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Transcript
Body parts are considered homologous if they have the same basic structure the same relationship to other body parts, and, as it turns out, develop in a similar manner in the embryo. It seems unlikely that a single pattern of bones represents the best possible structure to accomplish the functions to which these forelimbs are put. However, if we interpret the persistence of the basic pattern as evidence of inheritance from a common ancestor, we see that the various modifications are adaptations of the plan to the special needs of the organism. It tells us that evolution is opportunistic, working with materials that have been handed down by inheritance. Color the homologous bones according to the color scheme below: Green = humerus Red = radius Blue = ulna Yellow = carpels & metacarpals Purple = phlanges Many groups of organisms show similarities in appearance that cannot be explained by similarities in their functional requirements. For example, the forelimbs of a diverse group of mammals, including humans, cats, whales, and bats are all constructed from the same bones even though they perform vastly different functions. This puzzling similarity among mammals and a variety of other animals prompted Darwin to exclaim, "What could be more curious than that the hand of a man, formed for grasping, that of a mole for digging, the leg of a horse, the paddle of the porpoise, and the wing of the bat, should all be constructed on the same pattern, and should include the same bones, in the same relative positions?" Morphologically similar structures that perform different functions are called homologous structures. Perhaps the oddest kinds of homologous structures are vestigial organs, rudimentary structures that are of marginal or no use to an organism, or structures in which the shared form appears to be inefficient. Examples of vestigial structures include: Skeletons of some snakes retain the vestiges of pelvic girdle and leg bones of walking ancestors. Also, modern whales use hind limbs with bony supports. However, if we dissect a whale we find, at the appropriate place down the spine, a set of bones that are clearly homologous with the pelvis of any four-limbed vertebrate. Some blind, cave-dwelling fish have eye-sockets but no eyes. Humans have muscles that make our body hair stand on end when we get cold or excited. Did you ever wonder why we have goose bumps? Erectile fur that is important during aggressive encounters is found among mammals . Comparative Embryology Developmental biologists have discovered that closely related organisms go through similar stages during their embryonic development. For example, all vertebrate embryos go through a stage in which they have gill pouches on the sides of their throats, a post-anal tail, and segmented muscles. During the early stages of development the embryos of vertebrates are strikingly similar. However, as development continues, the various groups of vertebrates diverge considerably taking on the distinct characteristics of their classes. Among fishes, the gill pouches become gills. In humans, they develop into eustachian tubes that connect the throat and the middle ear. The embryonic similarities among vertebrates can be explained in terms of relative stasis in the environment for developing embryos, resulting in similar selection regimes. In the process of adapting to different environments, adult vertebrates have experienced different selective pressure and have undergone considerable degrees of divergence. 5. Molecular Biology Molecular biological studies have indicated that the structure of DNA and the nature of its hereditary materials are universal to all life. One way in molecular biology has confirmed this is by isolating the messenger RNA (a type of RNA that specifies a particular protein) for hemoglobin from a rabbit and injecting it into a bacterium. The bacterium does not normally make hemoglobin, but when injected with the messenger RNA for this product, it will produce rabbit hemoglobin. The bacterium is capable of manufacturing the rabbit protein because the machinery for decoding the message in the messenger RNA must be common to rabbits and the bacterium. Also, an examination of the structure of proteins and DNA for a vast diversity of organisms has revealed a high degree of similarity between organisms that are taxonomically closely related to one another. 6. Artificial selection Artificial selection refers to the selective breeding of domesticated plants (a variety of grains and vegetables that you buy at the local super market) and animals (e.g., dogs, pigeons, cattle, etc.) to encourage the occurrence of desirable traits. In artificial selection, a breeder selects the parents possessing traits that are considered desirable for each generation and culls, or destroys the undesirable varieties. If the breeder continues to select in a particular direction for an extended period of time the initial trait(s) will appear considerably different from their initial appearance. Broccoli and cauliflower have a common ancestor (wild mustard plant) and by accentuating different parts of this plant from generation to generation, plant breeders were able to generate the divergent features that are characteristic for each of these vegetables. 