The authors' misunderstanding of basic concepts is particularly obvious in their presentation of convergence. They treat the similarity of the bones musculature, nerves and development of hands in humans and moles as if it were no different than the gross similarity in the outlines of mole paws and mole cricket forelimbs. This sort of basic misunderstanding is what a biology textbook is supposed to clarify, not promulgate. The arguments presented in the discussion of convergence have no basis in the scientific literature, but trace back to the beginnings of modern creationism.
We determined above that homology, as defined by Darwin, is similarity in structure and position that occurs because species share a common ancestor that also exhibited the basic structural motif. If the similarity is not due to common ancestry, the structures would not be homologous. As Wagner (1988) pointed out, homologs are expected to have a similar position with respect to other structures in different species and their component parts are expected to have a similar position with respect to each other. Furthermore, homologs should be historically contingent in the sense that common descent is the only way to explain the presence of this invariant feature. Hence, before structures meet the criteria for homology, biologists evaluate alternative explanations of similarity, in particular, similarity as a result of function, common materials, and/or limitations of design.
The authors of Explore Evolution provide their evidence on pages 46-47 that function rather than ancestry may better explain the humerus-radius-ulna pattern of the vertebrate limb. Specifically that it’s a functional optimum for vertebrates with limbs to have one bone, the humerus, in the part of the limb closest to the trunk (body) and two bones, the radius and ulna, in the next portion of the limb. If the arrangement were reversed, functional problems would arise, particularly with range of motion.
The example the authors use to explain the restrictions placed on the two-to-one arrangement involves exploring the two differing arrangements using a ball of clay and two straws. However, a serious design flaw occurs when using their model. The example may recreate the structural arrangement of the bones, but provides an incredibly inadequate model of the connections between the bones. The ball and socket arrangement of the humero-scapular joint, the pivot of the radius-ulna, the hinge of the humero-radial joint are all important in determining function. Hence to really test their hypothesis, that function would be limited given another arrangement of bones, we would also have to rearrange the points of connection, which I suspect would rectify the problem. After all, the two-to-one arrangement works fine for the forearm, and allows the radius and ulna to rotate around the humerus.
Another problem with the authors’ argument is that vertebrate forelimbs actually function in a wide variety of habitats including running on land, swinging in trees, flying in the air, swimming in the water, and as the case is with penguins, flying in the water. It hard to believe that each of these habitats would place the exact same functional requirement on the design of the vertebrate limb. And in fact, to accommodate these different functional requirements, vertebrate limbs show incredible modifications. For example, the horse has a fused radius and ulna (see figure below), disproving the hypothesis that the one-two arrangement is a result of functional optimization for each vertebrate species. In fact, the vertebrate forelimb provides an amazing example of how function has influenced modifications to the same h-r-u pattern.
The pattern of limb bones called pentadactyl is found in all classes of tetrapods (i.e. from amphibians to mammals). It can even be traced back to the fins of certain fossil fishes from which the first amphibians are thought to have evolved. The limb has a single proximal bone (humerus), two distal bones (radius and ulna), a series of carpals (wrist bones), followed by five series of metacarpals (palm bones) and phalanges (digits). Throughout the tetrapods, the fundamental structures of pentadactyl limbs are the same, indicating that they originated from a common ancestor. But in the course of evolution, these fundamental structures have been modified. They have become superficially different to serve different functions in adaptation to different environments and modes of life. This phenomenon is clearly shown in the forelimbs of mammals. For example:
Common function can explain certain similarities of form, but cannot explain similar developmental pathways, or the particular components that make up certain structures in different species.
The discussion of functional constraints in Explore Evolution is nearly impossible to state in a way which does not refute itself. They do not deny the remarkable similarity between the structures of species within various taxonomic groups. They do not deny that one can produce a hierarchal (branching) arrangement of the ways these structures vary within and among these groups, and that the branching pattern is consistent regardless of which particular structure you examine. In other words, their response to the evidence of the branching pattern predicted by the tree of life is to agree that it is all accurate. They simply argue that it is possible to invoke special explanations for each such structure, multiplying causes needlessly. This practice violates basic scientific and logical principles. By treating structures in isolation, they obscure the actual evidence examined by scientists.
