Biogeography, contrary to what readers of Explore Evolution might think, encompasses more than just adaptive radiation on islands. Studying the biogepgraphic effects of rivers and mountain ranges also informs our understanding of evolution. Our understanding of relationships between distantly related groups is often informed by comparing the distributions of modern species and their fossil ancestors with our understanding of continental drift. Such comparisons allow scientists to predict the whereabouts of important fossils and to trace back the distant shared ancestry of modern groups.
Explore Evolution never discusses plate tectonics and its impacts on biogeographic study, and in some cases erroneously dismisses common ancestry based on the current distribution of continents. For the most part, the book focuses on the rapid diversification seen on many isolated island populations, but wrongly claims that evolution in these adaptive radiations has produced no novelties, and only represent loss of genetic information. In fact, studies on islands show the evolution of novel anatomical structures and complex adaptations to new ecological niches.
The vision of biogeography in Explore Evolution is shockingly narrow. The examples of biogeography discussed are: the Galá Islands, the Hawaiian Islands, and island continents Australia and ancient South America. The discussion of marsupial biogeography across South America and Australia bizarrely omits any discussion of plate tectonics, a central theme in any discussion of biogeography over long time scales. No discussion at all is offered of many crucial biogeographic concepts that bear on evolutionary biology.
Biogeography generally focuses on finding repeated geographical patterns across multiple taxonomic groups. For instance, in the 1850s, Alfred Russel Wallace (who independently discovered natural selection) found in his travels through the East Indies that there was a sharp line between the species found in Southeast Asia and the islands as far east as Borneo and Bali, while islands only a few miles away had communities of species with closer affinities to the Australian fauna. The same pattern could be found in a range of groups, including mammals and birds, indicating that some common process acted to allow diversification of groups within regions. He summarized the significance of this result by stating "Every species has come into existence coincident both in space and time with a closely allied species" (Wallace, 1855, "On the Law Which Has Regulated the Introduction of New Species," Annals and Magazine of Natural History 16:184-196.)
By comparing macroevolutionary patterns between different groups, we find that the same patterns repeat. This strongly suggests that the same forces drove the diversification of those different groups. This also makes it possible to compare rates of evolution of those groups. For instance, a recent study of the global biogeography of mites allows unique insights into the processes driving diversity within and across various groups of mammals.
The authors of the mite research explain that "To date, few conclusive empirical studies of the worldwide historical biogeography of terrestrial organisms are available, because the members of clades [groups containing all descendants of a single common ancestor] with global distributions tend to present dispersal abilities that obscure historical biogeographical patterns. To find a group of land organisms with an ancient global distribution, and therefore suitable for a study of historical biogeography on a global scale, one needs to look among the earliest colonizers of terrestrial environments" (Boyer, et al. 2007, "Biogeography of the world: a case study from cyphophthalmid Opiliones, a globally distributed group of arachnids," Journal of Biogeography, OnlineEarly Articles).
These data establish a context within which other groups' diversity can be examined. While the mites those researchers were studying have a long history, and can be traced back nearly to the beginning of life on land, other groups evolved much later, and exist only in the subset of the world that was connected at the time they evolved. Not only is biogeography evidence against the fixity of species, it is evidence against the fixity of larger taxonomic categories, since groups of much different taxonomic rank follow the same biogeographic patterns. Marsupial biogeography, discussed below, fits well with part of the pattern seen in the mites. The biogeography of extant mammals matches a large portion of the mite data, and the inclusion of fossilized mammalian ancestors results in a biogeography that matches yet more of the mite data. This biogeographic pattern can not be explained without reference to common descent.
Explore Evolution does not even mention major areas of biogeographic research such as gradients in species diversity found as one travels from the poles to the equator, or from sea level to the tops of mountains. Such studies are central to our understanding of the origins not just of individual species, but the evolutionary processes which generate species diversity, and the book's silence on these topics does students a disservice.