2. Biogeography When naturalists began to travel and study organisms from different parts of the globe, they observed interesting patterns in their geographical distributions. A number of patterns that emerged from examining the geographical distribution of species. Isolated islands harbor unique species of plants and animals. However, these organisms do bare some resemblance to species of plants and animals found on the nearest mainland. Islands with similar environments in different parts of the world do not appear to be populated with the same kinds of species. Instead, they are inhabited by species that are taxonomically closely related to the plants and the animals of the nearest mainland, where the environment is often quite different. Australia is comprised of a large number of species of marsupial mammals, but has relatively few species of placental mammals. 1. The Fossil Record The fossil record, documenting the history of life on Earth, is rather incomplete. The main reason that fossils are not too plentiful is that the conditions necessary for fossilization are very specific, depending on a linking of a series of improbable events: organisms must be buried under conditions that prevent the decay of their hard parts; organisms must be buried in sediments that subsequently become solidified into rock; rocks containing fossils must avoid erosion and metamorphosis for long periods of time; subsequently, rocks containing fossils must be eroded to expose them; the location of rocks containing fossils needs to be in an area where humans are likely to discover them. Given these precise circumstances, it may be a wonder that we find any evidence of past life forms on this planet! Despite these limitations, certain groups of organisms have left a reasonably rich fossil record, allowing scientists to characterize the patterns observed. A close examination of the fossilized organisms has uncovered an orderly history of life, with different groups originating at different times rather than all at once. For example, there is a geological succession of the different vertebrate classes, with fishes appearing first followed by the appearance of amphibians, reptiles, and mammals. The fossil record also has revealed the smooth sequence of intermediate fossil forms through geological time. Furthermore, it indicates that extinction is a common occurrence in the history of life and whether or not a group of organisms becomes extinct is independent of their emergence in the fossils record. Species that appear suddenly in younger rocks are just as likely to go extinct as fast or as slow as species that have been recovered from much older layers of rock. In addition, the fossil record has revealed the occurrence of organisms that possess characteristics that are intermediate between specific groups of organisms. These fossil organisms have sometimes been referred to as "missing links." For example, in 1861 several fossils were discovered exhibiting characteristics that were intermediate between reptiles and birds. The fossil, Archaeopteryx (pictured at left), had a number of reptilian features including teeth and a long tail, but it also had a number of birdlike features such as feathers and a wishbone. Perhaps even more revealing is the evolutionary transition to mammals from mammal-like reptilian ancestors. A classic example of the stepwise occurrence of organisms in the fossil record is provided by horses. The earliest fossils were very small and possessed four toes on its pad-footed front legs. The teeth were simple and were specialized for browsing on soft vegetation. During the approximately 60 million years since the occurrence of these early forms, horses have undergone substantial changes They became progressively larger and there was a gradual change in the configuration of the toes to the point where a single toe on the arch of the foot is used for locomotion (pictured at right). In addition, the teeth developed unique characteristics adapted for chewing hard abrasive grasses. The relationship between the first horses to appear in the fossil record and modern day horses is complete with a showcase of intermediate stages . However, it is important to understand that changes among the various forms of horses did not progress in a strictly linear fashion. There is evidence of a substantial amount of splitting or branching of groups with some individual branches maintaining fairly distinct features until they become extinct. The Nature of Science Science is a fallible enterprise. What does this mean? In part, it means that scientists, being human beings, will make mistakes. Crucial data will be overlooked, calculations will be in error, or worse, scientists will fudge their data to get a desired result. Science, in other words, is a human enterprise with all that entails. But science is fallible in a deeper sense. Science always only gives us tentative results. Science is a discipline that yields an understanding of reality given all available evidence. But of course the available evidence is not all the evidence. There could be another experiment, a new discovery, refined instruments, etc., any of which might prompt a revision of the current scientific understanding of reality. So, even when scientists avoid the errors mentioned above, scientific claims are always subject to revision in light of new evidence. (Once again, being human, scientists will sometimes claim their views are certain. To say this is to go beyond the limits of science.) Put another way, science does not deliver absolute, certain, indubitable truth. Neither, however, does science deliver mere conjecture, nor a view of reality with nothing to support it but blind faith. Certain truth and blind faith represent two ends of a continuum. Most beliefs we have lie somewhere between these two poles. One great value of science is that it provides us with methods by which we can