In its effort to debunk natural selection, Explore Evolution reiterates the debunked claims from the creationist book Icons of Evolution. That book claimed that textbooks misrepresented evolution by incorrectly characterizing certain popular experiments. Explore Evolution repeats the earlier book's arguments, reuses several of that books images without change (or attribution), and does not update its arguments to reflect more recent research.
The most fundamental error here is the claim that research on peppered moths and work on the Galápagos finches are the only, or at least major, examples offered for natural selection in textbooks. Those examples are frequently cited, but modern textbooks cite many other examples to show how natural selection works. Nor do modern textbooks cite those bodies of research for the purposes claimed in Explore Evolution. The treatment of natural selection in this book focuses exclusively on whether natural selection can generate biological novelty. That is an interesting topic, but not the only relevant topic for students to learn about natural selection, and Explore Evolution does students a disservice by treating an important and multi-faceted topic like natural selection through such a limited lens.
Turning to the details of the critiques offered for the peppered moth work and the Galápagos finch research, one finds that Explore Evolution describes that research inaccurately, and ignores recent work which directly contradicts the book's claims. For instance, it presents a graph of finch evolution which bears no relationship at all with any measurements reported by any researchers in the field, and criticizes the 50 year old work of Bernard Kettlewell on peppered moths without any discussion of research from the 1990s which tested several of the authors' criticisms of Kettlewell, and found that Kettlewell's results were unaltered by those criticisms.
Explore Evolution claims:
Biology textbooks cite two classic examples to support the claim that natural selection can produce small-scale change over a short time.Explore Evolution, p. 88
Campbell and Reece's Biology (6th edition) has a section in the chapter on evolution entitled, "Examples of natural selection provide evidence of evolution." It begins:
Natural selection and the adaptive evolution it causes are observable phenomena. As described in the interview at the beginning of this unit, Peter and Rosemary Grant of Princeton University are documenting natural selection and evolution in populations of finches in the Galápagos [Darwin's finches]. We will now look at two additional examples of natural selection as a pervasive mechanism of evolution in populations.Neil A. Campbell and Jane B. Reece, Biology, 6th ed.
Those examples include the evolution of HIV and insects in response to drugs and insecticides, yet neither HIV nor insecticide even rates a mention in the index of Explore Evolution. In Chapter 9, Explore Evolution addresses antibiotic resistance, but only to discuss the origins of mutations conferring resistance, not to point out that natural selection is what causes that resistance to spread.
Sickle cell anemia also makes an appearance in Chapter 9, again purely as an example of a mutation. In Raven and Johnson's Biology (5th edition), however, it is the first example of natural selection described in the section entitled "Natural selection explains adaptive microevolution." Explore Evolution mentions that sickle cell anemia can be beneficial under some circumstances, but misses the chance to either discuss how natural selection makes it more common in human populations traditionally occupying malarial areas, or to employ a truly inquiry based approach by inviting students to develop and test hypotheses about malarial resistance in order to actually explore evolution.
Earlier, Raven and Johnson discuss examples of natural selection including its ability to maintain persistent latitudinal gradients in the oxygen-carrying hemoglobin molecule in ocean fish, differences which make northern fish more efficient in cold water and southern fish more efficient in warmer water. They also describe how selection improves the camouflage of butterfly caterpillars and allows snail populations to adapt to different local ground coloration, as well as pesticide resistance in tobacco budworms and agricultural weeds. Only later do they discuss industrial melanism in peppered moths or the beaks of Darwin's finches.
That introductory textbook authors tend to focus on a few common examples does not detract from the fact that natural selection is commonplace, easy to observe, and widely documented. Specialized textbooks on evolutionary biology present an even wider array of examples of natural selection. For instance, the chapter on natural selection in Futuyma's Evolutionary Biology discusses how selection produces a north-south gradient in the frequency of alleles of a certain gene in Drosophila flies, a pattern repeated on multiple continents. Similar patterns exist for field crickets. Later, Futuyma describes how guppies in streams without predators have brighter coloration than closely related guppies in streams with predators. In the example of snail shells also used by Raven and Johnson, Futuyma points to paleontological studies showing that the genetic polymorphism seen in the population today has persisted for thousands or millions of years — clear evidence of stabilizing selection — and studies of broken snail shells allow an evaluation of rates of predation on various color morphs — allowing an assessment of the selective pressures acting on the population.
The emphasis on two examples of natural selection, and the complete disregard for the myriad other examples in active use by introductory and advanced textbooks, reflects a common creationist strategy. Jonathan Wells, a creationist author at the Discovery Institute, has made a career of attacking the Galápagos finches and the peppered moth, perhaps in the belief that all of the other examples of natural selection would go away if he could disprove one or two well-known examples.
