MACROEVOLUTION: SPECIES DIVERSITY AND EXTINCTION The process of speciation is the dividing line between two different areas of evolutionary study: microevolution and macroevolution. Microevolution, which we have already discussed, deals with changes in allelic frequency within a single population and so is the province of neontologists who study living species, e.g., population geneticists and evolutionary ecologists. Macroevolution, on the other hand, is the study of phylogenetic patterns which occur over long periods of time in taxa classified above the level of species; it is the province of paleontologists who study the evolutionary fate of species and their descendants. Neontologists and paleontologists view species differently within the context of the biological species concept (BSC). Neontologists view species as varying geographically across a single time dimension and so concentrate on variation in space rather than over time. Ernst Mayr emphasized this spatial- variation version of the BSC to explain how geographic variation among different populations of a species could lead to the evolution of a new species through the process of speciation. Mayr's peripatric speciation model, when projected into a time dimension, could explain the sudden origin of species during geologic time. Paleontologists view species as varying over time and ignore or minimize spatial variation, largely because their samples are too small to assess geographic variation. George Gaylord Simpson emphasized this temporal-variation version of the BSC to explain how species could change over time in the process of phyletic evolution. Since natural selection is believed to be the guiding force behind phyletic evolution, Simpson's view is consistent with that of Darwin's view of the origin of species. Strictly speaking, the BSC projects a multi- dimensional view of species which can change over both time and space. Each of these two versions emphasizes only one dimension and the difference between the two versions has contributed to a number of controversies that exist today in evolutionary biology. These controversies, like the two versions of the BSC described above, deal with a matter of emphasis and so are not mutually exclusive. The core of these controversies centers around the question of which microevolutionary force is responsible for shaping macroevolutionary phenomena. Your understanding of the major features of this course will be enhanced if you integrate the following concepts: (1) Mayr's spatial-variation version of the BSC with the peripatric model of speciation , punctuationalism, and a significant role in evolution for chance events due to mutation and genetic drift; and (2) Simpson's temporal- variation version of the BSC with phyletic evolution, gradualism, and the primacy of natural selection in producing adaptation (Darwinism). Gradualism vs Punctuationalism Paleontologists obtain their data from fossils. The fossil record is the only direct window to the past, but it is very incomplete and biased due to the vagaries of fossilization and collecting effort. Since more than 99% of all of the species known to have inhabited our planet are currently extinct, the problem of extinction is a major area of interest to macroevolutionists. Another area of focus is the relative importance of phyletic evolution and speciation in contributing to phylogeny. Is phylogeny primarily the result of gradual transformation of single species over time (phyletic evolution), or is it the product of speciation which increases the number of species over time and so explains species diversity? Gradualists argue that most phylogenetic change occurs as the result of phyletic evolution; punctuationalists see speciation as the principal cause. Both patterns exist and the difference between the two patterns is illustrated below. | GRADUALISM PUNCTUATIONALISM | | G H | | D. | E. TIME | | C. . . | B. | | | A. F. |___________________________________________ GRADATIONS IN MORPHOLOGY According to the gradualist model, species B and D are transformed versions of species A and species E also evolved from species C via the process of phyletic evolution. Species C, however, arose from species B through the process of speciation. The punctuationalist model emphasizes stasis (no change in morphology) over long time periods followed by brief episodes of speciation and extinction after which stasis returns. Thus, species F exhibits no morphological change until it disappears from the fossil record (extinction) and is replaced by two daughter species (G and H) which arose through speciation. Neither of these daughter species changes significantly until they become extinct themselves. The gradualist-punctuationalist debate concerns which pattern, phyletic evolution (also called anagenesis) or speciation (also called cladogenesis), is most prevalent in the history of life. The implications of these two regarding evolutionary theory are quite different. Gradualists see evolution as the result of strict Darwinism, i.e., that change is gradual and directed by natural selection. Punctuationalists, on the other hand, regard natural selection as only one of the factors influencing evolution. Since their model is based on speciation, they are quite receptive to the idea that chance might play a significant role in phylogeny. Mayr's peripatric model of speciation provides the microevolu- tionary basis of the