For instance, EE cites biologists from 150 years ago, biologists whose arguments were tested and found lacking.
Agassiz, for example, explained homologies as the result of the necessity of using similar structures to solve similar functional problems. On this view, the pattern we see in the vertebrate forelimb — a single bone closest to the trunk, two bones in the next segment, and a variety of bones in the segment farthest out — exists for important functional reasons.EE, p. 43
It is worth noting, to begin with, that vertebrate limbs do not have "a variety of bones in the segment farthest out." The number of fingers and toes is consistent. The numbers of bones in each finger and toe are consistent. The number of wrist bones is consistent. Even if functional constraints could predict the broad pattern, they do not explain why no living species has more than 5 fingers or toes, nor the consistency of the number and developmental histories of the wristbones.
Furthermore, functional constraints do not explain the broad pattern. Robotic arms do not typically have one element nearest the base, two further out, and a number in the "hand." They employ various sorts of joints and connections which do not exist in living species. The argument of functional constraints only make sense if you assume some evolutionary process acting on some common starting point. That explains vestigial fingers and toes in the legs of deer, it explains why our two legs, and all four legs in a deer, still have two bones in the middle segment, despite having no need to twist. It explains why no species has more than five fingers or toes, and why vestiges of all five can be found in vertebrates which seem to have fewer. These results would be surprising if there were not some common starting point, but are predicted and found because of evolutionary hypotheses.
The claim of consistency because of functional constraint also does not match the actual evidence. Because of the basic physics of flight, bird wings, bat wings and pterosaur wings must all be similarly shaped. If they were shaped much differently, flight would be (or have been) impossible. If the functional constraint hypothesis were the sole explanation for wing structure, we might predict that all three types of vertebrate wings would be similar in their anatomical structure, but this prediction fails. Pterosaurs have a wing consisting mostly of skin stretched between the 4th finger and the body, with the thumb and three fingers free of the wing. Bat wings consist of skin stretched across all 4 fingers and attaching to the leg, with the thumb free of the wing. Bird wings (like chicken wings you've eaten) do not have skin stretched across them, have fused the bones of the 2nd and 3rd fingers together for strength, and cover the structure with feathers.
Within each group, these traits are consistent, indicating that the wing can be treated as homologous within each group, but not across groups. All three wings share the same bones (the bones are homologous), but they are arranged very differently. Bats use all four fingers in the wing; birds use two, have lost one finger almost completely, and another is nearly functionless; pterosaurs used only one finger in flight, but adapted other fingers to different purposes. Since the anatomy of these wings is so different, this falsifies a prediction of the functional constraint hypothesis.
It confirms what we would expect from evolutionary explanations. Because vertebrates share a common ancestor, all three groups shared ancestors which possessed the basic vertebrate limb. Each group took steps toward flight at different times and from different ancestors with different initial traits. The differences in starting conditions meant that different sorts of genetic and structural changes were advantageous in each different group. Within each group, the structure of the wing is consistent, indicating common descent within pterosaurs, within bats and within birds. The differences in the structure of each type of wing indicates that wings evolved independently in each group. The similarity of the bones in the wings (and elsewhere in the body) indicate that all three groups share an ancestor farther back in their history. This nested pattern of shared characters is exactly what common descent predicts, and has not successfully been explained by other means.
Functional constraints cannot explain why vertebrate wings should consist of skin stretched over bone, since birds do not use skin for the flight surface. Indeed, insect wings do not have bones or skin, and are not derived from legs. Insect wings are structurally similar (homologous) to gills. Again, this pattern of similar structure is unexpected under the predictions of common functional constraint, but entirely predictable based on evolution and common descent.