It is noteworthy that several figures in this chapter are drawn from Jonathan Wells' Icons of Evolution, a creationist work aimed at critiquing the content of common biology textbooks and common examples used to illustrate and explain evolutionary processes. While Wells is not credited in this chapter, many of the arguments are the same as in his earlier work (critiqued by NCSE's Alan Gishlick), as are the illustrations. Explore Evolution repeats many of the errors previously identified in Wells' work. Just as the authors of Explore Evolution have a well-documented religious agenda which belies the scientific appearance of their book, Wells is famous for his religious reasons for obtaining a PhD in biology and attacking evolution, rooted in his involvement with the Unification Church (better known as "Moonies"), led by Sun Myung Moon, or, as Wells refers to him, "Father":
I asked God what He wanted me to do with my life, and the answer came not only through my prayers, but also through Father's many talks to us, and through my studies.…
He also spoke out against the evils in the world; among them, he frequently criticized Darwin's theory that living things originated without God's purposeful, creative activity.…
Father's words, my studies, and my prayers convinced me that I should devote my life to destroying Darwinism, just as many of my fellow Unificationists had already devoted their lives to destroying Marxism. When Father chose me (along with about a dozen other seminary graduates) to enter a Ph.D. program in 1978, I welcomed the opportunity to prepare myself for battle.
Explore Evolution, like Wells' earlier work, is rooted in a religious aversion to evolution, not in actual science. Scientists have sought to correct the erroneous claims displayed in Explore Evolution in their earlier incarnations, and the refusal to accept those corrections, or even to acknowledge those criticisms, recommends strongly against adopting this work into science classes.
…after the rains returned [to the Galápagos], the Grants notices that several separate species of finches were interbreeding. No only were no new species springing forth, but existing varieties actually seemed to be merging. Critics therefore conclude that the finch beak example of microevolution actually suggests that biological change has limits.Explore Evolution , p. 93
Given that this passage follows right after their complaints about the dangers of extrapolation, it is rich to find them claiming that anything that didn't happen in a 30 year study could never happen. The sole basis for the claim that limits exists seems to be that something did not occur in the course of a single study. By that standard, I could predict that I will never die, since I am thirty and have not been observed to die.
Furthermore, the citation the authors use to explain that hybridization occurred points out that genetic novelty accompanies such unusual matings, and explains why those matings do not mean that the species are merging. The ongoing changes in beak shape (shown in part F of the figure above) cannot be explained by natural selection alone. They can only be explained by invoking the combination of two of the 4 major evolutionary mechanisms: natural selection and gene flow. Natural selection explains the initial changes, but the flow of genes between species provided the ongoing evolutionary pressure towards blunter beaks. Peter and Rosemary Grant explain:
The proportionally greater gene flow from G. fortis to G. scandens than vice versa has an ecological explanation. Adult sex ratios of G. scandens became male biased after [the extremely hot and wet] 1983 as a result of heavy mortality of the socially subordinate females. High mortality was caused by the decline of their principal dry-season food, Opuntia cactus seeds and flowers; rampantly growing vines smothered the bushes. G. fortis, more dependent on small seeds of several other plant species, retained a sex ratio close to 1:1. Thus, when breeding resumed in 1987 after 2 years of drought, competition among females for mates was greater in G. fortis than in G. scandens. All 23 G. scandens females paired with G. scandens males, but two of 115 G. fortis females paired interspecifically. All their F1 offspring later bred with G. scandens because choice of mates is largely determined by a sexual imprinting-like process on paternal song.Grant, P. R. and B. R. Grant (2002), "Unpredictable evolution in a 30-year study of Darwin's finches." Science 296:707-711
There are two important things to understand about that. First, that the hybridization was a result of unusual environmental conditions and an excess in the number of males of one species. Those males competed for access to any female at all, and were prepared to overcome pre-mating barriers to hybridization. Second, the main force limiting hybridization is that different species of finches select mates with songs similar to those of their fathers. Elsewhere, the Grants explain:
Hybridization occurs sometimes as a result of miscopying of song by a male; a female pairs with a heterospecific male that sings the same song as that sung by her misimprinted father. On Daphne Major island, hybrid females bred with males that sang the same species song as their fathers. All G. fortis × G. scandens F1 hybrid females whose fathers sang a G. fortis song paired with G. fortis males, whereas all those whose fathers sang a G. scandens song paired with G. scandens males. Offspring of the two hybrid groups (the backcrosses) paired within their own song groups as well. The same consistency was shown by the G. fortis × G. fuliginosa F1 hybrid females and all their daughters, which backcrossed to G. fortis. Thus mating of females was strictly along the lines of paternal song.Peter R. Grant and B. Rosemary Grant (1997). "Genetics and the origin of bird species," Proceedings of the National Academy of Sciences, 94:7768–7775
The Grants go on to discuss how this helps explain the speciation of Galápagos finches.