punctuationalist model and, as you will recall, chance factors such as the founder effect, genetic drift, and mutation in the genetic revolution play a significant role in this speciation model. Gradualists see natural selection as the major directing force in evolution, but punctuationalists (while not denying an important role for natural selection - in fact they emphasize it as a major component in the speciation process) argue that there is ample room for chance events to shape evolutionary history. Consequently, not all structures should be interpreted as being adapted to the environment, since they might be neutral by- products of chance events. Interpretation of fossil patterns The fact that some paleontologists see far more examples of stasis in that fossil record than examples of gradual transition does not provide unambiguous support for the punctuationalist model and its microevolutionary implications. The reason is that the fossil record reveals only pattern, not process; and the same pattern can be variously interpreted as regards the process which produced it. Consider the following pattern revealed by sampling and dating successive strata in a single restricted locality: species A exists at one stratum and is not found in the next stratum (dated one million years later), but a closely related species (B) with more advanced (derived) character states of traits observed in species A is discovered in this more recent stratum. How should this phylogenetic sequence of A to B be interpreted as regards mechanism? A number of possible mechanisms could be hypothesized. 1. Species A gave rise to species B through phyletic evolution. If this interpretation is correct then intermediary species should exist in strata between the two different layers. 2. Species A gave rise to species B via the process of speciation and became extinct when species B invaded the range of species A. If this hypothesis is correct it would be predicted that both species A and species B should be found in the layer between the two original strata. 3. Species A became extinct and then species B, which lived elsewhere, migrated to the locality sampled and so only appears to have evolved from species A. This hypothesis would predict that neither species should be found in the intermediary layer. Since this hypothesis suggests that both species were allopatric contemporaries before the extinction of species A, it could support a biblical creationist view of phylogeny. Likewise, the absence of either species and possible intermediaries in the intervening strata would be consistent with the view of successive acts of creation. Since the observed pattern could be interpreted in more than one way, the pattern itself cannot be used as definitive evidence to support either gradualism (hypothesis 1), punctuationalism (hypothesis 2) or even creationism (hypothesis 3). Thus, the fossil record reveals pattern, not process, and the same pattern can be explained by more than one process. Mechanism behind Macroevolution In general higher taxonomic categories appear rather suddenly in the fossil record with no transitional forms suggesting their origin. This pattern could simply be due to sampling artifacts of an incomplete fossil record, or it could suggest that new forms evolve rather rapidly from small isolated populations and so transitional forms are unlikely either to be fossilized or found due to their limited extension in time and space. Creationists would argue that neither interpretation is correct and that the sudden appearance of new higher categories is evidence of successive creation rather than of macroevolution. The real problem of macroevolutionary study is to explain the mechanism behind the origin of evolutionary novelty as it appears in the fossil record. Is it due simply to gradual genetic differences produced by mutation and later selected by natural selection (anagenesis), or is it due to rather greater structural reorganization promoted by the factors behind the process of speciation (clado- genesis)? Either of these explanations would reduce the problem to one of microevolutionary mechanism. Some evolutionary biologists suggest that the differences between higher taxa are so great that new mechanisms must be behind their sudden origin. Developmental factors which result in rather large phenotypic changes with minor genetic ones have been suggested but little evidence for this mechanism exists at present. Some punctuationalists, e.g., Stanley and Gould, have suggested that species diversity could be shaped by species selection, a form of selection analogous to natural selection with the unit being the species rather than the individual. The concept of species selection is under debate largely because it can mean different things to different paleontologists. Are some species within a clade selected because they are better adapted to the prevailing conditions (a form of differential mortality), or is the selection process due to the ability of some species to speciate more easily than other congeneric species (a form of differential reproduction)? Species selection as a process of differential extinction would seem to be reducible to the individual level - species survival is a function of individual survival. On the other hand, species selection due to differential speciation (some species leave more descendent species simply due to a higher rate of speciation) opens the