An inquiry-based textbook could turn a discussion like that above into a fascinating exercise. Students could generate predictions based on various potential explanations, and then test them using data from various biological structures. Explore Evolution, despite its claim to be inquiry-based (and despite the nonsensical and meaningless exercise on pp. 46-47), does not invite students to examine any evidence at all, nor does it explain why students should ignore the research conducted in over a century since Agassiz defended his position. Inquiry-based instruction invites students to discuss the topic, but no useful discussion could possibly proceed from such a flawed foundation.
Similarity of the structure of mole cricket forelimbs and the paws of a mole does not, in any sense, suggest homology between those structures. No one has ever suggested such a thing. Homology consists of more than similarity of shape, but similarity of underlying structures.
It is worth noting in passing that only a few pages after complaining about the presentation of different fossil skulls without showing the different sizes of the skulls, every graphic in this chapter compares organisms and structures of very different sizes without any indication of the relevant scale. Human arms are 3 feet long, horse legs are generally around 5 feet long, bat wings are a foot or two long, and a whale flipper can stretch close to ten feet long (fig. 2:1). Mammal eyes are generally between half an inch and an inch deep, insect eyes are fractions of an inch across, and cephalopod eyes can be a foot and a half wide (fig 2:2). Ichthyosaurs were 3-6 feet long while the baleen whales EE compares them to are 40-50 feet long (fig. 2:3). A mole cricket is an inch long, a mole is closer to 8 inches long (fig. 2:4). This point is minor, but emphasizes the inconsistent approach Explore Evolution takes to its subject.
EE's attempt to claim some deep meaning for the similarities between the mole cricket's forelimbs and mole hands would be entertaining if it were not obvious that EE intends that comparison to be taken seriously. EE wonders:
are there some similarities not due to common ancestry? Surprisingly, nearly all biologists say there are. … The flippers of a whale and an ichthyosaur have very similar shapes, even though the whale is a mammal and the ichthyosaur was a reptile. … The forelimb of a mole cricket is very similar to a mole's forelimb, even though the mole is a mammal and the mole cricket is an insect.
Even biologists who take a monophyletic view of life's history will tell you that the similarity we see in these structures is not the result of common ancestry. They contend that the last common ancestor of these creatures did not possess the similar structure. In other words, the similar structures arose separately on independent lines of descent.EE, p. 46-48
It is not clear why anyone, let alone textbook authors, should be surprised that certain basic shapes recur in nature. The shape of a flipper or a wing or a digging implement is constrained not only by the evolutionary history of that structure, but by the nature of the work it is used to do. Limbs shaped more like a flipper tend to make animals better swimmers, whether they are birds (penguins), mammals (whales), or reptiles (ichthyosaurs). The question a biologist asks in assessing homology is not whether the shape of a structure is similar, but whether the composition and developmental origins of the structure are homologous. On that front, there is simply no basis for claiming any homology between cricket limbs and mole paws, any more than there is a homology between mole paws and a spade. An examination of the anatomy of a mole paw, cricket limb, shovel, and a human hand makes it clear why the similarities in the basic outline of three of these structures is less evolutionarily significant than the clear anatomical similarities between the human hand and the mole paw.
This example is so obvious an instance of functional constraints creating similarities in structures that biologists have been using it to illustrate this point since at least the 1950s. Michael Novikoff wrote in 1953:
the concept of analogy includes two different categories of phenomena. One clearly corresponds to the accepted definition of this term, according to which analogous forms are understood to be those that have been secondarily acquired by animals or plants in adaptation to similar environmental situations. As an appropriate example of such an analogy one may cite the front appendages of such widely different animals as the mole, Talpa europaea, and the mole cricket, Gryllotalpa vulgaris. The ends of the front legs of both the mammalian burrower and the insect burrower are shovel-like, well suited to digging underground. Another example, the acquisition of a snow-white fur or plumage by various animals of the northern regions, may be mentioned. This analogy is concerned with a purely physiological phenomenon and is therefore in contrast to homology, which is a morphological or phylogenetic phenomenon.Michael M. Novikoff (1953) "Regularity of Form in Organisms" Systematic Zoology 2(2):57-62.
