BIOGEOGRAPHY, COMPARATIVE ANATOMY AND EMBRYOLOGY Biogeography as Evidence for Evolution Scientists describe patterns in nature and then hypothesize the processes which produced the patterns. Biogeography is the study of animal and plant distribution patterns, both current and past (paleobiogeography), and the reasons for these patterns. One would expect that biogeography would contribute data in support of an evolutionary interpretation of current species diversity. Of the various patterns of animal and plant distribution described by biogeographers, however, only the pattern of oceanic island biogeography offers clear "evidence" for evolution, as Darwin was quick to recognize. Most other patterns can be interpreted as due to changing conditions on the planet (and so support a dynamic worldview), but in themselves do not unambiguously indicate transformation of species over time. Distribution patterns and a dynamic worldview Before we discuss the pattern of island biogeography which does have evolutionary significance, let us briefly survey biogeographic patterns which suggest a changing Earth, but not evolution. There are two major global biogeographic patterns: biomes and biogeographic realms or regions. Biomes are latitudinal belts which circle the planet and reflect differences in plant life-form observed from polar regions to the equator. The major biomes are: polar ice and snow, the tundra (low herbaceous growth), taiga (coniferous belt), deciduous forest (belt composed of tree species that lose their leaves in winter) and tropical rain forest. Biogeographic realms are divisions of the planet which correspond roughly to the distribution of the major continents and are land masses separated from each other by a significant barrier to dispersal. These realms or regions are: Palearctic (Europe, Asia north of the Himalaya and Nan Ling mountains, Africa north of the Sahara desert), Nearctic (North America), Ethiopian (Africa south of the Sahara), Oriental (Asia south of the Himalaya and Nan Ling mountains), Neotropical (Central and South America), and Australian (Australia and its nearby islands). Each biome or biogeographic region contains a collection of animal and plant species that is largely unique and so characteristic of that global zone. The general explanation for biome diversity is adaptation to a common climate (plants adapt most conspicuously to the prevailing long-term weather conditions; animals adapt to the plant life). Biogeographic realm diversity is explained largely by isolation produced by barriers to migration. Exceptions to each of these broad patterns provide a clue to the mechanism which produces them. That climatic features control biome life-form is dramatically indicated by the correlation between latitudinal and altitudinal gradients in life-form diversity. The same pattern of life-form sequence observed from the tropics to the poles is mimicked as one ascends a tropical mountain. Why is it that arctic lichens are found on high mountains in tropical and semitropical regions? The reason is not evolution; rather it is migration during periods of glaciation which pushed the biomes toward the equator. When the glaciers retreated as the climate warmed, plants stranded on tropical mountain tops remained because the climate at this altitude did not change. Thus, the altitudinal sequence of species diversity which differs from the lowland fauna (animal life) and flora (plant life) in each biome is best explained by past migration events rather than by the evolution or creation of species in their current location. Disjunct species distribution provides a clear exception to the integrity of biogeographic realms and so calls for an explanation. Why are tapirs found both in South America and Malaysia but nowhere else? Alligators and magnolias only in southeast United States and southeast China? Camels and their relatives (llamas and vicunas) only in Africa, Eurasia and South America? A static worldview would explain these disjunct distributions as the result of special creation, but this is really not an explanation, for it offers no knowable mechanism to account for the pattern. A dynamic explanation of this static pattern is provided by the fossil record. In each case the fossil distribution of the species in question is far more widespread than exists currently. This observation suggests that these species migrated to their current locations only to become subsequently stranded there by climatological or geological changes which caused their extinction elsewhere.. Surprisingly, the fossil distribution of camels indicates that they evolved in North America and migrated to their current locations before becoming extinct in their center of origin. Continental drift Active dispersal via migration is not the only explanation for disjunct distributions. Passive dispersal through continental drift offers a reasonable explanation for the global distribution of species which have little dispersal ability. Ratite or flightless birds (e.g., emus and cassowaries in Australia, rheas in South America and ostriches in Africa), lungfish and side-necked turtles (in South America, Africa and Australia) and marsupial mammals (in Australia and South America) provide good examples of this mode of transport. Each of these disjunct patterns is correlated with the timing of the breakup of Pangea about 250 million years ago. Alfred Wegener (1880-1930) was a climatologist who first proposed the idea of continental drift. His suggestion in 1915 was immediately rejected as pure fantasy but resurrected when paleomagnetic studies indicated that the Earth's magnetic field had changed direction often. In the early 1970s Vine and Matthews interpreted identical magnetic patterns on both sides of the mid-Atlantic ridge as evidence of sea floor spreading which pushed the continents apart. Subsequent study has revealed the presence of crustal plates on which the continents rest and these plates float on a sea of molten magma. The theory of plate