archipelago, off the mainland of Chile, the smallest known barnacle (seen with his microscope), which he named Cryptophialus minutus (Darwin 1854:23, 566–586, 2000:274–276, Richmond 1988, Keynes 2003:264–265, Stott 2003:xx–xxi, 62–63). C. minutus was a parasite that bored through the shell of a conch Concholepas peruviana and lived in its body. While overseeing the volumes describing his vast collections from his voyage and also writing his Journal of Researches, he did not pause to explore this oddity, but after those Beagle volumes appeared, he returned to this species and was soon studying all of the barnacles. His monographs mostly contain systematic descriptions and classifications of species (Winsor 1969a, b, Ghiselin and Jaffe 1973, Southward 1983, Richmond 1988); however their introductions are relevant. Barnacles ate “infusoria” (plankton), minute spiral univalves (snails), and crustacea, including larvae of other barnacles (Darwin 1851:45–46). Pedunculated barnacles (Fig. 2) extend over the whole world, and most species have large ranges, especially those that attach to floating objects. Of those species that attach to fixed objects or to littoral animals, one rarely finds more than three or four species in any locality (Darwin 1851:65–66). Cirripedes are usually bisexual, differing from all other crustacean; when sexes are separate, males are minute and permanently epizoic on females (Darwin 1854:15). Sessil barnacles (Fig. 3) live from latitude 74° 18′ North, south to Cape Horn. Charles Darwin, about 1854. Seward 1909:Frontispiece.. Pedunculated Pollicipes. By George Sowerby. Darwin 1851: from Plate 7, 1964. Sessile barnacle: larvae of Lepus australis. By George Sowerby. Darwin 1854: from Plate 30, 1964. The area between the north Philippine Archipelago and south Australia, extending to New Zealand on the right and Sumatra on the left, has a greater number of species than the rest of the world. Probably this is mainly due to the broken nature of the land, providing diversified habitats and due to much of the coast being rocky. There are more species on the rocky coast of western South America than on its sandy or muddy eastern coast. Coral reefs are unfavorable for all barnacles except Pyrgoma, and few barnacles are known from Pacific islands. Where they can live, species are few and individuals are infinite. No genus with more than one species is confined to the torrid zone. Pyrgoma species are confined to the torrid zone except for one species that is found from the Cape Verde Islands to England and Ireland (1854:159–160). James Dana's great work, Crustacea (1852–1855), has an excellent chart with isocrymal lines, showing mean temperature of waters along their course for the coldest 30 consecutive days in any season. He showed that these lines are most influential for the distribution of marine animals. These measurements were a further elaboration on the isothermal lines that Humboldt introduced into environmental studies (Egerton 2011:158). Dana divided the torrid and sub-torrid zones from the temperate zones at isocryme 68°, and the temperate zones from the sub-frigid and frigid zones at 44°. Darwin found no barnacles confined to frigid zones. Darwin knew 147 species, seven of doubtful habitat. Of the remaining 140, 37 inhabited both torrid and temperate zones, 46 were exclusively in torrid, and 57 were exclusively in temperate zones. The temperate zones, though smaller in area with considerably less lengthy coastlines, had the most species. There are two temperate zones, separated by torrid zones, and the number of species in any zone seems to depend on the isolation of sub-zones. Balanus was the largest known genus, with 36 species of known habitats: 9 in the torrid zone, 15 in temperate zones, and 12 in both zones (Darwin 1854:160–162). Darwin divided the oceans with barnacles into five provinces and listed the species in each province (Darwin 1854:164–171). His two volumes on living and two on fossil barnacles won the Royal Society Medal in 1854. D. T. Anderson, Barnacles: Structure, Function, Development and Evolution 1994, provides a modern perspective on Darwin's work. After publishing on barnacles, at Lyell's urging, Darwin returned to his natural selection project and was in the midst of writing a huge monograph, when he was interrupted by arrival in his mail of Alfred Russel Wallace's manuscript, “On the Tendency of Varieties to Depart Indefinitely from the Original Type,” in 1858. Wallace became a co-discoverer of evolution by natural selection, and Lyell and Joseph Hooker arranged for extracts of Darwin's work, along with Wallace's article, to be read on 1 July and published in 1859 by the Linnean Society of London (Darwin