The number of species on islands

July 27, 2020 • 1:00 pm

by Greg Mayer

[The following is a trivial, and speculative, discussion about a small part of an important paper in the history of ecology and evolutionary biology.]

There are fewer species of any given group of plants or animals on an island than on an equivalent area of the mainland; and the larger the island, the more species there will be. These two general rules of natural history have been known since at least the 19th century, and are known under the rubrics that island biota are depauperate, and the species-area relationship, respectively. There have been many, not always mutually exclusive, explanations for these phenomena, and in the early 1960s they were on the minds of at least several biologists.

The most important resulting paper was Robert MacArthur and Ed Wilson‘s classic “An equilibrium theory of insular zoogeography.” This paper, together with the expansion of the ideas contained within it into a book-length monograph, The Theory of Island Biogeography (1967), were extremely influential in setting out the questions to be asked, and how to go about answering them, across many areas of ecology and evolutionary biology, not just the phenomena of island life.

MacArthur and Wilson (1963).

In the paper, MacArthur and Wilson proposed (among other things) that the number of species on an island resulted from a dynamic equilibrium between ongoing immigration and extinction of species living on the island, and that the relation of these demographic processes to various physical properties of islands led to the species-area relationship. They illustrated this relationship in two figures showing the relationship between the area of an island or island group and the number of species of land and freshwater birds occurring on that island or island group. One figure was for islands in and near the Sunda group, the other for the Moluccas, Melanesia, and Polynesia. Both figures are interesting, but the second one concerns us today.

Figure 2 of MacArthur and Wilson (1963).

For reasons relating to some research projects I’m pursuing during the pandemic, I was led to look closely at this figure, including the sources of the data as indicated in the legend of the figure: three papers by Ernst Mayr, and a book by James C. Greenway. I have a copy of Greenway’s book (both editions, actually), and Mayr’s 1943 paper is readily available online to anyone from SORA (a wonderful free repository containing vast swaths of the ornithological literature).

Neither of the two earlier papers are readily available, but the University of Wisconsin, Madison, Library scanned a copy of the earliest, and sent me a pdf (kudos to the staff of the Library there for working very hard during the pandemic to keep the scholarly literature available); and I happened to have a reprint of the other, a paper from the modestly obscure Proceedings of the Sixth Pacific Science Congress. Mayr published two papers in the proceedings, and I have reprints of both; the second is the one cited by MacArthur and Wilson.

Mayr (1940a).


Mayr (1940b).

The provenance of my copies is of interest. By looking at the stamps and annotations, you can see that the copies were originally in the library of the Bird Department of the Museum of Comparative Zoology, which stamped them, and added the author and date notation at upper left. (Departments of major museums usually maintain libraries physically separate from the main library of the institution.) Reprints were the principal way in which scientific literature circulated in the 1940s, and it is most likely that the Bird Department obtained the copies shortly after publication, not when Mayr moved to the MCZ in 1953. As duplicates (and Mayr’s arrival may be why the Department had extra copies), they then passed into the possession of my friend and colleague Bob O’Hara; we were graduate students together in the 1980s, and he worked closely with Mayr. At some point, also in the 1980s, Bob gave them to me, and I penciled my name and a correction to the date of publication on them. (See Note on the date of publication below.)

So now we come to the matter at hand. I was trying to track down the actual numbers that went into MacArthur and Wilson’s Figure 2, and some of the data came from Mayr (1940b), the second of the proceedings papers. It’s a 20 page paper, with no table of species numbers by islands, so I was reading it carefully to find what numerical data I could. It’s all on page 202– species numbers for 11 islands or island groups that were included in Figure 2. And a little later, while copying the numbers on to a data sheet, I noticed a pencil mark next to the paragraph with the data. (It is the only mark in the paper, aside from those on the first page.)

A pencil mark highlighting species number data in Mayr (1940b).

The mark was not mine; and it would not be Bob O’Hara’s, who would not have been interested in the particular questions addressed by these data. So who would have been using the MCZ Bird Department Library’s copy prior to the 1980s to highlight data included in MacArthur and Wilson (1963)? It occurred to me, could it have been Ed Wilson himself?