tectonics attempts to explain the forces which cause the plates to move. As the plates move they cause the land masses on them to collide forming mountains such as the Himalayas and the Cascades. The Indian subcontinent is still being pushed up against mainland Asia and so the Himalayas are still rising. The recent eruption of Mount St. Helens and frequent earthquakes in California demonstrate that plate movement is still shaping the geography and geology of western North America. The shifting geology of this planet changes habitats as land becomes covered by seas, seas dry to form deserts, and mountains are formed and eroded. The wobbling of planet Earth causes periods of glaciation and continental drifting towards and away from the equator influences climate. Climate in turn influences plant life-form which in turn influences animal species diversity. Thus, geological changes provide strong evidence for a dynamic Earth and set the stage for change in plant and animal life through dispersal and evolution. The distribution patterns (biomes, biogegraphic regions and disjunct distributions) described above do not require the evolution of new taxa in their current locations to explain them. Then just why did Darwin use biogeographical evidence to support the case for evolution? The reason is that one biogeographic pattern does in fact provide compelling evidence for evolution, namely, the pattern of oceanic island biogeography. Oceanic island biogeography Islands are habitats surrounded on all sides by some barrier to active dispersal. We usually think of small land masses surrounded by water when we hear the term island, but a body of water bounded by land can also be an island, as can a valley surrounded by tall mountains. The characteristic of islands which makes them useful for evolutionary studies is their isolation. Species isolated on islands tend to vary considerably from their closest relatives. The most dramatic examples of interspecific variation (variation between different species) are found on clusters of oceanic islands called archipelagos. Formed by volcanic action in areas quite remote from the nearest continental land mass, these islands are eventually colonized by migration (either active or passive dispersal). Immigrant species with relatively poor dispersal abilities which arrive by accident are effectively stranded in their new habitats and evolve as they adapt to them. Once the colonists have established themselves and built up their population size, some individuals may migrate either to different habitats within the same island or to other nearby islands and become isolated long enough the evolve into a new species. We will study the details of this process of speciation later in the course. The pattern of interspecific variation on oceanic islands is called adaptive radiation which is interpreted as the consequence of multiple speciation by a single, generalized, ancestral species to produce a number of specialized, descendent species. Adaptive radiation results in variation around a common theme and Darwin was struck by the variation on different islands in the Galapagos Island group of finches and tortoises. The finches have an overall similar appearance, hence share a close phylogenetic relationship, but differ in beak shape indicating that they have adapted along different lines in the type of food they eat. Darwin reflected upon this pattern of spatial variation and made a momentous intellectual leap: if closely related species can vary over space, why can't they vary over time (evolve) to produce the observed pattern of spatial variation? The fact that these finches are found nowhere else in the world, have diversified into a large number of species, and inhabit a rather unique geographic formation (an archipelago) strongly suggested to Darwin the feasibility of evolution. Perhaps even more striking is the adaptive radiation of honey creeper and fruit fly species in the Hawaiian Island chain. The Hawaiian Islands were formed as the Pacific plate moved over a hot spot under the ocean floor which produced volcanoes by the upwelling of magma. The most northerly of these islands are not only the oldest, but also the flattest due to the effects of erosion after their formation and subsequent drifting from the hot spot. The "big island" or Hawaii is the newest of the archipelago, although an even younger one is now forming under the surface of the ocean south of Hawaii. The different Hawaiian islands harbor a variety of unique birds, called honey creepers, which also are found nowhere else in the world. Fossil evidence indicates that far more endemic species of honey creepers evolved on the Hawaiian Islands than currently exist. The case of the fruit flies (Drosophila) is quite amazing considering the large number of species that occupy these islands relative to their diversity elsewhere in the world. Again, it appears that all these Hawaiian species have descended from one or two colonizing species through speciation after isolation. The flow of lava during volcanic eruptions isolates patches of forest in which these flies live thereby isolating them long enough to evolve into different species. Unlike the honey creepers which would not find these lava beds impediments to movement, the small fruit flies do and so are far more frequently isolated; hence the larger number of fruit fly than honey creeper species. Islands as Natural Experiments Although investigators of the past cannot manipulate the phenomena they study, they can obtain similar results by looking for natural experiments, i.e., past conditions which produced data similar to those of designed experiments. What would happen if the fauna and flora of long ago were isolated and not exposed to species which evolved later? Nature has performed such an experiment as can be seen in the animals and plants of Australia, New Zealand and Madagascar which, due to continental drift, became isolated soon after the supercontinent of Pangea broke up. These