arrive at justified beliefs (i.e., beliefs nearer the certain end of the continuum). The methods available are as rich and varied as the sciences themselves--there really is no such thing as the scientific method--and the results these methods yield at any moment may be of greater or lesser significance. Consequently, it is perhaps better to speak of scientific theories as being confirmed or disconfirmed, and these may each be to a greater or lesser degree. Let us briefly explore the nature of confirmation. One important way in which theories are confirmed is by making successful predictions. The classic example of this is the triumphant discovery of Neptune. Newton's celestial mechanics were at odds with the observed orbit of Uranus. That is, given what was known at the time, Newton's laws predicted a different orbit for Uranus than was observed. At this point, one might suspect a disconfirming observation: Newton's theory got it wrong. But in the face of a failed prediction, one need not reject a theory! Instead, one might seek an explanation of the failure. This is just what John Adams and Urbain Leverrier did. These scientists hypothesized (independently) an unseen planet beyond Uranus. The existence of such a celestial body would then bring the observed orbit of Neptune into accord with Newton's laws. Using Newton's laws, the position of the planet was calculated, and Neptune was discovered right where Newton's laws predicted it would be. A stunning confirmation! Remarkably, there was a second opportunity for Newton's theory to shine as it had with Neptune. Like Uranus, the planet Mercury was discovered to deviate from the expected path given Newton's laws. Scientists again hypothesized an as yet undiscovered planet the existence of which would explain Mercury's observed deviation. This time, however, no such planet was discovered. Newton's theory failed in its prediction. Predictions, however, are not the sole test of a theory. Indeed, perhaps the most important test is the explanatory scope of a theory. That is, scientists are not simply in the business of collecting facts and charting predictive successes and failures. Science is a much more active enterprise seeking to unite these facts by explaining how the world is such that these facts might be expected. Newton's theory was remarkable for its capacity to unify what had heretofore been a disparate set of phenomena; the motion of celestial bodies was to be explained by the very same laws as the falling of an apple! In the end, what undermines a theory is not a false prediction, but a better successor theory (i.e., one that does a better job of explaining the facts). Thus, the mere failure to find a hidden planet in the case of Mercury did not lead to a rejection of Newton's theory. Mercury's motion was considered an unexplained anomaly until Einstein's theory came along and explained it. Newton's theory turned out to be false. Nonetheless, it was highly confirmed, and he and the rest of humanity were well justified in believing it (even in the face of failed predictions) until Einstein came along. Consider an analogy to a court of law. In a trial, the jury is presented with a complex set of facts, or evidence. Their task is to make sense of the evidence, to seek an understanding of how these facts can best be explained. Similarly, scientists seek to understand how the world works such that the observed facts are to be expected. Unlike jurors, however, scientists are not limited in their capacity to test their explanations as jurors are. Moreover, scientists must defend their explanations to a public community of scientists who have access to the very same resources. That is, while jurors can (and often do) justify their verdict based on gut instinct or feelings, scientists must appeal to publicly available evidence. Consequently, good scientific explanations are testable. (It is of singular importance that the testing of theories must bear a degree of independence from the theories themselves. The hypothesis that there was a hidden planet whose existence explained the orbit of Uranus needed testing beyond the observation of the orbit of Uranus, and it got it--by telescopic observation of the planet Neptune.) Such explanations generate predictions and raise questions that lead to new investigations. In the end, these investigations may lead to new and better theories (as was ultimately the case with Newton's celestial mechanics), but along the way the investigations may also offer confirming evidence of the theory (as was also true with Newton's theory in the case of Uranus). There is a further analogy with law. Sometimes in a court of law, there is nobody alive who witnessed the crime for which the jury sits in judgement; they are forced to arrive at a reasonable explanation of an event for which there is no eye-witness. Similarly, scientists often seek to explain that which no living person can observe. For example, geologists indicate that the Grand Canyon was formed by the erosive action of the flowing water over millions and millions of years. In one sense, we cannot test this hypothesis because we cannot directly observe the effects of water erosion over the period of time that is supposedly needed to produce the end result. However, we can measure erosion of sedimentary rocks over a much shorter period of time (months or years), and infer that if this process were extended over millions of years it could produce the patterns of erosion consistent with those that we find in the Grand Canyon. It is a circumstantial case, and scientists make use of this pattern of reasoning regularly. As