Experiments show that birds are less responsive to the songs of conspecifics from different islands than to songs from their own island. Even though the changes are small, the process of cultural drift is enough to begin isolating these populations. In addition, the finches show a preference for the morphology of birds from their own island to members of the same species from other islands, even independent of song differences. The forces driving natural selection on different islands will differ, and that will produce morphological differences, which will combine with differences in songs to make hybridization less likely.
Thus, the observation of hybridization does not provide evidence that two species will merge into one. Instead, it helps test the process by which species separate. Furthermore, the hybridization observed had an effect which directly contradicts the claim that novelty cannot originate through evolutionary processes. Because of genes flowing in from another species, G. scandens experienced substantial evolutionary change, and acquired novel traits.
The Grants point out that this sort of gene exchange is important to understanding speciation and evolution in general:
Introgressive hybridization [as seen in Darwin's finches] has the potential of leading to further evolutionary change as a result of enhancing genetic variances, in some cases lowering genetic covariances), introducing new alleles, and creating new combinations of alleles, some of which might be favored by natural selection or sexual selection. Svärdson believed that introgression in coregonid fishes has replaced mutation as the major source of evolutionary novelty. Introgression and mutation are not independent; introgressive hybridization may elevate mutation rates.
The relevance to speciation lies in the fact that regions of introgression are peripheral areas, which could become isolated from the main range of the species through a change in climate and habitat: they are potential sites of speciation.Grant, P. R. and B. R. Grant (1997)
Far from demonstrating limits to the power of evolution, the rare hybridizations between species demonstrate how strong pre-mating isolation is, illustrate an important source of variation, and provide evidence about the process of speciation and the origins of genetic novelty. The fact that Explore Evolution never mentions two of the four major mechanisms of evolution (gene flow and genetic drift), speaks poorly of the authors' commitment to a serious examination of evolutionary biology.
Critics question whether the peppered moth story shows that microevolution can eventually produce large-scale change. They point out that nothing new emerged.Explore Evolution, p. 93, emphasis original
Textbooks which present this example typically use it to illustrate the process by which biologists investigate natural selection, not to demonstrate the origins of biological novelty. In Raven and Johnson's Biology (5th edition), the discussion of peppered moths and industrial melanism is in a section titled "Natural selection explains adaptive microevolution," and never claims that the example illustrates anything other than the process by which scientists have investigated the effects of natural selection. Ridley's Evolution (2nd ed.) discusses peppered moths first in a section explaining how "Natural selection operates if some conditions are met," and later in a chapter entitled "The Theory of Natural Selection," in a section discussing how "the model of selection can be applied to the peppered moth." Ridley first demonstrates the reasons why natural selection was invoked by observers of a pattern, and then proceeds to describe the particular ways in which researchers investigated the hypothesis: determining the heritability of coloration, experimenting to determine the fitnesses of various genotypes under different conditions, and concluding with a discussion of ways in which "the details of the story are now known to be more complex."
As Ridley explains:
In conclusion, the industrial melanism of the peppered moth is a classic example of natural selection, and illustrates the one-locus, two-allele model of selection. The model can be used to make a rough estimate of the difference in fitness between the two forms of moth using their frequencies at different times; the fitnesses can also be estimated from mark-recapture experiments. However, the one-locus, two-allele model is only an approximation to reality. In fact, several alleles are present (and their dominance relations are not simple); selection is not simply a matter of bird predation in relation to camouflage; and it seems that migration, as well as selection, is needed to explain the geographic pattern of gene frequencies.Mark Ridley (1996), Evolution, 2nd ed., p. 109
Explore Evolution claims that "the experiment [does] not show what the story says it's supposed to," but misrepresents what scientists claim it illustrates. It is not an experiment meant to illustrate speciation, and Explore Evolution does not discuss those experiments, such as the examples in Drosophila discussed by Ridley in his chapter on speciation.
In fact, Explore Evolution does not even discuss the process by which melanism would have originated in peppered moths. The genetics of melanism have been well understood since the 1960s, when researchers showed how several different mutations to the same genes could produce similar sorts of melanism (Lees, David R., 1968, "Genetic Control of the Melanic Form Insularia of the Peppered Moth Biston betularia (L.)," Nature 220(5173):1249-1250). Natural selection is the process by which those mutations increased in frequency over several generations, exactly what scientists and textbook authors claim this example demonstrates.