species enjoyed an evolutionary history quite separate from their relatives which lived on the major continents and so allow scientists to obtain information that would otherwise be lost to history. The existence of evolutionary novelties, e.g.,marsupial mammals and monotremes in Australia; relics of extinct, primitive groups (reptiles, frogs and plants) in New Zealand; lemurs and boas in Madagascar, is interpreted as due to the fact that they have been protected from competition through their geographic isolation and so have been saved from the fate of extinction suffered by their continental relatives. That native placental mammals are absent from the Australian region is explained by the fact that this region became isolated before the placental mammals evolved. If this is so, why is it that placental mammals coexist with marsupials in North and South America? This pattern is explained by migration between North and South America through the Isthmus of Panama - a fairly recent land connection between the two continents. In fact, the fossil record reveals far greater marsupial diversity in South America than currently exists, suggesting that the immigration of placental mammals (which evolved in Laurasia after it separated from Gondwanaland) resulted in extinction of many marsupial species due to interspecific competition. The clear exception is the opossum, an adaptable species that not only avoided competition but even thrived to migrate into North America from South America. It could be argued that the pattern of species variation described above is simply a consequence of the island habitat occupied by these species and so has no evolutionary significance. That such is not the case can be seen by comparing oceanic islands with continental islands, e.g., Great Britain, located close to the nearest continent. Continental islands possess a flora and fauna remarkably similar to the nearby continent and, unlike oceanic islands, contain many poor dispersing species, such as moles, salamanders and carnivores which find the ocean an impenetrable barrier. Continental islands were once connected to the mainland and so came to have the same species before they were isolated by changes in the level of the ocean. Thus, comparison of continental and oceanic islands allows us to conclude that the unique pattern of species variation on oceanic islands is not due simply to their status as islands. Comparative Anatomy as Evidence for Evolution An evolutionary perspective of organic diversity views all living species as endpoints in a single multibranching lineage which started some 3.5 billion years ago. The details of this tree of life (phylogeny) are largely unknown due to limitations in the fossil record discussed earlier. But sufficient fossil evidence does exist to hypothesize that living species are descendants of ancestral species which no longer inhabit the Earth. What if the fossil record did not exist? Could we ever arrive at the concept of evolution simply by observing existing species? The answer is yes due to the diversity of extant species and the fact that some are more complex than others. By assuming that rates of evolution are unequal, so that some descendants more closely resemble the common ancestor than others, and by using the principle of sequences we could arrive at the concept of evolution. Careful analysis of the structural features of living species enables us to reconstruct a hypothetical phylogeny. As we discussed earlier in the course, classification based on structural similarities and difference can reveal relationships among living species which reflect their evolutionary history or phylogeny. The pattern of similarity and difference did not just happen by chance, and if the analysis is based on homology rather than analogy, it can be interpreted as the product of descent with modification from some distant common ancestor. Hence, the degree of similarity and difference is an indirect indicator of the degree of genetic relationship between species which followed separate evolutionary paths after diverging from a common ancestor. Comparison of structural variation between different species has revealed three different patterns which have been interpreted as the product of evolution: vestigial structures, divergence and convergence. Note again that this evidence is based on an interpretation of the process which produced the patterns and that Vestigial structures During the course of evolutionary history homologous structures can change. One such change is a loss of function when selective pressures promoting the structure relax. This results in vestigial structures which serve no function and are considered to be excess baggage passed on from ancestor to descendant. The existence of such rudimentary organs as nonfunctional ear muscles, body hair and tail bones in humans is explained by evolutionists as due to genes inherited from ancestors which possessed these organs as functional structures. Boas and pythons possess spurs around their cloacas which are interpreted as vestigial hindlimbs inherited from their lizard ancestors. Vestigial structures make little sense from a creationist point of view (why design a structure with no function?) but can be explained within an evolutionary paradigm. Pattern of divergent evolution Another form of change in homologous structures is the direct product of environmental forces acting as selective pressures in producing adaptation. Such change in different but related species produces a pattern known as divergent evolution. The basic ground plan of bone arrangement in mammalian limbs has been modified by evolution to produce a variety of adaptations to different environments: flippers in whales, digging claws in moles, grasping hands in monkeys, wings in bats and streamlined legs for running in horses and antelopes. This pattern of divergence, also called adaptive radiation, reflects variation between a generalized ancestor and its specialized descendants who adapted to