possibility that many nonadaptive traits might be passed along through frequent speciation (remember that chance can play a significant role in fixing characters in isolated populations) and so species selection can be yet another source for chance events in evolution. Creationists use the inability of macroevolutionists to explain the origin of higher taxa satisfactorily to support their viewpoint. Since we have not existed long enough as a species to observe macroevolutionary phenomena (other than extinction), we must rely on very indirect evidence to explain the mechanisms behind macroevolution. The prevailing reductionist view is that microevolutionary phenomena, adjusted to allow for differences in rates of change to accommodate both gradualist and punctuationalist viewpoints, can explain macroevolutionary pattern; some macroevolutionists and certainly creationists disagree and so the origin of higher taxa is subject to much debate. Extinction of the Dinosaurs The phenomenon of extinction has attracted the recent attention of macroevolutionists, since extinction is the most common end point of evolution with about 99.9% of all known species currently extinct. The boundaries of the geologic time scale are set based on catastrophic geological upheavals and mass extinctions which have punctuated the history of our planet. The most notable extinction events occurred at the boundary between the Permian period of the Paleozoic era and the Triassic period of the Mesozoic era, between the Triassic and Jurassic periods of the Mesozoic, and between the Cretaceous period of the Mesozoic and the Tertiary period of the Cenozoic era. The Permian-Triassic (P-T) extinction was the most devastating in terms of number of species that became extinct, the Triassic- Jurassic extinction opened the door for dinosaur evolution, and the Cretaceous-Tertiary (K-T) extinction marked the celebrated demise of the dinosaurs. The cause of dinosaur extinction has fascinated paleontologists ever since the event was discovered in the fossil record. The dinosaurs were immensely successful in an evolutionary sense as they were the dominant form of vertebrate life for more than 140 million years. This fact can only be appreciated by comparing dinosaur history with our own (hominid history) which extends back only about 3.5 million years. The number of proposed explanations for this extinction event are legion and include: epidemic disease, egg predation by mammals, explosion of a supernova, high level of volcanic activity and climatic change. None of these explanations enjoyed wide support, nor were they legitimate scientific hypotheses because they could not be tested. The Alvarez Hypothesis of Dinosaur Extinction The latest explanation, proposed by Luis Alvarez et.al., suggests that the K-T extinction event was triggered by the collision with the Earth of an extraterrestrial body, either an asteroid or a comet(s). This hypothesis is far more respectable than all the others because it (1) is supported by evidence, (2) makes testable predictions, and (3) has potential general applicability in explaining other major extinction events. Support for this hypothesis comes from the observation of iridium enhancement in the clay layer which separates the limestone beds of the Cretaceous and Tertiary periods. Iridium is a rare metal of the platinum group on the periodic table and is more common in extraterrestrial bodies and cosmic dust than on the surface of our planet. The high levels observed in the clay could have been produced by (1) volcanic action which released iridium from our molten core, (2) a supernova or exploding star, or (3) the impact of an extraterrestrial body. The fact that this clay layer also contains scarred quartz crystals (shocked quartz) eliminates the first two possible explanations and supports the extraterrestrial collision hypothesis. The Alvarez hypothesis predicts (1) that iridium and shocked quartz should occur in strata before rather than after the extinction event and (2) that most, if not all, K-T boundary samples should contain evidence of iridium enhancement and/or shocked quartz crystals. So far these predictions have been verified. Periodicity of Extinctions and Nemesis In analyzing change in taxa associated with extinction, Raup and Sepkoski announced a definite periodicity associated with extinction events of approximately 26 million years. If this result is not a statistical artifact, it suggests a pattern which might link all extinctions together with a common cause. To explain this regularity Muller postulated the existence of a sister to our sun which has an orbit which approaches our solar system every 26 million years. Nemesis, as this hypothesized star is called, disturbs the Oort cloud and sends a rain of comets moving toward the sun. If by chance some of these strike the Earth, the impacts could propel enough dust into the atmosphere to darken the sky globally long enough to stress some species beyond their limits to survive. Muller's Nemesis model not only explains the regularity of extinctions by replacing Alvarez's asteroids with comets, but also makes a prediction: Nemesis will return in about 15 million years. Catastrophism vs. Uniformitarianism The discovery of the phenomenon of extinction contributed to a dynamic worldview by demonstrating that the species composition of the Earth had changed over time. What the significance of this observation