Explore Evolution states:
Critics note that the examples of mockingbirds in the Galápagos and fruit flies in the Hawaiian Islands show only small scale variations in existing traits. … Since critics of the argument from biogeography see no evidence of large-scale change, or of a mechanism that can produce the new genes needed to cause such change, they doubt that the biogeographical distribution of animals supports Universal Common Descent.Explore Evolution, p. 77
The issue here of Universal Common Descent is a bit of a red herring. Biogeography is a powerful way to illustrate the power of evolutionary processes, but since all of the parts of the planet have been connected at one point or another, the earliest biogeographic evidence tends to be obscured by subsequent extinction and evolutionary change. Biogeography does reveal the speed with which evolution can operate, and comparing the biogeographic histories of different groups, we can better understand the timing and processes by which various groups evolved. As with the research on mite biogeography discussed earlier, researchers can test predictions of common ancestry by examining overlapping biogeographic patterns, and research on island adaptive radiations provide powerful examples of the power of evolution to generate evolutionary novelties. Explore Evolution claims that these radiations do not demonstrate a mechanism which can "transform one type of animal into a fundamentally different type of animal" (p. 77), but never offers a definition of "fundamentally different." By any reasonable standard, though, island radiations do indeed show exactly such novelty.
Most of the examples that Explore Evolution discusses are poor examples of the breadth of biogeography, since they are really illustrations of adaptive radiation (and not the most striking examples of that phenomenon, either). For instance, rather than addressing the classic case of the adaptive radiation of Darwin's finches on the Galápagos, Explore Evolution focuses on the less diverse Galápagos mockingbirds. Of the 14 species of finches which evolved from an ancestral population blown to the islands several million years ago, the range in sizes is vast, and the ecologies range from vegetarianism to carnivory, from tool-using insect hunters to bills which can crush seeds and to bills that probe into tiny holes to draw out insects. The range of ecologies generated from a small starting population in a few million years is tremendous. Similar ecological diversity evolved among a population of finches which blew onto the Hawaiian islands between 5 and 10 million years ago. Their descendants, known as honeycreepers, show a range of variation in morphology and ecology which falsifies any claim that evolution on islands does not produce fundamental differences. Some authors consider that group to represent a separate family of birds which evolved in a their short time in Hawaii, others regard it as a subfamily within the finches; all agree that their diversity is stunning.
Similarly, in three lakes of Africa's Rift Valley, a member of a family of fish named cichlids has evolved a range of ecologies and sizes unmatched anywhere else. Those lakes are known to have formed no later than 1.5-2 million years ago, and the hundreds of species of fish in those lakes occupy ecological niches, and exhibit biological forms, unheard of elsewhere. (One species specializes in eating the eyes of other fish.) The range is greater than what you might find at a coral reef, and all from a small number of evolutionary starting points.
In Hawaii, there are about at least a thousand species of flies — many still waiting to be described — in the genus Drosophila and they all share a common ancestor that separated from the mainland Drosophila tens of millions of years ago. Islands known to be less than 500,000 years old have species which exist only on that island, and which must have evolved in less than half a million years. Those flies represent roughly one third of the members of the genus in the world, and the species found in Hawaii exhibit evolutionary novelties: anatomical traits and behaviors seen nowhere else. This diverse group is not the only adaptive radiation on the islands.
In most of the world, damselfly larvae are aquatic hunters of other invertebrates, breathing through gills on their tails. In Hawaii, some have evolved to live on land, hunting through the leaf litter. In the course of their evolution from aquatic to terrestrial habitats, their gills evolved into air-breathing structures, and representatives of various intermediate stages in this transformation can be found on the islands. Others have adapted to live near the water that collects at the bottom of leaves, but actively avoid being actually soaked in that water, requiring a different set of adaptations. Again, these are structures which are fundamentally different than anything found in other damselflies, and these adaptations have occurred in remarkably short amounts of time.
Though Explore Evolution constrains itself to discussing animals, adaptive radiations can also be found in the plants of Hawaii. The silverswords, three genera in the sunflower family found in Hawaii have evolved a range of structures, ranging from short plants with spiky leaves to trees, shrubs, vines and low ground-cover. As Futuyma explains, "They vary greatly in the form and anatomy of the leaves and in the size, color and structure of the flowers. In many features their range of variation exceeds that among families of plants, yet almost all of them can be crossed, and the hybrids are often fully fertile. They all appear to have been derived from a single ancestor that colonized the Hawaiian Islands from western America" (Futuyma, 1997, Evolution, Sinauer Associates, Sunderland, MA, p. 118).