Wilson was at the MCZ with Mayr, and would have had access to this copy. In their paper, he and MacArthur thank Mayr (and a few others) for “material aid and advice during the course of the study.” There is thus no question that they consulted Mayr, and used data from a number of Mayr’s papers and books (in Figure 1, as well as Figure 2). If they looked up the data themselves (as opposed to Mayr directly telling them the species numbers), then this particular copy, now in my possession, is a likely copy for them to have used, and Wilson is the likely person to have made the mark highlighting the data that was used. This then, is my speculative (and trivial) suggestion: that Wilson used this copy in the preparation of his and Robert Mac Arthur’s classic and influential paper.

MacArthur and Wilson’s (1963) Figure 2 plots data for 26 islands or island groups. In the cited references, I can find data for only 21 of them; I do not know where they got the data for Tonga, Kei, Tanimbar, Buru, and Ceram. Jurgen Haffer (2007:163), Mayr’s biographer (and himself an accomplished contributor to ornithological science) records the following interesting tidbit:

Mayr (pers. comm.) had copious data on island sizes, distances from mainlands or other islands, number of species, etc. When he tried to determine relations among all these figures he got into mathematical problems and turned this material over to a graduate student with mathematical abilities. However this student got sidetracked into other problems and this material was never exploited.

This account was recalled to Haffer by Mayr decades later, and what data was compiled, when it was compiled, and who the student was, isn’t known.

Haffer (2007: 170) also states that in the 1933 and 1940b papers Mayr “clearly discussed what became later known as the equilibrium theory of insular biogeography”, a claim that has been echoed by other authors. This is not quite correct. Mayr discussed a number of relevant factors contributing to the characteristics of island faunas in 1940b, and the paper is well worth reading and studying today. But he did not formulate in any clear way, even verbally, MacArthur and Wilson’s later theory.  (My very limited German does not allow me to properly assess the 1933 paper, but Mayr’s two 1940 papers in English seem to parallel closely the earlier paper in German.)

For example, Mayr (1940b:215) does note that on a smaller island a species will have a smaller “effective breeding population” and thus be vulnerable to extinction; this is a striking formulation, obviously influenced by theoretical population genetics (likely gotten from Theodosius Dobzhansky). But in discussing extinction on New Caledonia he is clearly discussing evolutionary events stretching over much of the Tertiary (i.e. tens of millions of years), and not the turnover in ecological time of insular populations contemplated by MacArthur and Wilson’s equilibrium theory. The latter has been demonstrated to occur over annual and decadal scales on, for example, smaller islands in the British Isles.

While not excluding the possibility of a MacArthur and Wilson style ecological equilibrium (because he does not consider the situation), Mayr is clearly discussing the origin and persistence (or not) of endemic forms (species, genera, even families) over evolutionary time. Mayr himself (quoted in Haffer, 2007: 163) offers a much more nearly accurate appreciation of his views, stating that his own ideas embraced the “basic thesis” of MacArthur and Wilson; he did not “clearly discuss” the equilibrium theory.

Note on the date of publication. The Sixth Pacific Science Congress was held in 1939, but the Proceedings were published later, from 1940-1943.  Greenway (1958), who knew Mayr well, cited them as 1941, and this may have been the source of my handwritten correction, but I cannot now recall. Haffer’s (2007) definitive list, based on Mayr’s own lists, cites them as 1940.

Greenway, J.C. 1958. Extinct and Vanishing Birds of the World. American Committee for International Wild Life Protection, New York.

Haffer, J. 2007. Ornithology, Evolution, and Philosophy: The Life and Science of Ernst Mayr 1904–2005. Springer, Berlin.

MacArthur, R.H. and E.O. Wilson. 1963. An equilibrium theory of insular zoogeography. Evolution 17:373-387.  pdf

MacArthur, R.H. and E.O. Wilson. 1967. The Theory of Island Biogeography. Princeton University Press, Princeton, N.J.

Mayr, E. 1933. Die Vogelwelt Polynesiens. Mitteilungen aus dem Zoologischen Museum in Berlin 19:306-323.