you read through the evidence section of this site, you will see just how extraordinarily well Darwin's theory of evolution by natural selection fares. It is certainly one of the most highly confirmed theories science has ever enjoyed. It is unifying, predictively successful and certainly has and continues to raise challenging questions for investigation. Does its success in these respects show it to be certain? No. While we have every reason to believe it is true, and so are justified in believing it, the same thing once was true of Newton's theory. Perhaps, then, there is an Einstein to Darwin's Newton awaiting us in the future. In the meantime, let us explore the extent to which evolutionary theory is confirmed by the evidence currently available. Introduction http://www2.evansville.edu/evolutionweb/evidence.html Modern biologists recognize that evolution is the process that has transformed life on Earth from its early beginnings to the diversity of forms that characterizes it today. Evolution is regarded as the most pervasive concept in biology, providing deeper meaning and understanding to all aspects of the discipline. The celebrated geneticist, Theodosius Dobzhansky, vividly captured the essence of this perspective when he said "nothing in biology makes sense except in the light of evolution." Our modern understanding of the evolution of life began in 1859, with the publication of Charles Darwin's On the Origin of Species. In this book, Darwin presented a convincing case for evolution. He argued that species were not specially created in their present forms, but had evolved from ancestral species. Darwin coined the phrase "descent with modification" to encapsulate the idea that all organisms on this planet are related to one another through descent from an unknown ancestor that lived in the remote past. A critical component to Darwin's concept of evolution was his proposal of a mechanism to explain how species change through time and how they become better adapted to their local environments. Many of Darwin's predecessors suggested that life on Earth had evolved, but their theories were speculative and failed to adequately explain how the evolutionary process operates. The mechanism for evolution proposed by Darwin is called natural selection. According to Darwin's concept of natural selection, populations change over time because individuals with certain heritable traits leave more offspring than others. Interestingly, while some form of descent with modification was readily accepted by most of the scientific community, Darwin’s explanation for this process (natural selection) was being rejected. Some of the main objections to natural selection were: it lacked an explanatory theory of heredity; it attributed evolution to chance; and it could not explain all the patterns (e.g., gaps between forms) that were evident in the fossil record. Darwin also indicated that evolution proceeded as a branching tree; new species arise from small populations that branch from an ancestral species.. Although Darwin's theory of evolution by means of natural selection was a profound interpretation of the natural world, it was also a rather simple one. Thomas Huxley, who became Darwin's main public defender, is reported to have said, "How extremely stupid not to have thought of that!" The essence of Darwin's theory can be summarized as follows: There is a high degree of heritable variation in most natural populations. That is, populations exhibit a substantial amount of variation in form and function and this variation is passed on through the generations. Populations typically produce more offspring than can survive and reproduce successfully. The survival and reproductive success of individuals in a population are not equal. If some offspring have traits that give them an advantage under a particular set of conditions those organisms will be more likely to survive and pass on those traits to future generations. As the differences accumulate from generation to generation, this will lead to gradual change in the population such that populations of organisms will diverge from their ancestors, becoming reproductively isolated and eventually forming new species. Behind Darwin's rather simple explanation for evolutionary change and organismal adaptations was a substantial amount of evidence supporting his claims. In the 140 year period since the publication of On the Origin of Species, evidence in support of Darwin's theory of evolution continues to accrue. This essay is intended to present evidence for an evolutionary way of life. Before the evidence of evolution is presented, it will be useful to introduce a short discussion on science and the scientific method. There are certain criteria and avenues of inquiry that scientists must adhere to when attempting to interpret the natural world objectively. We certainly want to be sure that the evidence used to support evolution is in compliance with them. Indeed, many of the objections to modern evolutionary theory, result from misunderstandings about the nature of science and the scientific method. NEXT The coelacanth is a "living fossil" with much the same limb bone structure as the fish which gave rise to terrestrial vertebrates. The bones of this fish are homologous to the limb bones of all terrestrial vertebrates. Through reduction, duplication and/or modification, evolutionary pressures have given rise to organisms which have diverged to occupy different ecological niches. The fin of the coelacanth, the leg of the frog and the wing of the bird indicate evolutionary relationships between these organisms.