It is not the least surprising that certain shapes recur in nature, coming from disparate origins. The surprise expressed by Explore Evolution is truly disappointing. A textbook is not supposed to encourage confusion among students, but to clarify misconceptions. EE's befuddled efforts to cast doubt on evolution merely reflect the authors' own misunderstandings; misunderstandings that no school board nor teacher should wish its students to perpetuate.
There is nothing mysterious about convergence. Species facing similar selective pressures would be expected to be similar in certain ways. There is no reference to any particular scientist who would support the claims EE advances, but it is an argument commonly advanced in the creationist literature.
Explore Evolution says the following about convergence:
No citation is given to the scientists who are supposedly skeptical about the ability of natural selection to explain convergence. Indeed, it's difficult to imagine how anyone who thinks a structure could evolve in one lineage in response to a set of selective pressures would be surprised that the same selective pressures would produce a functionally similar structure in another lineage. Furthermore, in the examples of convergence offered in EE, the convergence does not involve any complexity. The mole paw is a simple modification of shape of the paws found in other mammals. Similarly, the mole cricket's forelimbs are simple modifications of the basic anatomy of the insect forelimb. No new structures are involved, merely the rearrangement of pre-existing structures.
Even though scientists cannot be readily identified making the argument EE presents, that argument is easy to locate in the writings of creationists. In 1925, young earth creationist George McCready Price made a very similar argument to that in EE:
we become convinced that these many similar or identical structures, which must have been evolved quite independently (if evolved at all), make too great a draft on our credulity. At least, these hundreds of examples of "parallel evolution" greatly weaken our confidence in homology, or similarity of parts and organs, as a proof of blood relationship.
There are several distinct types of eyes, each type being quite efficient as organs of seeing. But if we take the eye of the higher animals, we become amazed to find an almost identical structure in the cuttlefish or devilfish, which is really a mollusk. Its eye has all the parts found in the human eye, a retina, a sclerotic, a choroid, a vitreous humor, an aqueous humor, and an adjustable lens, just as in the eye of one of the higher vertebrates. Now I can believe that these similar organs could have been created independently for these very distinct classes of animals. But I cannot believe that this marvelous organ was evolved independently in these two instances by any process of natural development or evolution. … I do not think that [Darwin's] mental equilibrium would have been restored if he had considered that this organ must have been evolved quite separately in at least these two instances. Indeed, this process must have been repeated also once more; for the pecten, another kind of shellfish wholly different from the cuttlefish, has two rows of almost equally perfect eyes around the edge of its body. I cannot force myself to believe that these complete organs of sight were separately and independently evolved by any natural development in these three instances.
The argument has been repeated many times by many creationists. In 1970, Evan Shute wrote:
Many resemblances between animals and plants of different genera, families and orders defy evolutionary explanation. There are both differences and similarities between creatures of different kinds. The evolutionist must decide what features are useful as true species criteria and what features are spurious or misleading. A small but interesting sampling of strange similarities between widely diverse living forms is given here, from a study of spinal tracts, ears, placentae, electric organs, kidney function, fern vessels, milk, brown fat, sweat glands, and other systems: It is asserted that these puzzling resemblances are best explained by special creationism rather than by evolutionary convergence.Evan Shute (1970) "Puzzling Similarities," Creation Research Science Quarterly, 7(3):147-151.
More recently, the newsletter for the old earth creationist group Reasons to Believe claimed:
No known evolutionary mechanism can account for the nature of biological convergence. Convergence has been far too common throughout life’s history, has involved exceedingly complex structures, and has occurred in situations in which the forces of natural selection have been vastly different. Biological convergence is an important component in the argument that life, throughout earth's history, is a result of the supernatural activity of a Creator.