In these experiments the moths were placed onto trunks and branches at dawn, not day time, and allowed to take up their own resting places, as described in Kettlewell's 1958 paper "The importance of the micro-environment to evolutionary trends in the Lepidoptera" (Entomologist, 91:214-224). This is exactly what the moths do naturally. In one experiment, as a control, Kettlewell released moths earlier, and allowed them to fly on to the trees themselves. The recapture patterns from this experiment were no different from the recapture patterns with the moths placed on branches and trunks (Kettlewell, 1956, "Further experiments on industrial melanism in Lepidoptera" Heredity, 10: 287-301).
As well, some of the moths that were released in the mark-recapture-experiments stayed out for two nights before being captured. That is, they had been flying free at night and had found their own location during the morning. The distribution of those moths that did freely choose their own resting places is no different from those that were placed on trunks and branches (as shown in Kettlewell, 1956, and in his 1955 paper "Selection experiments on industrial melanism in Lepidoptera," (Heredity, 9: 323-342).
While there were legitimate reasons why scientists did criticize Kettlewell’s experiments (including Bruce Grant's 1999 paper "Fine tuning the peppered moth paradigm," Evolution 53. 980-984 and Michael Majerus's 1998 Melanism: evolution in action, Oxford University Press, Oxford, chapters 5 and 6), none of these criticisms (density and resting place choice) involve the moths being sleepy or sluggish, and no serious experimenter suggested that Kettlewell’s results were invalid. Indeed, subsequent experiments to test these criticisms broadly confirmed Kettlewell’s results (again, see Grant, 1999, Majerus 1998, and Majerus' 2007 talk "The Peppered Moth: The Proof of Darwinian Evolution," given at the ESAB meeting in Uppsala on 23 August – also available as Powerpoint, as well as his 2009 paper "Industrial melanism in the peppered moth, Biston betularia: an excellent teaching example of Darwinian evolution in action," Evolution: Education and Outreach 2(1):63-74). Further details of these experiments are discussed in "Where Peppered Moths Rest," below.
Kettlewell was aware that peppered moths rested on both trunks and branches. In Kettlewell’s experiments, he actually placed the moths on trunks and branches, in relatively unexposed locations, thus covering the natural resting places of the peppered moth.
In a comprehensive study of peppered moth resting places in the wild, fully 25% of moths were found resting on trunks (Majerus, 1998, cited above). Of the remainder, roughly 25% were found on branches, and 50% at branch/trunk junctions. Furthermore, in the branch/trunk junction category, the moths are actually resting on the trunks, 2-3 inches below the branch. In a later, extensive 6 year study 37% of peppered moths were found on trunks (Majerus, 2007, cited above).
It is important to note that Kettlewell performed several different experiments; direct observations, filmed observations of birds taking moths from exposed trunks, indirect observations of moth predation where moths were released onto relatively unexposed trunks and branches and allowed to chose their resting places, and mark-recapture experiments, where again moths were released onto relatively unexposed trunks and branches to choose their own resting places (Kettlewell, 1955, 1956, both cited above). So when Kettlewell put his moths on trunks and branches (Kettlewell, 1955, 1956), he was placing them where the majority of all moths rest naturally, as far as we can tell (even more if we count the trunk-resting moths at the trunk/branch junctions).
Michael Majerus has repeated Kettlewell’s experiments using moths resting on the undersides of branches (Majerus 1998, 2007). In both cases, differential predation was found that confirmed Kettlewell’s original observations. Furthermore, in Majerus’s 6-year experiment, measured predation intensity at the experimental sites predicted the population frequencies of moths found in the wild (Majerus, 2007).
While the authors of Explore Evolution could not have been expected to have had access to Majerus’s 2007 results, Majerus’s 1998 results, as well as Kettlewell’s description of the original experiments (Kettlewell, 1955, 1956) alone are enough to show that Explore Evolution is completely wrong on this point.
Since Kettlewell's original experiments were published, they have been independently replicated at least 6 times (See for example Grant 1999, Majerus 1998, and Majerus 2007, all cited above, for reviews). All of these experiments have addressed one or more criticisms of the original study, and all have broadly confirmed Kettlewell's experiments. Thus we can say that Kettlewell's experiments have stood the test of time.
An inquiry-based book could have used this history of successive investigations to explore the practice of science as a self-correcting enterprise, and the importance of replicability to the scientific process. Students could have been asked to devise their own experiments based on criticisms of Kettlewell's early work, and then teachers could reveal data from experiments like those performed by Majerus to evaluate the results of those new experiments. Instead, students are presented with erroneous critiques of Kettlewell's work, given none of the more recent vindicating evidence, and instructed to believe that this flawed exploration demonstrates a weakness in natural selection. In fact, it reflects only the weaknesses of Explore Evolution, and of its authors' approach to evolution and science in general.