Thoeries of Evolution

Thoeries of Evolution Evolution is the process by which living organisms originated on earth and have changed their forms to adapt to the changing environment. The earliest known fossil organisms are the single-celled forms resembling modern bacteria; they date from about 3.4 billion years ago. Evolution has resulted in successive radiations of new types of organisms, many of which have become extinct, but some of which have developed into the present fauna and flora of the world (Wilson 17). Evolution has been studied for nearly two centuries. One of the earliest evolutionists was Jean Baptiste de Lamarck, who argued that the patterns of resemblance found in various creatures arose through evolutionary modifications of a common lineage. Naturalists had already established that different animals are adapted to different modes of life and environmental conditions; Lamarck believed that environmental changes evoked in individual animals direct adaptive responses that could be passed on to their offspring as inheritable traits. This generalized hypothesis of evolution by acquired characteristics was not tested scientifically during Lamarck's lifetime. A successful explanation of evolutionary processes was proposed by Charles Darwin. His most famous book, On the Origin of Species by Means of Natural Selection (1859), is a landmark in human understanding of nature. Pointing to variability within species, Darwin observed that while offspring inherit a resemblance to their parents, they are not identical to them. He further noted that some of the differences between offspring and parents were not due soley to the environment but were themselves often inheritable. Animal breeders were often able to change the characteristics of domestic animals by selecting for reproduction those individuals with the most desirable qualities. Darwin reasoned that, in nature, individuals with qualities that made them better adjusted to their environments or gave them higher reproductive capacities would tend to leave more offspring; such individuals were said to have higher fitness. Because more individuals are born than survive to breed, constant winnowing of the less fit-a natural selection-should occur, leading to a population that is well adapted to the environment it inhabits. When environmental conditions change, populations require new properties to maintain their fitness. Either the survival of a sufficient number of individuals with suitable traits leads to an eventual adaptation of the population as a whole, or the population becomes extinct. Evolution proceeds by the natural selection of well-adapted individuals over a span of many generations, according to Darwin's theory(Microsoft 96). The parts of Darwin's theory that were the hardest to test scientifically were the interferences about the heritability of traits because heredity was not understood at that time. The basic rules of inheritance became known to science during the turn of the century, when the earlier genetic works of Gregor Mendel came to light. Mendel had discovered that characteristics are transmitted across generations in discrete units, known as genes that are inherited in a statistically predictable fashion. The discovery was then made that inheritable changes in genes could occur spontaneously and randomly without regard to the environment. Since mutations were seen to be the only source of genetic novelty, many geneticists believed that evolution was driven onward by the random accumulationof favorable mutation changes. Natural selection was reduced to a minor role by mutationist such as Vries. Morgan, and Bates. While mutation was replacing Darwinism, the leading evolutionary theory, the science of population genetics was being founded by Sewall Wright, J.B.S. Haldine, and several other geneticists, all working independantly. They developed arguments to show that even when a mutation that is immediately favored appears, its subsequent spread within a population depends on such variables as the following: the size of the population the length of generations the degree to which the mutation is favorable the rate at which the same mutation reappears in descendants Furthermore, a given gene is favorable only under certain environmental conditions. If conditions change in space, then the gene may be favored only in a localized part of the population; if conditions change over time, the gene may become generally unfavorable. Because different individuals usually have different assortments of genes, the total number of genes available for inheritance by the next generation can be large, forming a vast store of genetic variability. This is called the gene pool. Sexual reproduction ensures that the genes are rearranged in each generation, a process called recombonation. Mutations provide the gene pool with a continuous supply of new genes; through the process of natural selection the gene frequencies change so that advantageous genes occur in greater proportions(Ardrey 24). As the new evolutionary theory became enriched from such diverse sources, it became known as the synthetic theory. Three American scientists made controbutions that were especially important. The German-born Ernst Mayr, a zoologist, showed that new species usually arise in geographic isolation, often following a genetic turn that quickly changes the contents of their gene pools. George Simpson, a paleontologist, showed from the fossil record that rates and modes of evolution are correlated. G. Ledyard Stebbins, a botanist, showed that plants display evolutionary patterns similar to those of animals, and especially that plant evolution has demonstrated diverse adaptive responses to environmental pressures and opportunities. In