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Thoeries of Evolution Evolution is the process by which living organisms originated on earthand have changed their forms to adapt to the changing environment. The earliestknown fossil organisms are the single-celled forms resembling modern bacteria;they date from about 3.4 billion years ago. Evolution has resulted insuccessive radiations of new types of organisms, many of which have becomeextinct, but some of which have developed into the present fauna and flora ofthe world (Wilson 17). Evolution has been studied for nearly two centuries. One of theearliest evolutionists was Jean Baptiste de Lamarck, who argued that thepatterns of resemblance found in various creatures arose through evolutionarymodifications of a common lineage. Naturalists had already established thatdifferent animals are adapted to different modes of life and environmentalconditions; Lamarck believed that environmental changes evoked in individualanimals direct adaptive responses that could be passed on to their offspring asinheritable traits.
This generalized hypothesis of evolution by acquiredcharacteristics was not tested scientifically during Lamarck's lifetime.A successful explanation of evolutionary processes was proposed byCharles Darwin. His most famous book, On the Origin of Species by Means ofNatural Selection (1859), is a landmark in human understanding of nature.Pointing to variability within species, Darwin observed that while offspringinherit a resemblance to their parents, they are not identical to them. Hefurther noted that some of the differences between offspring and parents werenot due soley to the environment but were themselves often inheritable. Animalbreeders were often able to change the characteristics of domestic animals byselecting for reproduction those individuals with the most desirable qualities.Darwin reasoned that, in nature, individuals with qualities that made thembetter adjusted to their environments or gave them higher reproductivecapacities would tend to leave more offspring; such individuals were said tohave higher fitness. Because more individuals are born than survive to breed,constant winnowing of the less fit-a natural selection-should occur, leading toa population that is well adapted to the environment it inhabits.
Whenenvironmental conditions change, populations require new properties to maintaintheir fitness. Either the survival of a sufficient number of individuals withsuitable traits leads to an eventual adaptation of the population as a whole, orthe population becomes extinct. Evolution proceeds by the natural selection ofwell-adapted individuals over a span of many generations, according to Darwin'stheory(Microsoft 96). The parts of Darwin's theory that were the hardest to testscientifically were the interferences about the heritability of traits becauseheredity was not understood at that time. The basic rules of inheritance becameknown to science during the turn of the century, when the earlier genetic worksof Gregor Mendel came to light.
Mendel had discovered that characteristics aretransmitted across generations in discrete units, known as genes that areinherited in a statistically predictable fashion. The discovery was then madethat inheritable changes in genes could occur spontaneously and randomly withoutregard to the environment. Since mutations were seen to be the only source ofgenetic novelty, many geneticists believed that evolution was driven onward bythe random accumulationof favorable mutation changes. Natural selection wasreduced 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. Theydeveloped arguments to show that even when a mutation that is immediatelyfavored appears, its subsequent spread within a population depends on suchvariables as the following:the size of the populationthe length of generationsthe degree to which the mutation is favorablethe rate at which the same mutation reappears in descendants Furthermore, a given gene is favorable only under certain environmentalconditions. If conditions change in space, then the gene may be favored onlyin a localized part of the population; if conditions change over time, the genemay become generally unfavorable.
Because different individuals usually havedifferent assortments of genes, the total number of genes available forinheritance by the next generation can be large, forming a vast store of geneticvariability. This is called the gene pool. Sexual reproduction ensures thatthe genes are rearranged in each generation, a process called recombonation.Mutations provide the gene pool with a continuous supply of new genes; throughthe process of natural selection the gene frequencies change so thatadvantageous 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 madecontrobutions that were especially important. The German-born Ernst Mayr, azoologist, showed that new species usually arise in geographic isolation, oftenfollowing a genetic turn that quickly changes the contents of their gene pools.George Simpson, a paleontologist, showed from the fossil record that rates andmodes of evolution are correlated. G.
Ledyard Stebbins, a botanist, showedthat plants display evolutionary patterns similar to those of animals, andespecially that plant evolution has demonstrated diverse adaptive responses toenvironmental pressures and opportunities. In additon, these biologistsreviewed a broad range of genetic, ecological, and systematic evidence to showthat the synthetic theory was strongly supported by observation and experiment. During the establishment of the synthetic theory of evolution, thescience of heredity underwent another drastic change in 1953, when James Watsonand Francis Crick demonstrated the way genetic material is composed of twonucleic acids, deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Nucleicacid molecules contain genetic codes that dictate the manufacture of proteins,and the latter direct the biochemical pathways of development and metabolism inan organism. Natural selection can then operate to favor or supress aparticular gene according to how strongly its protein product contributes to thereproductive success of the organism.