The appearance of such diversity from a known starting population demonstrates the incoherence of Explore Evolution's "criticisms". Adaptive radiations have often generated variations exceeding that seen within whole families within a geological flash, yet the authors call these "only small-scale variations in existing traits." By making such a sweeping and imbalanced generalization, Explore Evolution misinforms students about the actual evidence at hand. By making the claims without explaining the basis for them, Explore Evolution makes it impossible for students to explore these ideas in any additional depth, once again hindering inquiry, rather than encouraging and supporting true scientific investigation. If such new structures can be generated in the space of a few milllion years, it's not hard at all to envision the same processes producing the diversity of all life in the space of billions of years.
The marsupial faunas of South America and Australia are at least as ecologically diverse as placental mammals worldwide (with some exceptions, see the discussion of developmental constraints in our response to chapter 8). The convergent evolution of Australian mammals and placentals found in comparable habitats elsewhere shows the power of evolution to adapt species to similar conditions. That they have similar adaptations to those found in placentals, but achieve such adaptations by different means, indicates how flexible evolutionary processes can be. Because of the ecological diversity of South American and Australian marsupials, and the biogeographic history which made such diversity possible, marsupials could serve as a useful exploration of the interplay of evolution and biogeography.
Unfortunately, the discussion of marsupial biogeography in Explore Evolution is laughably bad: too brief to education, and so inaccurate as to be utterly useless. It begins with a mischaracterization of the evolution of marsupials:
The first mammals with the marsupial's distinctive mode of reproduction arose on the ancient southern super-continent of Gondwanaland. Later, after this great land mass broke up into separate continents, the ancestors of marsupials were separated from other mammals and evolved in isolation on the new continents of Australia and South America.Explore Evolution, p. 75.
This is a straw man. Textbooks and researchers in the field do not claim that marsupials originated in Australia or South America. The best evidence is that marsupials originated Asia, migrated to North America via a land bridge, and that the co-existed with placental mammals in the northern hemisphere for some time. Marsupials colonized first South America, and from there moved on to Antarctica and then Australia. The marsupial populations in Asia and North America went extinct, possibly as a result of competition with placental mammals among other factors, and the populations on southern continents remained in those safe havens.
Explore Evolution's perfunctory and inaccurate coverage of this basic biogeography does students a disservice. Students cannot be guaranteed to have a background to know what Gondwanaland was, nor to appreciate that the supercontinent was already largely broken up by the time marsupials were crossing landbridges between the continents. If students do not have the background to appreciate the interplay of diversification and continental drift, the book's explanation will not help.
Having created the straw man, Explore Evolution proceeds to knock it down:Critics of the marsupial argument insist that it, too, fails to establish Universal Common Descent or even the descent of all marsupials. At best, it shows that various groups of marsupials first originated in the same general area in the Southern Hemisphere and were then distributed more widely as the Southern continents separated from one another. But even this is questionable, some critics say. They point out that marsupials are not restricted to the southern continents of Australia and South America. Marsupials such as the opossum live in the northern hemisphere. And, in a recent development, paleontologists have unearthed the oldest marsupial fossil of all … in China.Explore Evolution, p. 77-78.
Marsupials exist in North America because of migration, a process described in this very chapter, but ignored now when the authors find it inconvenient. Around 3 million years ago, a combination of continental drift and the rising Andes brought South America into contact with North America. For the first time in 50 million years, North American species and South American species came into contact. Some marsupials (like the opossum) spread north. Some placentals from North American spread south. For reasons that scientists continue to investigate and discuss, the ability of North American placentals to persist and diversify in South America was much greater than the ability of South American marsupials to diversify in the north, or to outcompete the northern invaders (Stehli and Webb, eds. 1985. The Great American Biotic Interchange. Plenum Press: New York). Today, half of South American mammals are descended from North American ancestors. South American descendants represent no more than 20 percent of modern North American species, and most of those are in Central America, near the point of initial contact (Marshall, et al. 1982. "Mammalian evolution and the Great American Interchange," Science 215:1351-1357). This helps explain the confusion of the apparently biogeographically illiterate authors of Explore Evolution about why "marsupials such as the opossum live in the northern hemisphere." It is because of migration, a process they described as uncontroversial only a page earlier. It is not a mystery, and it is unclear why the authors would regard it as such.