Mayr, E. 1940a. Borders and subdivisions of the Polynesian Region as based on our knowledge of the distribution of birds. Proceedings of the Sixth Pacific Science Congress 4:191-195.

Mayr, E. 1940b. The origin and history of the bird fauna of Polynesia. Proceedings of the Sixth Pacific Science Congress 4:197-216. (This paper was reprinted in Mayr (1976) but Mayr sometimes updated papers in that collection, and for my purposes I needed to see the original.)

Mayr, E. 1976. Evolution and the Diversity of Life. Harvard University Press, Cambridge, Mass.

Mayr, E. 1943. The zoogeographic position of the Hawaiian Islands. Condor 45:45-48. pdf

A visit with Dick Lewontin

July 19, 2019 • 2:00 pm

by Greg Mayer

While on the East Coast to attend the Evolution 2019 meetings in Providence, Rhode Island, I also stopped for a few days at the Museum of Comparative Zoology (Jerry’s and my alma mater), and got a chance to visit with Dick Lewontin, Jerry’s Ph.D. advisor, and my de jure Ph.D. advisor (my actual advisor, E. E. Williams, was retired, and so could not officially be my advisor). WEIT readers may recall that Jerry posted greetings for Dick’s 90th birthday earlier this year. I went to see Dick with Steve Orzack, another one of Dick’s Ph.D. students, who took the two pictures below.

Visiting with Dick Lewontin, Cambridge, Mass., 21 June 2019. Picture by Steve Orzack.

We chatted for an hour or two about various things. Steve and I both had some things we wanted to ask Dick about, one of mine being whether Dick’s advisor, Theodosius Dobzhansky, was Russian or Ukrainian. (Wikipedia claims he’s Ukrainian, and I once had a Ukrainian complain to me about an exhibit about Dobzhansky that I curated that referred to him as Russian.) Dick was adamant that Dobzhansky was Russian, noting that he spoke Russian at home with his wife, thought of himself as Russian, and had Russians as his lab assistants and technicians. Historians, friends, and colleagues of “Dodik/Doby” have always called him Russian, so I was not surprised by Dick’s response.

Dick hams it up for the camera. Picture by Steve Orzack.

Dick also regaled us with stories of when he worked with Buckminster Fuller on geodesic domes back in the ’50s, when Dick was at North Carolina State. Bucky, he assured us, did not understand the geometry of solids! Dick mentioned that he considered leaving academia to work full time with Fuller, but was now glad he hadn’t, as Fuller’s company went under a few years later. Steve replied that if Dick had joined the company full time, Dick could have saved the company!

Dick has given up essentially all his space at the Museum, and most of his papers (correspondence, etc.) have been taken by the American Philosophical Society, (which also has a considerable trove of Dobzhansky material), and Dick has given his books to the Ernst Mayr Library– the library of the Museum of Comparative Zoology. This is appropriate, as Ernst Mayr, while Director of the MCZ from 1961-1970, engaged in correspondence with Dick on “genetical problems” (Haffer , 2007:265), and pushed for the building of the Museum of Comparative Zoology Laboratories, the MCZ’s lab wing, completed in 1973 (Mayr, 1973), of which Dick’s fly lab was one of the first inhabitants, arriving at Harvard in that opening year. (Dick mentioned that the proximity of the MCZ to his summer place in Vermont, which he’d had to travel to by train from Chicago, was a consideration in moving from the University of Chicago to Harvard.)

Dick’s books are being sorted, and I looked though several of them, finding a number of interesting inscriptions. First, a set of inscriptions from Mayr himself. These show that Mayr was presenting Dick with his books as early as March 1969, prior to Dick’s arrival at Harvard. I’m not sure if discussions involving Dick’s movement to the MCZ had begun this early.

To Dick Lewontin, | evolutionary geneticist, |who appreciates the importance of systematics, | in friendship | Ernst | March 1969

The ISBN stamp on the following cover page (and some further below) are from a cataloging effort in Dick’s personal library, not from the MCZ Library.