In the scientific literature, convergence is far from surprising. In Futuyma's Evolutionary Biology, the second of seven "principles of evolutionary change" is "homoplasy [convergence] is common in evolution":
When a similar character (or character state) in two organisms has not been derived from a corresponding character (or state) in their most recent common ancestor, it is said to be homoplasious. An example of a homoplasious character is the superficially similar eye of vertebrates and of cephalopods (squids, octopods). Both have a lens and retina, but their many profound differences indicate that they evolved independently: for example, the axons of the retinal cells arise from the cell bases in cephalopods, but from the cell apices in vertebrates.
Three more or less arbitrarily distinguished kinds of homoplasy are recognized. In convergent evolution (convergence), independently evolved features are superficially similar, but arise by different developmental pathways. The eyes of vertebrates and cephalopods are an example. Parallel evolution (parallelism) is thought to involve similar developmental modifications that evolve independently (often in closely related organisms, because they are likely to have similar developmental mechanisms to begin with). … Evolutionary reversals constitute a return from a [derived] character state to a more … ancestral state.
Homoplasious features are often (but not always) adaptations by different lineages to similar environmental conditions. In fact, a correlation between a particular homoplasious character in different groups and a feature of those organisms' environment or niche is often the best initial evidence of the feature's adaptive significance.Douglas Futuyma (1998) Evolutionary Biology 3rd ed., Sinauer Associates:Sunderland, MA. p. 110-111
There is nothing the least bit surprising about convergence or homoplasy in general. Shared selective pressures ought to produce some degree of similarity in structures. Characteristics used to identify evolutionary relationships are selected to avoid traits likely to be result of convergence, so homoplasy is not, in general, a problem for reconstructing evolutionary history. Convergence, far from being a surprise, is a predicted result of evolutionary processes.
This is another instance where Explore Evolution uses ambiguous language to confuse students, rather than bring clarity to a subject. It is entirely predictable that shared selective pressures would produce superficial similarities from dissimilar anatomical structures. We expect the underlying anatomy to reveal underlying evolutionary relationships; superficial similarities are not expected to be evolutionarily informative.
Explore Evolution claims:
For other scientists, the phenomenon of convergence raises doubts about how significant homology really is as evidence for Common Descent. Convergence, by definition, affirms that similar structures do not necessarily point to common ancestry. … But if similar features can point to having a common ancestor — and to not having a common ancestor — how much does "homology" really tell us about the history of life?EE, p. 48
It is not clear who the "scientists" are who advance this argument. As discussed above, scientists see nothing surprising about similar selective pressures producing superficial similarities between structures. The similarities that scientists consider evidence of common descent are similarities of underlying structures and developmental processes.
The point that EE raises here is not a terribly complex point, and closing the chapter with that question is hardly educational. Indeed, EE here passes up an opportunity for genuinely inquiry-based learning. It would not be difficult to prepare an exercise in which students would be asked to examine actual organisms, and to propose investigations which would test whether certain traits are homologous. Scientists perform such tests routinely, and a simplified example would allow students to understand that process. In doing so, students would come to understand that identifying similarities or differences between a particular structure in two species is the first step in a scientific inquiry. Students would learn that testing a hypothesis about homology requires comparison with other structures in other species. Students would also see that certain traits tend to vary rapidly in response to an organism's environment (coloration, for instance), while other traits are remarkably consistent (the number of bones in the limb). Students could then be given a new set of organisms to examine, and see how scientists use knowledge from previous research to inform new assessments of homology and homoplasy.
Instead of encouraging scientific inquiry, EE misuses terminology, makes false claims about the current state of the science, and then closes the discussion with a question to the students, without having given any indication of how students ought to go about addressing the question. This is a poor model of how science works, and a poor way of teaching any subject.