The claim that North American opossums are evidence against the common ancestry of marsupials bears striking similarities to a critique of biogeography by young earth creationist Kurt Wise:There are very few examples of macrobiogeographical evidences for macroevolution, and none of them is very strong. The best-known claim is the concentration of marsupials in Australia. But there are several reasons that marsupials in Australia are actually a poor example. First, all marsupials are not in Australia. The Virginia opossum of North America, for example, is a marsupial. It is thought to have come from South America, not Australia. Thus not all similar organisms are known from every continent. Third, marsupials are the oldest fossil mammals know from Africa, Antarctica and Australia—in that order. The fossil record seems to show a migration of marsupials from somewhere around the intersection of the Eurasian and African continents and then a survival in only the continents farthest from their point of origin (South America and Australia).source Wise, Kurt (1994) "The Origins of Life's Major Groups," ch. 6 in J. P. Moreland (ed.) The Creation Hypothesis: Scientific Evidence for an Intelligent Designer, Intervarsity Press:Downers Grove, IL, p. 223).
Wise's confusion over the status of the Virginia opossum perhaps reflects his confusion of the New World order Didelphimorphia — commonly called opossums — with the Australian order Diprotodontia — some of which are commonly called possums (without the first "o"). The groups are morphologically and molecularly distinct, with well-established paleontological histories. If the opossum truly had roots in Australia, it would indeed be a biogeographic conundrum. In fact, the only close link between opossums and Australia is Wise's typo.
Similar misunderstandings plague the discussion of the marsupial fossil record in Explore Evolution and its creationist source material. It is not surprising, the breathless tone of Explore Evolution notwithstanding, that "paleontologists have unearthed the oldest marsupial fossil of all … in China." The authors wonder "If the ancestors of the marsupials originated in the Southern Hemisphere, why has the oldest known member of the group been discovered in the Norther Hemisphere?" The answer is simple: paleontologists do not claim that marsupials originated in the Southern hemisphere, only that they migrated there.
Marsupials and placental mammals separated roughly 125 million years ago, according to the most recent fossil data. Several lines of evidence indicate that the marsupials originated in Asia, spread to North America over a land bridge, which can be seen in the figure. The ages and characteristics of fossils found in Europe, South America, Africa, Australia and Antarctica suggest that marsupials spread to Europe and South America from North America. South America, Africa, Australia and Antarctica still had linkages at that time, allowing species on the Southern continents to spread easily.
Despite the feigned confusion of Explore Evolution's authors, the fossil record gives a very clear picture of the biogeographic history of marsupials, though there are many questions scientists continue to investigate. The earliest marsupial fossils (and the earliest placental fossils) are found in Asia. Fossilized marsupials are found in North America in rocks that are only a few million years younger than the Chinese fossils. During that period in geological history, plate tectonics had brought North America and Asia close enough together to forge a land bridge, allowing many species to migrate between those continents.
At that time, the southern continents were all connected, North America and Europe were still very close, and South America had not drifted far from North America, allowing dispersal during periods when ocean levels dropped. Marsupials related to North American species colonized Europe briefly, through a northern land bridge, and others colonized South America. Africa was in the process of separating from the supercontinent which also included Antarctica, Australia and South America, so the presence of marsupial fossils in Africa gives a good measure of how quickly they entered South America and dispersed across the supercontinent Gondwana. Fossilized marsupials in Antarctica also allow us to track their dispersal to Australia. This pattern is consistent with the fossil record of placental mammals, and with other lines of evidence (John P. Hunter and Christine M. Janis. 2006. "'Garden of Eden' or 'Fool’s Paradise'? Phylogeny, dispersal, and the southern continent hypothesis of placental mammal origins," Paleobiology, 32(3):339–344).