different ecological conditions and modes of life. On a finer scale adaptive radiation can be seen in the variety of marsupial mammals which inhabit Australia and in the diversity of Galapagos finches and Hawaiian honey creepers - all of which appear to have radiated or diverged from a common ancestor. Pattern of convergent evolution Whereas divergence can be explained by similar genes being exposed to different selective pressures, another pattern, called convergence, results when dissimilar genes are exposed to the same selective pressure. Convergent evolution produces similarity that is based on analogy and so does not reflect the degree of phylogenetic relationship suggested by the degree of superficial similarity. The similarity in body shape and appendages observed in sharks (cartilage fish), whales (mammals), swordfish (bony fish), penguins (birds) and ichthyosaurs (extinct reptiles) is due to locomotor adaptation to their common marine environment. Other examples of convergence are the possession of wings by birds, bats and the now extinct pterosaurs, and the existence of ecological equivalents among placental and marsupial mammals (moles, wolves and flying squirrels of North America and Australia). Ecological equivalents are unrelated species which possess the same adaptations in different ecological communities because they have been exposed to the same environmental pressures. The anatomical patterns of divergence, convergence and vestigial structures can be interpreted as the product of descent with modification and so provide evidence for an evolutionary origin of species diversity. Creationist interpretation Creationists also observe anatomical similarity and difference among species and argue that the differences cannot be bridged by genetic changes. They further suggest that morphological similarity is due to physical constraints imposed on living entities by the laws of physics. Some go even further by suggesting that the creator had a limited set of structural plans in mind in designing species. These plans, which they call archetypes, represent common themes upon which variations exist and so the data of comparative anatomy simply reveal the archetypal plans of the creator. As for vestigial structures, creationists deny that they are without function. They argue that further study will reveal the function, which may be unknown at present, and cite the thymus gland as an example of an organ whose function (producing antibodies) was discovered only relatively recently. Evolution and creation are different interpretations of the process which produced the anatomical patterns observed in nature. Creationists raise the objection that complex structures, perfectly adapted to promote a particular function, e.g., the vertebrate eye, could not have arisen by chance and ask how an ancestor could have survived with a less-than-perfect structure before it evolved into its final form. How did primitive mammals feed their young before the mammary gland fully evolved, if evolution rather than creation was responsible for this gland's origin? As Gould points out in his Panda principle, perfect adaptation masks the path of history and so provides more evidence for creation than for evolution. Imperfections (e.g., vestigial structures) and transitional states (e.g., the nipple-less mammary gland of monotremes) argue for evolution. Comparative Embryology as Evidence for Evolution A comparison of developmental patterns in different species reveals anomalies and similarities which are suggestive of species origin by descent with modification. Three different lines of embryological evidence are presented below. Recapitulation and the biogenetic law In 1828 Karl von Baer described ontogeny (individual developmental history) as the gradual development of specialized characters from generalized ones. He noted that the embryos of advanced species were similar to the embryos of more primitive ones during the early stages of development but depart progressively in appearance from them as development proceeds. Darwin was quick to interpret this observation as a consequence of descent with modification and so used comparative embryology as evidence for evolution. Despite the protestations of von Baer who did not agree with this interpretation, Ernst Haeckel, a staunch supporter of Darwin, carried this line of argument even further to suggest that each embryo reveals its phylogenetic history as it develops so that "ontogeny recapitulates phylogeny". This famous statement became known as the biogenetic law and for a while embryology was taken as powerful evidence in support of evolution. Later, however, severe doubt was cast on the biogenetic law, in part because Haeckel suggested that as evolution proceeds the adult stage gets pushed further back in development, and in part because with the rise of genetics no mechanism for recapitulation could be conceived. Also, Haeckel's recapitulation theory strongly suggested the Lamarckian concept of organisms ascending the ladder of life. Consequently, comparative embryology lost favor as evidence for evolution and even today reference to embryological data as suggestive of evolution is viewed with suspicion. This latter perspective is indeed unfortunate because if interpreted properly, comparative embryology does fit well within the evolutionary paradigm. We will review three patterns of comparative embryology which support an evolutionary interpretation but are quite inexplicable in terms of creationism. Haeckel's strong claim of recapitulation may have some validity after all. In a recent article in Bioscience (May 1990, Vol. 40, No. 5) Lawrence W. Swan suggests that there exists a remarkable similarity between the stages of human embryonic development and phylogenetic events occurring over geological time. When a graph of the first appearance of structural features in human development (plotted against day of development) is compared with a graph of the first appearance of the same structure in