and Wallace 1859). Darwin then abandoned his large manuscript and wrote a more readable abridgment, On the Origin of Species 1859. His longer manuscript was partly used in later books, but those parts not so used are now published (Darwin 1975) and provide many citations to his sources not included in Origin. Darwin presented his theory in the first four chapters, followed by nine chapters on diverse supporting evidence. Origin chapters 1–2 presented noncontroversial evidence that variation occurs in both domestic and wild populations of species. …it breeds when thirty years old, and goes on breeding till ninety years old, bringing forth three pair of young in this interval; if this be so, at the end of the fifth century there would be alive fifteen million elephants, descended from the first pair. …it breeds when thirty years old, and goes on breeding till ninety years old, bringing forth six young in the interval, and surviving till one hundred years old; if this be so, after a period of from 740 to 750 years there would be nearly nineteen million elephants alive, descended from the first pair. But since there would never be nineteen million elephants, descended from one pair, alive at the same time, there must be checks on the growth of populations of all species. To illustrate the complexity of such checks, Darwin explained the interrelationships of red clover, humble bees, mice, and cats: only humble bees pollinate red clover, but field mice eat humble bees, and cats eat mice. Therefore, the success of red clover fields might depend on the local population of cats (1859:73–74). Although we now know that this food chain is more complex than Darwin realized (Egerton 2007a:52–53), his conclusion is still valid: if we speculate on those checks and their magnitudes, “It will convince us of our ignorance on the mutual relations of all organic beings…” (1859:78). Revolutionary paradigms, such as Origin, reorient sciences and uncover new problems to study (Kuhn 1970). When we see leaf-eating insects green, and bark-feeders mottled-grey; the alpine ptarmigan white in winter, the red-grouse the colour of heather, and the black-grouse that of peaty earth, we must believe that these tints are of service to these birds and insects in preserving them from danger. Grouse, if not destroyed at some period of their lives, would increase in countless numbers; they are known to suffer largely from birds of prey; and hawks are guided by eyesight to their prey—so much so, that on parts of the Continent persons are warned not to keep white pigeons, as being the most liable to destruction. Hence I can see no reason to doubt that natural selection might be most effective in giving the proper colour to each kind of grouse, and in keeping that colour, when once acquired, true and constant. He is speaking here of protective coloration, though without naming it. It is interesting to contemplate an entangled bank, clothed with many plants of many kinds, with birds singing on the bushes, with various insects flitting about, and with worms crawling through the damp earth, and to reflect that these elaborately constructed forms, so different from each other, and dependent on each other in so complex a manner, have all been produced by laws acting around us. Sessile acorn barnacles Balanus. By George Sowerby. Darwin 1854: from Plate 1, 1964. Darwin’s study and laboratory, Down House. Photo, John Webb. Karp 1968:6. Dissected flower of early orchis Orchis mascula. Drawn by G. B. Sowerby. Darwin 1862: Fig. 1. An explanation of all these figures accompanied this illustration. Darwin substituted a sharpened pencil for a bee and poked it into an Orchis mascula flower and found that the pollinium attached to it (A), then bent at a right angle to be in position to pollinate the next orchid the bee entered. Drawn by G. B. Sowerby. Darwin 1862: Fig. 2. Darwin's theory might have undermined the related concepts of the economy of nature and the balance of nature, but this last quotation shows why it did not (Egerton 1973:341–342, Pearce 2010). “The theory of evolution by natural selection is an ecological theory—founded on the ecological observations by perhaps the greatest of all ecologists” (Harper 1967:247). It is no wonder, then, that one of Darwin's most fervent disciples, Ernst Haeckel (1834–1919), realized the need not only for a new science of evolutionary biology, but also for a new science of ecology, which he named and defined in his Generelle Morphologie der Organismen (two volumes, 1866). Haeckel dedicated Generelle Morphologie to Goethe, Lamarck, and Darwin, and sent a copy to Darwin. Although Darwin had spent some time teaching himself German, he found Haeckel's writings