As the southern continents drifted apart as well, the marsupial faunas in each isolated continent followed a different path. As Antarctica drifted south towards its current polar position, it became colder and colder, ultimately driving its resident marsupials and palm trees extinct. South American and Australian marsupials produced diverse radiations which filled many of the same ecological niches occupied by placental mammals elsewhere. In the northern continents, which were periodically linked by land bridges, biotic interchanges resulted in periods of intense competition, which seem to have driven the native marsupials extinct.
When South America drifted north again and connected with North America around 3 million years ago, the Great American Biotic Interchange had the same devastating effect that biotic interchanges had on other marsupial faunas.
The same pattern of diversification and migration seen in marsupials can also be seen in other groups of plants and animals. That consistency between biogeographic and evolutionary patterns provides important evidence about the continuity of the processes driving the evolution and diversification of all life. This continuity is what would be expected of a pattern of common descent. The creationist orchard scheme gives us no reason to predict this pattern.
True biological novelty can be found in many of the adaptive radiations that Explore Evolution describes. Despite this, the authors insist "There are many examples of isolated islands that are home to flightless birds and insects that have clearly lost some of the genetic information necessary to produce the traits possessed by their ancestors. Large-scale macro-evolutionary change requires the addition of new genetic information, not the loss of genetic information" (p. 77). No evidence is offered of research on the genetics of flightlessness, and it is far from obvious that flightlessness must represent a loss of information. It is not generally true that loss of a structure involves loss of genes; eyeless cave fish lose their eyes because certain genes are over-expressed. Furthermore, there is no basis for their claim that macroevolution requires the addition of new genetic information, nor is new genetic information beyond the capacity of normal evolutionary processes. For a fuller discussion of the problems with Explore Evolution's treatment of "information," please see chapter 8.
We saw previously that the variation within Hawaiian Drosophila and other adaptive radiations is far greater than the variation found within some much broader taxonomic groups. It is difficult to say what genetic information the authors of Explore Evolution believe was lost. For instance, in the example of damselflies above, here is how one researcher put it in 1970:
This change from aquatic to semi-aquatic to arboreal to terrestrial habit has demanded considerable morphological and physiological change in the gills, and there is a beautiful transition series displayed by the gills of the various species from the long, thin, delicate, highly tracheated gills of the aquatic forms to the short, thick, opaque, densely hairy gills of the terrestrial species. There must also be changes in the function of the spiracles.Elwood C. Zimmerman (1970) "Adaptive Radiation in Hawaii with Special Reference to Insects." Biotropica, 2(1):32-38.
"It would appear," he concludes, "that it is from such extraordinary adaptive radiation that new major taxa might be produced, and the phenomenon is here demonstrated most lucidly before our eyes."
While the full set of genetic changes underlying this evolution are not fully known, there is no reason to believe it required any new genes, nor that any existing genes were lost along the way. Like many cases of the evolution of new structures (for instance, those discussed by Armin Moczek. 2008. "On the origins of novelty in development and evolution," Bioessays, 30(5):432-447), the evolutionary process most likely operated by rearranging and reusing existing genes and regulatory systems, making changes to the places where genes were expressed, or the times when they turned on or off. Such subtle changes can produce dramatic effects on the final form of an organism.
Understanding the precise genetic basis for those sorts of changes has been an important area of research over the last few decades, as new technology made it possible to examine the ways that genes control development. Explore Evolution presents such open areas of research as a reason to abandon all hope of resolving the underlying issues, but this is not how science works. Scientists are actively investigating the ways in evolution actually works, and students who hope to participate in the active research under way as researchers, doctors, or patients need to understand the process by which scientists produce and evaluate new knowledge. A competent textbook would use these areas of active research to invite true exploration of novel ideas. The fact that Explore Evolution despairs of finding explanations for unresolved issues in science is a damning indictment of the book's inadequacies.