To Dick Lewontin | fellow worker in the evol. vineyard, | in the hope that he will crack | some of the nuts that were too hard for me! | With best wishes | Ernst | Christmas 1976


To Dick Lewontin | to whom I owe so much intellectual | stimulation | in friendship and admiration | Ernst


For Dick Lewontin | whose deep understanding of genetics | I admire beyond words (and song!) | in friendship | Ernst Mayr

[I am unsure of my transcription of the final word of the third line, “song”.]


For Dick Lewontin | in friendship and admiration | from the non-Marxist dialectic materialist | Ernst Mayr

The following is Dick’s MCZ bookstamp, which appears in many, though not all, of his books from his MCZ years.

The following is of interest, coming from Tom Schopf, one of the “young Turks” of paleontology in the early 1970’s, whom I mentioned in my tribute to David Raup.

To Dick Lewontin | I hope you will enjoy this effort to make invertebrate paleontology a “creative, chancy young man’s game” | As you will see from citations to your | work, you have had a large influence. | And I look forward to your continuing | analysis of problems critical to paleontologists. | Tom | December 2, 1972

The following is an inscription to Dick from a younger colleague, Jonathan Losos, on his book Lizards in an Evolutionary Tree. Note that Dick had been Jonathan’s intro bio professor!

Dick, | With great appreciation for the influence you have had on my | career through your writings, your teaching | of my introductory biology class) of which | I have vivid recollections) and your | helpful conversations with specific | reference to points herein. | Jonathan

And finally, some inscriptions from Dick’s Ph.D. advisor, Theodosius Dobzhansky. These I got from Dick several years ago, when I visited him while preparing my exhibit on Dobzhansky. I used all three inscriptions in the exhibit. The first is on a copy of the third edition of Dobzhansky’s classic Genetics and the Origin of Species. It was published in 1951, which is about the time Dick went to Columbia to work with Dodik. (Dick was in the Harvard class of 1950, but since he had been “rusticated” for a year, he actually graduated in 1951.) The inscription isn’t dated, but it seems to be earlier than the other two, referring to Dick’s “scientific youth”, and his “coming” eminence. According to Dick, Dodik referred to finishing graduate students as “soon to be professor” (as did Dick himself), so this inscription is probably early in Dick’s grad school days.

To Dick Lewontin, the coming | eminent geneticist, in his scientific | youth, with best wishes of continued | success | Th Dobzhansky

The next inscription is on a bound set of numbers I to XX of Dobzhansky’s monumental series of paper on “The Genetics of Natural Populations”. These 20 papers were published from 1938 through 1952. It is interesting that Dodik refers to the greater success of succeeding generations; the inscription was made only 5 years before Dick published his groundbreaking papers with Jack Hubby on allozyme polymorphism, confirming Dodik’s long-argued view that genetic variation was abundant and “normal” in natural populations.

Progress of science means that | succeeding generations do better than | preceding generations— and this | is what is to happen when the | genetics of natural populations | is investigated by my old | friend and spiritual son, | Prof. R. Lewontin! | Th Dobzhansky | New York, February 4, 1961

The final inscription is on a bound set of numbers XXI-XL of “The Genetics of Natural Populations”, published from 1953 through 1968. This inscription is undated but necessarily postdates initial Dick’s work on allozymes. There were three more papers in the series to come, published from 1969 through 1976; for the last, Dobzhansky was a posthumous coauthor, having died in December,1975. (The notation “GNP | XXI-XL” was made by me on the copy I made, and is not on the original.)

These lucubrations of the | old age of Th. Dobzhansky | dedicated to the super-star | R.C. Lewontin

You can see in these inscriptions the development of Dobzhansky’s appreciation of Dick as a scientist, from promising “youth”, to “old friend and spiritual son” (Dobzahnsky had one child, a daughter), and finally to “super-star”. You can also see Dodik’s colloquial phrasing and sense of humor, also evident in his  aphorism, “Heaven is where, when the experiment is over, you don’t need statistics to figure out what happened.” (Which Dick reconfirmed, on my latest visit, was indeed Dodik’s.)

Haffer, J. 2007. Ornithology, Evolution, and Philosophy: The Life and Science of Ernst Mayr 1904-2005. Springer, Berlin

Hubby, J. L., and R. C. Lewontin. 1966. A molecular approach to the study of genic heterozygosity in natural populations. I. The number of alleles at different loci in Drosophila pseudoobscura. Genetics 54:577-594.

