Why Evolution is True is a blog written by Jerry Coyne, centered on evolution and biology but also dealing with diverse topics like politics, culture, and cats.
Our third installment of Teaching Evolution is a paper by George Gaylord Simpson, the most influential paleontological contributor to the Modern Synthesis, and one of its key figures. In this paper, Simpson discusses a wide variety of phenomena revealed in the fossil record– parallelism, mosaic evolution, convergence, adaptation, conservatism, variation of evolutionary rates over time, variation of evolutionary rates among taxa, and variation of evolutionary rates among characters, to name a few– as exemplified by a group of South American hoofed mammals, the Notoungulata. The paper analyses these subjects in the context of an earlier discussion of variation and evolution in wasps by Alfred C. Kinsey, in which Simpson finds much to admire:
Kinsey’s review of this subject is the most recent and in many respects the most complete, and it is based on a remarkably thorough and profound study, of an exceptionally large mass of data.
Kinsey was later a famed sex researcher, and few are aware that he originally made his name as an entomologist studying wasps. Besides Simpson, Theodosius Dobzhansky, Ernst Mayr, and Julian Huxley– all influential contributors to the Synthesis– cited Kinsey’s work approvingly. (And Mayr, as he told me himself years later in a conversation in which he related Kinsey’s interviewing methods, was one of Kinsey’s subjects for his first studies of human sexual behavior!)
George Gaylord Simpson (1902-1984) was an American paleomammalogist and one of the crowning figures of the Modern Synthesis. In Tempo and Mode in Evolution (1944), Simpson showed that the patterns and rates of evolution and variation revealed in the fossil record are consistent with the mechanisms of inheritance and evolution that had been elucidated by geneticists and systematists in studies of extant taxa. In particular, Simpson argued that the distinction between microevolution and macroevolution was purely one of scale, and not one of evolutionary process. Educated at the University of Colorado and Yale, he spent his career at the American Museum of Natural History, the Museum of Comparative Zoology, and the University of Arizona. His books include The Meaning of Evolution (1949, revised 1967), The Major Features of Evolution (1953, a major reworking and updating of the themes of Tempo and Mode), Principles of Animal Taxonomy (1961), This View of Life (1964, a collection of popular articles) and The Geography of Evolution: Collected Essays (1965). His life and work are treated in his autobiography Concession to the Improbable (1978) and Leo Laporte’s George Gaylord Simpson: Paleontologist and Evolutionist (2000).
Reading:
Simpson, G. G. 1937. Supra-specific variation in nature and in classification from the view-point of paleontology. American Naturalist 71 (734):236-267. (This link will allow you to read it online with a JSTOR account (which is free to anyone).)
Study Questions:
1. Simpson argues that evolutionary rates vary within the Notoungulata. What evidence does he use?
2. To what other group of mammals do the notohippids show parallel evolution? Why does Simpson think the characters undergoing parallel evolution are adaptive?
3.What does the variability of Henricosbornia reveal about the relationship between infraspecific and supraspecific variation?
4. What do the terms “habitus” and “heritage” mean? What does Simpson do with these concepts?
[The other installments of Teaching Evolution can be found by clicking ‘MOOC’ under “filed under” or “tags” just below.]
While in Colombia last week, Jerry directed my attention to a paper by Roy Britten (abstract only free) in that week’s issue of the Proceedings of the National Academy of Sciences. Britten is a venerable figure in evolutionary molecular genetics, one of the pioneers of DNA-DNA hybridization who helped elucidate the structure of the genome long before sequencing was possible. The paper was indeed interesting. This post is a bit longer and more data-and-analysis-laden than usual, but I think the paper merits discussion.
Britten summarizes his latest paper’s conclusions succinctly:
The aim of this paper is an explanation for the high speed of evolution of the human lineage, which has been exceptional compared with other animals. The high speed of evolution of human lineage brain size is recognized by comparison of fossil brain sizes (1, 2). Evolution of the lineage leading to humans during the last several million years was striking. … A major source of variation [for brain evolution] has been the insertion of transposable elements (TEs).
He goes on to note that besides rapid brain evolution, humans have many TE insertions. For him
This is an extraordinary correlation. Human evolution has been rapid, particularly brain evolution in the last several million years. It is the only species known to make such rapid evolutionary progress. Now it is shown that human is the only species studied to have so many TE insertions. Recognition of this correlation leads to the concept that Alu insertions underlie rapid human evolution.
