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Come to Me, My Melancholic Baby!

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Come to Me, My Melancholic Baby! Evolutionary Anthropology 3 CROTCHETS & QUIDDITIES Come to Me, My Melancholic Baby! KENNETH WEISS Some famous textbook cases of evolution aren’t unambiguously true. Does it CATCHING EVOLUTION matter? IN THE ACT Of Birds and Bugs here’s not much point in writ- the comparison was wrong. He said ing a column about things we the horse was bigger. Of course, we Come to me, my melancholy baby, Talready know, but there is value have to presume that he himself had Cuddle up and like your hue ... in looking at things we thought we actually seen a fox terrier. All your fears are foolish fancy, already knew. One way to do this is to As so often happens, something maybe see how textbooks illustrate basic similar came up coincidentally in an You know dear, that I’ll be tree to principles. I was led to write this col- unrelated conversation, and is closer you. umn when my daughter took an intro- to anthropology. In 1940 the promi- ductory bioanthropology course this nent geneticist A. H. Sturtevant sug- Every smog may have a silver lining, year and asked me something about gested that tongue-rolling ability illus- But until the sun shines through, natural selection. Her class had been trated the genetic control of human Smile, my honey dear, discussing the example of industrial traits, and we’ve used it as a classroom While I kiss away each tear, melanism and the peppered moth. I example ever since (are you TT or tt?). ‘Cause I’ll be staying melancholic found that curious, because I thought It costs nothing to test in students, too!’ I had seen several years ago that that and if you assume a dominant gene in example had been undermined by Hardy-Weinberg equilibrium you can contradictory evidence from color always estimate the allele frequency. A quick check of the web confirmed changes in the same moths in North However, in 1965 Sturtevant retracted my recollection that numerous prob- America. How could it still be in text- his suggestion that the trait was ge- lems had been raised about industrial books? netic, saying “I am still embarrassed melanism as an example of natural This brought to mind Stephen Jay to see it listed in some current works selection observed in action (and Gould’s making fun in Natural History as an established Mendelian case.” touted by creationists as disproving of the classic description of the early Tongue rolling has still not been com- the evolutionary edifice) (see Majerus horse Eohippus (or Hyracotherium)as pletely displaced as a classroom ex- and B. S. Grant references). What are being the size of a fox terrier (Figure ample of Mendelian inheritance, but these issues? 1). This comparison was first used in if it is genetic at all, it’s neither a sim- In 1896 J. W. Tutt suggested protec- 1904 by the paleontologist H. F. Os- ple single-locus trait nor even a single tive coloration against bird predation born in reconstructing horse evolu- biological trait, and may in fact be as an explanation of the rapid recent tion. The simile was then copied from something most people can acquire rise in the frequency of melanic forms textbook to textbook by authors—and during a critical developmental pe- of the peppered moth Biston betularia for readers—who, like me, probably riod. in Britain. The selective force was pre- had little idea even what a fox terrier These whimsical examples of text- dation facilitated by industrial air pol- looked like. And according to Gould, book echoing illustrate Lewis Carroll’s lution. Most moths had been of the promise in The Hunting of the Snark light-colored typica form with black that “what I tell you three times is speckles, and enjoyed protective col- true.” But this may not always be triv- oration on lichen-covered trees where Ken Weiss is Evan Pugh Professor of An- ial, so it is worth looking at cases in they were hard for birds to see, be- thropology and Genetics at Penn Univer- sity. which questions have been raised cause they dramatically resemble the about the two classic examples used mottled pattern of lichens (Figure 2). to buttress core concepts of evolution- But soot from the industrial revolu- Evolutionary Anthropology 12:3–6 (2003) ary biology, which could have serious tion blackened the trees in the 19th DOI 10.1002/evan.10040 Published online in Wiley InterScience practical consequences if the exam- century, killed the lichens, and de- (www.interscience.wiley.com). ples are wrong. prived the moths of their protective 4 Evolutionary Anthropology CROTCHETS & QUIDDITIES is also evidence of differential mortal- ity in the expected direction, but of pre-adult moths, which cannot be at- tributed to protective coloration. Fi- nally, I was able to confirm my recol- lection