Life originated more than 3.4 billion years ago, when the earth'senvironment was much different than that of today. Especially important was thelack of significant amounts of free oxygen in the atmosphere. Experiments haveshown that rather complicated organic molecules, including amino acids, canarise spontaneously under conditions that are believed to simulate the earth'sprimitive environment. The earliest organisms that still exists would be cells, resemblingmodern bacteria. These simple unicellular forms(procaryotes) were at firstanaerobic, but they diversified into and array of adaptive types from whichblue-green algae descended, including aerobic photosynthesizers. Advnced cells(eucaryotes) may have evolved through the amalgamation of a number of distinctsimple cell types.
A large ingesting cell may have incorporated as symbiontssome small blue-green algal cells that evolved into chloroplast and some tinyaerobic bacteria that evolved into mitochondria(Reader 45). In order for complex animal communities to develop, plants must firstbecome established to support herbivore populations, which in turn may supportpredators 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 millionyears. Dinosaurs may well have been more active, and certainly were larger,than their mamalian contemporaries, which were small and pssibly nocturnal.
Themammals, however, survived a wave of extinction that eliminated dinosaurs about65 million years ago, and subsequently diversified into many of the habitats andmodes of life that formerly had been dinosaurian(Gribbon 211). Humans belong to an order of mammals, the primates, which existed beforethe dinosaurs became extinct. Early primates seem to have been tree dwellingand may have resembled squirrels in their habitats. Many of the primateattributes, the short face, overlapping visual fields, grasping hands, largebrains, and even alertness and curiosity, must have been acquired as arborealadaptations. Descent from tree habitats to forest floors and eventually to moreopen country, however, was associated with the development of many of the uniquefeatures 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 widevariety of trends and patterns. Lineages may evolve slowly at one time andrapidly at another time, they may follow one pathway of change for sometimeonly to switch to another pathway, and they may diversify rapidly at one timeand then shrink under widespread extinctions. The key to many of these patterns is the rate and nature ofenvironmental change.
Species become adapted to the environmental conditionsthat exist at a given time, and when change leads to new conditions, they mustevolve new adaptations or become extinct. When the environment undergoes aparticularly rapid or extensive change, waves of extinction occur. These arefollowed by waves of development of new species. The times of mass extinctionare not yet well understood. Although the most famous one is that of thedinosaurs, about 65 million years ago, such events appear in the fossil recordas far back as Precambrian time, when life first arose.
In fact, five massextinctions on the scale of that at the end of the age of dinosaurs are knownover the past 600 million years. Some scientists also claim to havedemonstrated a definite periodicity to smaller periods of mass extinction, andin particular a 26-million-year cycle of eight extinctions over the past 250million years(Wilson 34). Controversy has arisen over the proposal made by some geologists thatmass extinctions are related to periodic catastrophes such as the striking ofthe earth's surface by a large asteroid or comet. Many paleontologists andevolutionary theorists reject such hypotheses as unjustified. The feel thatperiods of mass extinctions can be accounted for by less spectacularevolutionary processes and by more earthbound events such as cycles of climaticchange and volcanic activity.
Whatever proposals may eventually prove true,however, it seems fairly certain that periodic waves of mass extinction dooccur. Species adapted to live in environments that are changeable in the shortterm have broad tolerances, which may better enable them to survive extensivechanges. Human beings are uniquely adapted in that they make and use tools anddevices and invent and propogate procedures that give them extended control overtheir environments. Humans are significantly changing the environment itself.The effects are most complex and cannot be predicted, and yet like thelikelihood is that evolutionary patterns in the future will reflect theinfluence of the human species(Microsoft96).Works CitedArdrey, Robert. The Hunting Hypothesis: A Personal Conclusion Concerning the Evolutionary Nature of Man. New York: Antheneum, 1976.Encarta 96.
Computer Software. Microsoft, 1995.Gribbon, John and Cherfas, Jeremy. The Monkey Puzzle: Reshaping theEvolutionary Tree. Philly: Pantheon, 1982.Reader, John. Missing links: The Hunt for Earliest Man.
Boston: Little,1981Schwartz, Jeffery H. The Red Ape: Orang-Utans and Human Origins. San Francisco: Houghton, 1987.Wilson, Peter J. The Domestication of the Human Species. Oxford: Yale, 1991.
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