the fossil record (plotted against millions of years) a strong agreement results which, although not perfect (there exists a 20 million year discrepancy in comparison of the two curves), is suggestive of some form of recaptulation. Garstang's theory of paleogenesis The second pattern is a milder version of recapitulation suggested by Garstang in 1922 to correct Haeckel's erroneous correlation of embryonic stage sequence with adult phylogenetic stage sequence. Garstang emphasized that comparison of species should be made between similar embryonic stages, not between the embryonic stages of descendants and the adult stage of ancestors. Accordingly, he proposed his theory of paleogenesis which states that descendent ontogenies tend to recapitulate ancestral ontogenies. Garstang argued that early developmental stages are very conservative and departures during development occur in the later stages. Thus, at a very early stage in development embryos of all vertebrates are very similar with noticeable differences occurring only in the later stages. The more closely related the species, the longer will be the similarity during their developmental history. The more distantly related two species are in a phylogenetic sense, the earlier will be the developmental stage when their embryos begin to diverge in appearance. Evolutionists interpret the early embryonic similarity as a consequence of genetic similarity. Apparently, the early developmental pattern is resistent to variation so that any mutations which did occur in the past were eliminated. Consequently, those genes which influence early development are very similar in all vertebrates because deviations from them over geologic time did not produce viable individuals. Development of individual organs The final pattern of comparative embryology concerns the development of particular organs in advanced species from precursors which characterize the adult stages of primitive species. Two examples among many possible ones will serve to illustrate this pattern: kidney nephron development and aortic arch blood vessel development. Nephric tubule development in the kidney. The vertebrate kidney tubules, which filter the blood to produce urine, develop from a segmented mass of mesoderm (middle germ layer tissue) called the nephrotome. The nephrotome in mammals, birds and reptiles differentiates sequentially in three regions called the pronephros (most forward or anterior section), the mesonephros (middle section) and the metanephros (most posterior section), each of which produces tubules during development. In these three vertebrate classes the adult kidney develops from tubules produced by the metanephros while the tubules produced by the other two segments degenerate. The mesonephric tubules, however, are functional for a while in the embryo. Adult fish and amphibian kidneys develop from the mesonephros, but the pronephros also produces tubules which degenerate early in development. Finally, in the most primitive of vertebrates, the jawless cyclostomes, e.g., lamprey and hagfish, the adult kidney develops from the pronephros with no nephrotomic differentiation of mesonephric and metanephric segments. The puzzling feature of vertebrate kidney development (see Figure A) is why the pronephros develops in all but the cyclostomes, and the mesonephros develops in reptiles, birds and mammals. From an energy standpoint it seems highly inefficient to produce tubules and have them degenerate before the kidney tubules which will remain in the adult are produced. If the expression of genes inherited from common ancestors are modified in later developmental stages but not earlier ones, this pattern makes some sense. If each class of vertebrates was independently created, it makes no sense at all. Aortic arch development. Aortic arches are blood vessels which arise from the aorta after it leaves the heart and in fish supply the gills through which oxygen passes into the blood. A phylogenetic survey of vertebrates shows that arches are lost, reduced or modified in advanced vertebrates (see Figure B). Nevertheless, the full complement of fish aortic arches appears early in the development of advanced vertebrates only to be resorbed later in life. Again, why should the primitive pattern appear as a precursor only to disappear as development proceeds? These patterns of comparative embryology can be interpreted within the evolutionary paradigm to provide "evidence" for evolution. Creationists emphasize the differences among vertebrate adults and embryos and tend to downplay their similarities. Because the mechanisms behind development are still largely unknown, some creationists have suggested that development cannot be explained in terms of physiochemical process and so developmental patterns simply reflect the wisdom of the creator. Others argue that the archetypal groundplan used by the creator requires certain developmental constraints which accounts for similarities. The evolutionary interpretation presented above is heavily process-laden with such terms as selective pressures, genetic change and adaptation to a given environment and draws heavily on Darwin's theory of evolution by means of natural selection. Variation in structure between species provides evidence for evolution only in the sense that the patterns described above make sense from an evolutionary perspective. A creationist perspective sheds no light whatsoever on why these patterns exist. FIGURE A. COMPARATIVE KIDNEY DEVELOPMENT Nephrotome pronephros mesonephros metanephros become adult nephric tubules Jawless fish Bony fish & amphibians Reptiles, birds & mammals degenerate in embryo FIGURE B. AORTIC ARCH DEVELOPMENT Organ supplied by aortic arches Chordate #1 #2 #3 #4 #5 #6 amphioxus gill gill gill gill gill gill shark spiracle gill gill gill gill gill lungfish head gill gill gill gill lungs salamander ---- ---- head body reduced lungs lizard & ---- ---- head body ---- lungs bird mammal ---- ---- head ---- ---- lungs body (left side only)