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Friday, October 7, 2011

Evolution

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A long path leads from the origins of primitive ?life,? which existed at least .5 billion years ago, to the profusion and diversity of life that exists today. This path is best understood as a product of evolution. Contrary to popular opinion, neither the term nor the idea of biological evolution began with Charles Darwin and his foremost work, On the Origin of Species by Means of Natural Selection (185). Many scholars from the ancient Greek philosophers on had inferred that similar species were descended from a common ancestor. The word ?evolution? first appeared in the English language in 1647 in a non-biological connection, and it became widely used in English for all sorts of progressions from simpler beginnings. Some common errors about the theory of evolution are illustrated by such questions as ?If man evolved from the apes, then why are there still apes?? The theory of evolution is not equivalent to Darwinism. The term Darwin most often used to refer to biological evolution was ?descent with modification,? which remains a good brief definition of the process today. The extensive evidence in support of both fact and theory of evolution is an interrelated set of now well confirmed hypotheses including descent with modification, natural selection, and genetics as being the mechanisms behind the evolutionary process itself.


The descent with modification component of evolutionary theory is that all life forms can trace their lineages back to earlier classes of life forms in a branching, ?nested? hierarchy (forming what looks like a bush or tree, as in the ?Tree of Life?), which can ultimately be traced back to the beginning of life on earth (a point that would likely have been the culmination of a long period during which the distinction between living and non-living matter would have been difficult to make). During the long period since that time, changes in body plans have accumulated in diverse directions to make all the differences we now see between all life forms. To continue the tree analogy, a person can start from the tip of any arbitrarily chosen twig and follow it back to the point where it joins another twig, the ?common ancestor? of the two (or more) twigs. That now thicker twig or branch can then be traced back to where it joins another branch. This process can be repeated until that person ultimately reaches the trunk. Applied to evolution, each twig and branch represents a particular lineage; the points where there is a joining of those twigs and branches represents the common ancestor of all the lineages that can be traced back to that point. Note that the descent with modification hypothesis can be tested independently of any ideas about how it happened, the mechanism behind it. Of course, the mechanism introduced by Darwin was that of natural selection (along with the additional view that natural selection proceeded in a gradual, steady fashion).


Darwin proposed that evolution could be explained by the differential survival of organisms following their naturally occurring variation, a process he termed ?natural selection.? According to this view, the offspring of organisms differ from one another and from their parents in ways that are heritable; that is, they can pass on the differences genetically to their own offspring. Furthermore, organisms in nature typically produce more offspring than can survive and reproduce given the constraints of food, space, and other environmental resources. If a particular off spring has traits that give it an advantage in a particular environment, that organism will be more likely to survive and pass on those traits. As differences accumulate over generations, populations of organisms diverge from their ancestors.


A particularly compelling example of speciation involves the 1 species of finches studied by Darwin on the Galapagos Islands, now known as Darwin?s finches. The ancestors of these finches appear to have emigrated from the South American mainland to the Galapagos. Today the different species of finches on the island have distinct habitats, diets, and behaviors, but the mechanisms involved in speciation continue to operate. A single year of drought on the islands can drive evolutionary changes in the finches. Drought diminishes supplies of easily cracked nuts but permits the survival of plants that produce larger, tougher nuts. Droughts thus favor birds with strong, wide beaks that can break these tougher seeds, producing populations of birds with these traits. It has been estimated that if droughts occur about once every 10 years on the islands, a new species of finch might arise in only about 00 years.


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Darwin?s original hypothesis has undergone extensive modification and expansion, but the central concepts stand firm. Studies in genetics and molecular biology, fields unknown in Darwin?s time, have explained the occurrence of the hereditary variations that are essential to natural selection. Genetic variations result from changes, or mutations, in the nucleotide sequence of DNA, the molecule that genes are made from. Such changes in DNA now can be detected and described with great precision.


Genetic mutations arise by chance. They may or may not equip the organism with better means for surviving in its environment. But if a gene variant improves adaptation to the environment (for example, by allowing an organism to make better use of an available nutrient, or to escape predators more effectively, such as through stronger legs or disguising coloration), the organisms carrying that gene are more likely to survive and reproduce than those without it. Over time, their descendants will tend to increase, changing the average characteristics of the population. Although the genetic variation on which natural selection works is based on random or chance elements, natural selection itself produces ?adaptive? change; the very opposite of chance.


Evolutionary change continues to this day, and it will proceed so long as life itself exists. In recent years, many bacterial pathogens have evolved resistance to antibiotics used to cure infections, thereby requiring the development of new and more costly treatments. In some frightening cases, bacteria have evolved resistance to every available antibiotic, so there is no longer any effective treatment. In the case of HIV, which causes AIDS, significant viral evolution occurs within the course of infection of a single patient, and this rapid evolution enables the virus to evade the immune system. Many agricultural pests have evolved resistance to chemicals that farmers have used for only a few decades. As we work to control diseases and pests, the responsible organisms have been evolving to escape our controls. Moreover, scientists can perform experiments to study evolution in real time, just as experiments are used to observe dynamic processes in physics, chemistry, and other branches of biology. To study evolution in action, scientists use organisms like bacteria and fruit flies that reproduce quickly, so they can see changes that require many generations.


The extensive evidence in support of both fact and theory of evolution comes primarily from studies of descent with modification, natural selection, and genetics. Evolutionary biology is a strong field of science. A theoretical framework that encompasses several basic mechanisms is consistent with the patterns seen in nature; and there is abundant evidence demonstrating the action of these mechanisms as well as their contributions to nature. Hence, evolution is both a theory and a set of established facts that the theory explains. Like every other science, there is scientific debate about some aspects of evolution, but none of these debates appear likely to shake the foundations of this field. There exists no other scientific explanation that can account for all the patterns in nature, only non-scientific explanations that require a miraculous force, like a creator. Such super-natural explanations lie outside of science, which can neither prove nor disprove miracles. Science provides us with a compelling account and explanation of the changing life on Earth.


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