Lewontin, R. C., and J. L. Hubby. 1966. A molecular approach to the study of genic heterozygosity in natural populations. II. Amount of variation and degree of heterozygosity in natural populations of Drosophila pseudoobscura. Genetics 54:595-609.

Losos, J.B. 2009. Lizards in an Evolutionary Tree: Ecology and Adaptive Radiation of Anoles. University of California Press, Berkeley.

Mayr, E. 1969. Principles of Systematic Zoology. McGraw-Hill, New York.

Mayr, E. 1973. Museums and biological laboratories. Breviora 416, 7pp. BHL

Mayr, E. 1976. Evolution and the Diversity of Life. Harvard University Press, Cambridge, Mass.

Mayr, E. 1988. Toward a New Philosophy of Biology. Harvard University Press, Cambridge, Mass.

Mayr, E. 1991. One Long Argument. Harvard University Press, Cambridge, Mass.

Mayr, E. 1997. This is Biology: the Science of the Living World. Harvard University Press, Cambridge, Mass.

Schopf, T.J.M., ed. 1972. Models in Paleobiology. Freeman Cooper, San Francisco.

The National Academy honors Ernst Mayr

November 24, 2014 • 8:06 am

by Greg Mayer

Ernst Mayr (1904-2005) was one of the greatest biologists of the 20th century, an architect of the “Modern Synthesis” of evolutionary biology which harmonized Mendelism and Darwinism and showed that the Mayr NAS Biog Mem coverphenomena of paleontology, systematics, and genetics formed a mutually consistent and coherent whole. Mayr in particular identified and explicated the importance of the discontinuities in the diversity of life we identify by the name species, characterized the nature of species through the biological species concept, and forcefully argued for the importance of geographic isolation as a key ingredient in the origin of species. Although some of his greatest contributions were yet to come, he was elected to the National Academy of Sciences in 1954.

The Academy honors its members who have died in its Biographical Memoirs, and last week they released the Memoir for “Uncle Ernst” (as he was affectionately known to graduate students at the Museum of Comparative Zoology). The Memoir, by Walter J. Bock, perhaps Mayr’s most distinguished graduate student, was previously published in 2006 in the equivalent series of the Royal Society, the Biographical Memoirs of Fellows of the Royal Society. It consists of a short, fact-filled biography, highlighting both Mayr’s life and scientific contributions, a chronological list of his various professional appointments and numerous awards and honors, and a selected bibliography of his most important books and papers. The Academy has made the Memoir available as a free pdf (as has the Royal Society), and it serves as a nice introduction to Mayr and his work. (There is an amusing typo on p. 10, uncorrected from the Royal Society version: referring to Mayr’s dissatisfaction with certain aspects of his positions in New York, Bock writes “…he became more and more reckless in his situation in New York City”; “restless” is obviously intended– Mayr soon left for the Museum of Comparative Zoology at Harvard.)

The first full biography of Mayr (the manuscript of which was available to Bock) appeared in 2008: Ornithology, Evolution, and Philosophy: The Life and Science of Ernst Mayr 1904–2005, by Juergen Haffer. Haffer, who passed away in 2010, was a field geologist by profession, but also an accomplished avocational biologist, well known for his monograph on speciation in Amazonian birds. He was a friend of Mayr’s, and his biography includes much information provided by Mayr himself over many years of interviews and discussions. The biography is, in fact, more akin to a primary document, and will be a rich resource for future biographers.