that there was a parallel change in color variants in North American moths, and parallel pat- terns of industrialization, but lichens were not involved here and the change may be continuing in regions that do not share comparable pollution differ- ences. It is disturbing that so many aspects of what seemed such a classically sim- ple exemplar of evolution could be cred- Figure 1. Horse and Hounds? (Not to scale.) Courtesy www.nature.ca/nature_e.cfm and ibly questioned. However, it appears www.akc.org. that the problems, though serious, do not entirely invalidate the story. If soot and lichens were not involved in the cover. This favored a formerly rare (and hence most exposed) places on color pattern changes in North Amer- melanic variant whose peppering the tree trunks; normally these moths ica, there was a correlation here, as in black spots were so large that the hide in the branches, and are active Britain, with air pollutants like SO2 that affect reflectance on trees. That’s not wings were essentially all black, con- only at night when birds aren’t. Ket- the same as mimicking lichens, but it is ferring reversed protective coloration tlewell stressed the role of lichens be- a plausible source of protective colora- against the newly blackened trees. cause they appeared to provide an ex- tion. Although several studies showing The carbonaria form rose rapidly in ceptionally cryptic background for the differential predation by birds of con- frequency. As pollution waned after peppered form, but there has been de- spicuous moths had various design Clean Air Acts were passed in the 20th bate about whether moths preferen- century, the variant frequencies re- problems, their findings have been con- tially seek lichens that match their sistently in the expected direction. It is verted towards the lighter peppered own color, which would obviate pre- form. probably true that protective coloration dation based on color. Even the cor- This selective hypothesis was exam- is at most one of several factors that will relation of morph frequency with the ined theoretically by the leading pop- explain this story. For example, the fact presence of lichens is imperfect in ulation geneticist J. B. S. Haldane and that color changes in two continents Britain, so geographic anomalies be- others in the 1920s. In the 1950s Ber- have paralleled the history of air pollu- tween pollution and melanic fre- nard Kettlewell undertook a series of tion is compelling, but we have to be- famous experiments to demonstrate it quency have had to be explained after ware that we not fall into the common directly. Among other tests, he placed the fact by invoking gene flow. There trap of equating correlation with causa- moths on trees and observed cryptic to non-cryptic predation rates by birds. This became, and remains, the classic direct demonstration of evolu- tion by natural selection. However, many aspects of this work have been questioned. I mention some of them to illustrate the problems. To the human eye the moths seemed to enjoy cryptic protection, but evidence suggests that birds can see into the UV light spectrum, which means that protective coloration may be less effective to them than it is to the scientists suggesting the hypothe- sis. Kettlewell performed his experi- ments under various artificial condi- tions. He used unnatural moth densities in his experiments, and he released the moths in daylight, when Figure 2. Liking lichens? Kettlewall’s Biston betularia. Lichen covered tree with dark and light they quickly landed on the nearest (lower left) variants. (From Kettlewell, 1956.) CROTCHETS & QUIDDITIES Evolutionary Anthropology 5 time their study began. Strong selec- tion in times of drought also did not exhaust variation, as was shown by the birds’ ability to respond when rains returned. Overall, the phenotype changes or states over these 30 years were not predictable from the changes that could have been observed in a single season or short time span. For exam- ple, data from the first 5 years could not have been used in population ge- netics models to predict the 30-year results. Selection certainly occurs, and if your beak is too small you can’t eat big tough seeds and won’t do well Figure 3. Darwin’s finches. (From Darwin’s The Voyage of the Beagle, 1845.) if there aren’t any small seeds around. The Grants’ major point was a plea for more long-term observational studies, tion. Air pollution has been associated Rosemary Grant have observed the because typical small, short-term ob- with many things, and we may not Galapagos finches in detail. During servational studies may be insufficient know them all. this time there have been changes in to understand how evolution is actu- Above all, however, the changes in climate, including droughts that af- ally working.
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