Importantly, he states up front that:
We believe the brain evolution was due to natural selection and genomic variation.
He is thus not seconding Colin Blakemore’s unwarranted claim that brain size is a neutral character, conferring neither selective advantage nor disadvantage, that must therefore have evolved via genetic drift. Britten is definitely not saying this, and is thus not open to the criticisms of Blakemore made by Jerry (here and here) and John Hawks. Britten thinks a big brain is advantageous.
So, how well supported are the claims he does make? I’d first note that one data point does not a correlation make, especially not an extraordinary one. TE’s, by increasing mutation rates, can certainly increase evolutionary rates, but all sorts of other singular characteristics apply to the Homo lineage. In addition to having many TE’s, they were savannah-dwelling, nearly hairless, bipedal, etc. Which of these correlates with rapid brain growth is the important one? I don’t know, but a more forceful argument than simple occurrence in the same lineage would be needed to establish which is the most likely causal factor.
And what about the factual points? Much of the paper is devoted to establishing the prevalence of TE’s in the human lineage, and I, as at least a first approximation, would yield to Britten’s expertise on this point. What about the rate of brain size evolution? (To be fair, Britten takes his cue here from the literature; the high rate is a premise, not a conclusion.) G.G. Simpson, one of the founders of modern evolutionary biology, spent a major part of his career documenting the variability of evolutionary rates. He showed that there is great variability of evolutionary rates between lineages, among characters within lineages, and within lineages at different times. The following figure is based on an original in Simpson’s 1953 Major Features of Evolution. It shows that the rate of evolution in lungfish was high about 300 million years ago, but not so much at other times (i.e. variation within lineages at different times).
Rate of lungfish evolution, from Mark Ridley's Evolution, 2004.
Historically, claims of human exceptionality have tended to become less exceptional when examined more closely. Huxley’s debate with Owen over the brain is perhaps the classic example: contra Owen, Huxley “showed that the brains of apes and humans were fundamentally similar in every anatomical detail.” Knowing this, I decided to check on this important premise of Britten’s paper. Is the speed of evolution of the human brain “striking”, and “exceptional compared with other animals”? In a word, no: over the last several million years, human brain size has evolved at rates which are typical of paleontologically measured evolutionary rates.
Here’s a graph by John Hawks (using the same or similar data as Lee and Wolpoff, 2003) showing the pattern of change in cranial capacity (on the vertical axis, in cubic centimeters) over the last 2 million years or so.
There are a number of ways of measuring evolutionary rates of morphological features such as cranial capacity. One useful measure is the haldane, developed by Phil Gingerich (1993) of the University of Michigan Museum of Paleontology, based on a suggestion made in 1949 by the original most interesting man in the world, the great geneticist-physiologist-soldier-pacifist-communist-Hindu-atheist-patriot-expatriate J.B.S. Haldane, one of the founders of modern evolutionary theory. The haldane is the change, on a logarithmic scale, of the feature in question in units of the standard deviation (a measure of how variable the feature is), per generation.
Using data from three papers on modern human cranial capacity, I found the average to be 1345 cubic centimeters (cc), calculated as the unweighted average of males and females from the measured populations from Korea, Turkey, and Nigeria (a total sample of 1151). 1.8 million years ago, the cranial capacity of the Homo lineage was 702 cc, calculated as the average of skulls of that age from Perning, Kenya (one each, given by Lee and Wolpoff, 2003), and Dmanisi (three skulls, given by Gabunia et al., 2000, for two of them, and the median of two estimates by Lee, 2005, for the third).
The logarithmic standard deviation is well approximated by the coefficient of variation (CV: the untransformed standard deviation divided by the mean; Lewontin, 1966). The unweighted average CV for the modern humans was .0955, and for the five early Homo it was .1149; averaging, we get .1052.
So, the amount by which cranial capacity has changed on the log scale is ln(1345)-ln(702)=.6502; dividing this by the estimated logarithmic standard deviation, .1052, gives 6.181. In the last 1.8 million years, our cranial capacity has increased about 6 standard deviations. We don’t know generation time for early Homo, but we do know it for modern humans (about 25-30 years) and chimps (19-24 years; Matsumura and Forster, 2008). Using 25 years as an estimate for the whole lineage, we get 72000 generations in the last 1.8 million years, giving an evolutionary rate for brain size of .00008584 (or 10^-4.066) haldanes.