Jerry has written two important papers on Mayr, one being Mayr’s obituary for Science. Written under a tight time deadline, I recall worrying with Jerry about getting certain details right: who did send Mayr on his momentous, life-changing expedition to the South Pacific in 1928? Looking back at our correspondence, I see that I suggested that Walter Bock would know, but there was no time to make inquiries. It turns out that what Jerry eventually wrote is about right, that the full answer is rather complex, and Walter Bock did know. And by reading the memoirs — and even more so Haffer’s book — everyone can know.

h/t Neil Shubin


Bock, W.J. 2006. Ernst Walter Mayr 5 July 1904 — 3 February 2005. Biographical Memoirs of Fellows of the Royal Society 52:167-187. pdf

Bock, W.J. 2014. Ernst W. Mayr 1904-2005. Biographical Memoirs of the National Academy of Sciences 29 pp. pdf

Coyne, J.A. 1994. Ernst Mayr and the origin of species. Evolution 51:19-30. pdf

Coyne, J.A. 2005. Ernst Mayr (1904-2005). Science 307:1212-1213.

Haffer, J, 1974. Avian speciation in tropical South America, with a systematic survey of the Toucans (Ramphastidae) and Jacamars (Galbulidae). Publications of the Nuttall Ornithological Club 14. Buteo

Haffer, J. 2008. Ornithology, Evolution, and Philosophy: The Life and Science of Ernst Mayr 1904–2005. Springer, Berlin. Amazon

The late Ernst Mayr speaks

October 13, 2010 • 10:19 am

by Greg Mayer

Ernst Mayr was one of the greatest biologists of the 20th century, an architect of the evolutionary synthesis, and chief exponent of the biological species concept and geographic speciation. Jerry and I both had the privilege of knowing him. I was fortunate to be able to attend his 100th birthday party in 2004 (at the end of which he gave a brief impromptu talk on his life and achievements, which he summarized by saying “I’ve had a wonderful life.”), and Jerry wrote what I regard as the canonical obituary in Science (not open access, unfortunately) after his death in 2005. A biography of Mayr (the first, but probably not the last) by Jurgen Haffer was published in 2007

I was greatly pleased, therefore, to run across, more or less accidentally, Web of Stories, a website which features videos of long interviews with interesting people, including Ernst Mayr. The Mayr interview, conducted in 1997 by Mayr’s student Walter Bock, a famous biologist in his own right, consists of 150 high quality video segments, each a few minutes long, with their content identified by subject matter.

Here’s a clip in which Mayr discusses the founding of the Society for the Study of Evolution and its journal, Evolution. (For a full scholarly account of this, see Betty Smocovitis’s paper on thesubject.) You’ll have to click the link to see it; I’ve tried very hard to embed the video, but the provided code doesn’t work.

In lieu of embedding the Web of Stories video, here’s another, shorter interview done in 2000.

Caturday felid

March 14, 2009 • 5:20 pm

by Greg Mayer

Until Jerry settles back in there’ll be a bit of overlap in our posting, so I’m providing this Caturday’s felid. Actually it’s two felids: the lion and the tiger (both of these links come from a wonderful page maintained by Virginia Hayssen of Smith College), both photographed today at the Racine Zoo in Wisconsin.

Two young lions at the Racine Zoo
Two young lions at the Racine Zoo

The tiger, unfortunately, sat back out of useful range of the camera I had with me, so I had to settle for this.

Tiger sign at Racine Zoo
Tiger sign at Racine Zoo

In captivity hybrids between lions and tigers, called ligers (male lion X tigress) and tigons (male tiger X lioness), can be produced, which are healthy and vigorous.  As Jerry explains in chapter 7 of WEIT, species are defined by their reproductive relationships: members of the same species will interbreed with one another, while members of different species are kept from successfully reproducing by one or more reproductive isolating barriers. Why, then, do we consider lions and tigers different species?

Most people think of lions as being from eastern and southern Africa, but within historic times lions ranged across north Africa and southeastern Europe through southwest Asia to northern India.  One population of Asiatic lions still survives, in the Gir Forest, closely protected by the Indian government.

Historic distribution of the lion in north Africa, Europe, and Asia
Historic distribution of the lion in north Africa, Europe, and Asia

Tigers were widespread in Asia, from the Caucasus to Siberia in the north and Java and Bali in the south. Until man began to decimate them, lions and tigers broadly overlapped in southern Asia, but remained distinct, without interbreeding. Thus, in nature, lions and tigers did not interbreed. And the full definition of a species, given by Ernst Mayr in 1940, is that species are groups of actually or potentially interbreeding populations in nature, reproductively isolated from other such groups.