difficult to understand (Desmond and Moore 1991:541–542, Browne 2003:269–270), and might not have read Haeckel's discussions of “oecologie.” In the 23 years after publishing Origin, Darwin published 10 more books and numerous articles. His subjects were diverse, but they all had the same objective of expanding and elaborating proofs for his theory. As we saw in part 37 (Egerton 2010b), despite having made a diligent collection of plants on the Galapagos Islands and elsewhere, Darwin thought of himself as a zoologist and geologist during and immediately after his voyage on the Beagle, and his new book-length publications after his Beagle volumes were on barnacles. Nevertheless, his post-Origin research and publications were as much or more on plants as on animals (Allan 1977, Morton 1981:413–419, Ornduff 1984, Magnin-Gonze 2004:188–192). He was usually assisted by his son Francis (1848–1925), a trained botanist (Desmond 1977:173–174, Browne 2003:434–435, 465–469, 2004, Junker 2004), and sometimes by his son George (1845–1912), who also had scientific interests (Browne 2003:434–436). However, Darwin's close relationship and correspondence with botanist Joseph Dalton Hooker (1817–1911) after 1843 was a most valuable source of information and judgment (Browne 1978, 1983:197, 215–216, 1995:518–521, 2003:see index). Head and proboscis of Acontia luctuosa with seven pair of pollinia of Orchis pyramidalis attached to its proboscis. Darwin 1877b:31. Those which had the largest or and which most would be by and would be and so in the would the Those which had their and in to the and of the particular insects which so as to in any the of their from flower to would be or A with a By George Darwin. Darwin It was Darwin's to that the of many orchid were not a of or but of natural selection His orchid book first specific species were and then to the evolved between and insect to between than of his and in the number of its in Browne his there was no scientific explanation for the or and of some Darwin explained both and were the insect chapters and illustrate different of and their He began with a early Orchis and to its parts and their the of to which he this was a and this book was a history of Darwin's theory. It was by his but convinced few of his Darwin that had not collected information on the between and but sometimes he could his as he did with Orchis mascula. Using a pencil as a (Darwin 1862: Fig. he showed that the pollinium to the then 30 at a right which it to the of the next orchid the bee studies to (Darwin that with were by bees or and those with by or F. sent Darwin 23 species of that he had Orchis having pollinia to their After Darwin published his orchid suggested that was insects the through the large on the and crawling by one of the two close to the and (Darwin Darwin this by into the of C. without but when he a into the it as The has a to 12 and Darwin that there must be a with a proboscis to its (Darwin In found a with a proboscis a was discovered and has been The book a major in and in as a It completely our of and to literature on ecology. that has been on the of and related has been or by this John it of the most books of all Darwin listed in the of (Darwin of publications that had since he published the first to this Darwin's next On the and of (Darwin first as parts of the Journal of the Linnean Society of London (Freeman and only as a book in the (Darwin He was in the evolution of this and its for these include plants in which the around another and The in most species which he were from or flower but he also from by and (Darwin included and including and and F. to or with their He did not study the of plants on their and he did not the that its Francis Darwin Browne Junker later his book in a of studies on this (Darwin in 1 and and to and Darwin a Darwin (Darwin in the same as the of but it was a more substantial book by Francis and Francis Darwin was his and during the (Browne Junker saw in part (Egerton that had in that the and all but he did not their or Darwin had read in 1839 2009). Darwin first the in the of when he on the of the them on but he had to understand the to and had on to other subjects by (Browne He returned to the on 23 (Darwin and a and for this research (Browne Darwin he had off in with his (Allan of the two of flower showing the to Darwin of the book He and son Francis to various and to the to and also the to various did not the that plants live in do not for further The remaining chapters the mechanisms of the and a few M. of had his of the of that and and had and that a of than not (Darwin plants to both botanists and and modern literature and these species a

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