Is this a high rate or a low rate? Neither– it’s absolutely typical for evolutionary rates measured over this generational time scale. Gingerich (2001) compiled a large data set on rates of evolution, measured in haldanes, over a wide variety of time scales. He states:
Macroevolutionary studies yield rates on the order of 10^-2–10^-6 haldanes calculated over intervals of geological time ranging from 10^2–10^6 generations.
If we look more precisely, at about 72000 (10^4.86) generations, we find measured rates of about 10^-3.5 to 10^-6.5. So the rate of human brain evolution is above the median, but nothing “exceptional”. Could this unexceptional result be due to the particular initial time (1.8 mya) selected? What if we looked further back in time? I redid the analysis using an average chimpanzee cranial capacity of 383 cc (McKee et al., 2005), and a divergence time of 6 million years. Using the same logarithmic standard deviation and generation time, we get [ln(1345)-ln(383)]/.1052 = 11.940 standard deviations, nearly twice the 6 standard deviation change in the last 1.8 million years. Dividing by the 6 million years/25 years per generation = 240000 generations gives .00004975 (or 10^-4.3) haldanes. Again, above the median, but nothing exceptional.
So, there’s nothing much remarkable about the speed of human brain size evolution. If the TE’s (or hairlessness or bipedalism or whatever) of the Homo lineage had an effect on human evolution, it was not expressed as an unparalleled increase in the rate of evolution of cranial capacity.
Acer, N., M. Usanmaz, U. Tugay, and T. Ertekin. 2007. Estimation of cranial capacity in 17-26 years old university students. Int. J. Morphol. 25:65-70. pdf
Britten, R.J. 2010. Transposable element insertions have strongly affected human evolution. Proceedings of the National Academy of Science 107:19945-19948.
Gabunia, L., et al. 2000. Earliest Pleistocene hominid cranial remains from Dmanisi, Republic of Georgia: taxonomy, geological setting, and age. Science 288:1019–1025.
Gingerich, P. D. 1993. Quantification and comparison of evolutionary rates. American Journal of Science 293A: 453-478. pdf
Gingerich, P. D. 2001. Rates of evolution on the time scale of the evolutionary process. Genetica 112-113: 127-144. pdf
Haldane, J.B.S. 1949. Suggestions as to quantitative measurement of rates of evolution. Evolution 3:51-56.
Hwang, I.-L., et al. 1995. Study on the adult Korean cranial capacity. Journal of Korean Medical Science 10:239-242. pdf
Matsumura, S. and P. Forster. 2008. Generation time and effective population size in Polar Eskimos. Proc. R. Soc. B (2008) 275:1501–1508. pdf
McKee, J.K., F.E. Poirier, and W.S. McGraw. 2005. Understanding Human Evolution. Pearson, Upper Saddle River, New Jersey.
Lee, S.-H. 2005. Is variation in the cranial capacity of the Dmanisi sample too high to be from a single species? American Journal of Physical Anthropology 127:263–266. pdf
Lee, S.-H. and M.H. Wolpoff. 2003. The pattern of evolution in Pleistocene human brain size. Paleobiology 29:186-196. pdf
Lewontin, R.C. 1966. On the measurement of relative variability. Systematic Zoology 15:141-142.
Odokuma, E.I., P.S. Igbigbi, F.C. Akpuaka, and U. Esigbenu. 2010. Craniometric patterns of three Nigerian ethnic groups. African Journal of Biotechnology 9:1510-1513. pdf
Simpson, G.G. 1953. Major Features of Evolution. Columbia University Press, New York.
George Gaylord Simpson (1902-1984) was an American paleomammalogist and one of the crowning figures of the Modern Synthesis. In Tempo and Mode in Evolution (1944), Simpson showed that the patterns and rates of evolution and variation revealed in the fossil record are consistent with the mechanisms of inheritance and evolution that had been elucidated by geneticists and systematists in studies of extant taxa. In particular, Simpson argued that the distinction between microevolution and macroevolution was purely one of scale, and not one of evolutionary process. Educated at the University of Colorado and Yale, he spent his career at the American Museum of Natural History, the Museum of Comparative Zoology, and the University of Arizona. His books include The Meaning of Evolution (1949, revised 1967), The Major Features of Evolution (1953, a major reworking and updating of the themes of Tempo and Mode), Principles of Animal Taxonomy (1961), This View of Life (1964, a collection of popular articles) and The Geography of Evolution: Collected Essays (1965). His life and work are treated in his autobiography Concession to the Improbable (1978) and Leo Laporte’s George Gaylord Simpson: Paleontologist and Evolutionist (2000).