additon, these biologists reviewed a broad range of genetic, ecological, and systematic evidence to show that the synthetic theory was strongly supported by observation and experiment. During the establishment of the synthetic theory of evolution, the science of heredity underwent another drastic change in 1953, when James Watson and Francis Crick demonstrated the way genetic material is composed of two nucleic acids, deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Nucleic acid molecules contain genetic codes that dictate the manufacture of proteins, and the latter direct the biochemical pathways of development and metabolism in an organism. Natural selection can then operate to favor or supress a particular gene according to how strongly its protein product contributes to the reproductive success of the organism. Life originated more than 3.4 billion years ago, when the earth's environment was much different than that of today. Especially important was the lack of significant amounts of free oxygen in the atmosphere. Experiments have shown that rather complicated organic molecules, including amino acids, can arise spontaneously under conditions that are believed to simulate the earth's primitive environment. The earliest organisms that still exists would be cells, resembling modern bacteria. These simple unicellular forms(procaryotes) were at first anaerobic, but they diversified into and array of adaptive types from which blue-green algae descended, including aerobic photosynthesizers. Advnced cells (eucaryotes) may have evolved through the amalgamation of a number of distinct simple cell types. A large ingesting cell may have incorporated as symbionts some small blue-green algal cells that evolved into chloroplast and some tiny aerobic bacteria that evolved into mitochondria(Reader 45). In order for complex animal communities to develop, plants must first become established to support herbivore populations, which in turn may support predators and scavengers. Land plants appeared about 400 million years ago, spreading from lowland swamps as expanding greenbelts(Gribbon 208). Dinosaurs and mammals shared the terrestrial environment for 135 million years. Dinosaurs may well have been more active, and certainly were larger, than their mamalian contemporaries, which were small and pssibly nocturnal. The mammals, however, survived a wave of extinction that eliminated dinosaurs about 65 million years ago, and subsequently diversified into many of the habitats and modes of life that formerly had been dinosaurian(Gribbon 211). Humans belong to an order of mammals, the primates, which existed before the dinosaurs became extinct. Early primates seem to have been tree dwelling and may have resembled squirrels in their habitats. Many of the primate attributes, the short face, overlapping visual fields, grasping hands, large brains, and even alertness and curiosity, must have been acquired as arboreal adaptations. Descent from tree habitats to forest floors and eventually to more open country, however, was associated with the development of many of the unique features of the human primate, including erect posture and reduced canine teeth, which suggest new habitats of feeding(Schwartz 78). The history of life as inferred from the fossil record displays a wide variety of trends and patterns. Lineages may evolve slowly at one time and rapidly at another time, they may follow one pathway of change for sometime only to switch to another pathway, and they may diversify rapidly at one time and then shrink under widespread extinctions. The key to many of these patterns is the rate and nature of environmental change. Species become adapted to the environmental conditions that exist at a given time, and when change leads to new conditions, they must evolve new adaptations or become extinct. When the environment undergoes a particularly rapid or extensive change, waves of extinction occur. These are followed by waves of development of new species. The times of mass extinction are not yet well understood. Although the most famous one is that of the dinosaurs, about 65 million years ago, such events appear in the fossil record as far back as Precambrian time, when life first arose. In fact, five mass extinctions on the scale of that at the end of the age of dinosaurs are known over the past 600 million years. Some scientists also claim to have demonstrated a definite periodicity to smaller periods of mass extinction, and in particular a 26-million-year cycle of eight extinctions over the past 250 million years(Wilson 34). Controversy has arisen over the proposal made by some geologists that mass extinctions are related to periodic catastrophes such as the striking of the earth's surface by a large asteroid or comet. Many paleontologists and evolutionary theorists reject such hypotheses as unjustified. The feel that periods of mass extinctions can be accounted for by less spectacular evolutionary processes and by more earthbound events such as cycles of climatic change and volcanic activity. Whatever proposals may eventually prove true, however, it seems fairly certain that periodic waves of mass extinction do occur. Species adapted to live in environments that are changeable in the short term have broad tolerances, which may better enable them to survive extensive changes. Human beings are uniquely adapted in that they make and use tools and devices and invent and propogate procedures that give them extended control over their environments. Humans are significantly changing the environment itself. The effects are most complex and cannot be predicted, and yet like the likelihood is that evolutionary patterns in the future will reflect the influence of the human species(Microsoft96).