was caused a debate among geologists known as catastrophism vs. uniformitarianism. The catastrophists held that Earth's history followed no pattern and was the result of unique, rare and unpredictable events which changed the appearance of the planet and caused species extinction. The uniformitarians, on the other hand, considered the past as explainable in terms of processes which were uniform over time and could be studied today. Due to the influence of Lyell and Darwin, the uniformitarians won out and extinction came to be viewed as a continuous process dominated by selective pressures imposed by the biotic (competition and predation) and abiotic (climate changes) components of the environment. By positing a single root cause of the major extinction events which appear to follow a regular cycle, the ideas of Raup, Sepkoski and Muller fit well with the uniformitarian perspective. Survival through adaptation or preadaptation Why do some species survive major extinction events? According to uniformitarians, the selectivity of extinction events can be explained by the adaptive ability of the survivors and the inability of the many species to cope genetically with the environmental stresses imposed by geological upheavals during periods of mass extinction. These episodes of extinction can be constructive in a Darwinian sense if they result in the extinction of species which have an adverse or limiting effect on the evolutionary development of other species which survive due to chance alone. For example, mammals coexisted for millions of years with the dinosaurs but never underwent extensive adaptive radiation presumably due to the pressure imposed on them by the dinosaurs. Once the dinosaurs became extinct, however, this pressure was lifted and mammals diversified to produce the wide range of species that exist today. In either case, preadapta- tion or chance survival can lead to constructive evolution in accordance with Darwin's theory of evolution. Some paleontologists refuse to accept the idea that the dinosaurs were eliminated in one fell swoop by a chance, catastrophic collision. They believe they have evidence for a gradual decline in dinosaur species diversity long before the hypothesized impact 65 million years ago. The impact, if it occurred at all, would have been the final blow to a trend towards extinction which, they argue, was due to gradual climate change and would have happened even without the celebrated impact. Chance and the history of life The history of evolutionary thought has demonstrated over and over that old ideas seldom die; they keep returning as interpretive models to explain new data. Catastrophism is no exception and has been recently revived by Stephen Jay Gould. Paleontological analysis of Precambrian fossils from Australia and elsewhere (the Ediacara assemblage) and Cambrian fossils from the Burgess Shale in British Columbia have revealed an amazing level of species diversity with many forms possessing structures whose function is totally unknown. All of these forms have become extinct, most without any recognizable descendants. One notable exception, however, is the oldest known chordate (Pikaia) whose descendants include all the vertebrates including ourselves. Gould maintains that the few species among these assemblages which survived later extinction events did so simply due to chance. He finds no compelling evidence to suggest that the surviving species possessed structures better adapted to withstand the catastrophic events which produced this extinction than those which succumbed. Since only surviving species can evolve, Gould attributes our existence to just plain luck due to the fact that Pikaia or one of its descendants did in fact survive. How different might have been evolutionary history had Pikaia become extinct and other Cambrian species survived, or if the dinosaurs had survived and mammals became extinct! Gradualism vs punctuationalism, uniformitarianism vs catastrophism, anagenesis vs cladogenesis are all related controversies in the field of macroevolution based on two different patterns of fossil species diversity: sequences with intermediaries and stasis followed by sudden change. Since both patterns exist, the debate centers around the question of which pattern is of greater importance, relative to the other, in explaining macroevolution. The implications of both explanations, however, are quite different. Gradualism emphasizes anagenesis (or phyletic evolution), the reductionist position that natural selection is the major force shaping higher category evolution, and a uniformitarian view of evolutionary history. It also is very consistent with an adaptationist view of the origin of evolutionary novelty. Punctuationalism, on the other hand, emphasizes cladogenesis (or speciation), a holistic view of selection which includes species selection, and a catastrophist view of evolutionary history. It is quite receptive to the idea that chance events play an important role in shaping evolutionary history and allows for the existence of neutral or nonadaptive phenotypic traits which may or may not be shaped later by natural selection to produce adaptive characteristics. The controversies over macroevolutionary pattern and process continue today as vigorously as in the past and (as always seems to be the case) await more definitive evidence which hopefully will be literally "unearthed" by future investigators.