РефератыИностранный языкUnUntitled Essay Research Paper BODYINTRODUCTION TO EVOLUTION

Untitled Essay Research Paper BODYINTRODUCTION TO EVOLUTION

Untitled Essay, Research Paper


BODYINTRODUCTION TO EVOLUTION


What is Evolution? Evolution is the process by which all living thingshave developed from primitive organisms through changes occurring overbillions of years, a process that includes all animals and plants. Exactly howevolution occurs is still a matter of debate, but there are many differenttheories and that it occurs is a scientific fact. Biologists agree that all livingthings come through a long history of changes shaped by physical andchemical processes that are still taking place. It is possible that all organismscan be traced back to the origin of Life from one celled organims.The most direct proof of evolution is the science of Paleontology, orthe study of life in the past through fossil remains or impressions, usually inrock. Changes occur in living organisms that serve to increase theiradaptability, for survival and reproduction, in changing environments.Evolution apparently has no built-in direction purpose. A given kind oforganism may evolve only when it occurs in a variety of forms differing inhereditary traits, that are passed from parent to offspring. By chance, somevarieties prove to be ill adapted to their current environment and thusdisappear, whereas others prove to be adaptive, and their numbers increase.The elimination of the unfit, or the "survival of the fittest," is known asNatural Selection because it is nature that discards or favors aarticular being. Evolution takes place only when natural selection


operates on apopulation of organisms containing diverse inheritable forms.


HISTORY


Pierre Louis Moreau de Maupertuis (1698-1759) was the first


topropose a general theory of evolution. He said that hereditary material,consisting of particles, was transmitted from parents to offspring. His


opinionof the part played by natural selection had little influence on other


naturalists.


Until the mid-19th century, naturalists believed that each


species wascreated separately, either through a supreme being or through


spontaneousgeneration the concept that organisms arose fully developed from soil or


water. Thework of the Swedish naturalist Carolus Linnaeus in advancing the


classifying ofbiological organisms focused attention on the close similarity between


certainspecies. Speculation began as to the existence of a sort of blood


relationshipbetween these species. These questions coupled with the emerging


sciences ofgeology and paleontology gave rise to hypotheses that the life-forms of


the dayevolved from earlier forms through a process of change. Extremely


important wasthe realization that different layers of rock represented different time


periods andthat each layer had a distinctive set of fossils of life-forms that had


lived in the past.


Lamarckism


Jean Baptiste Lamarck was one of several theorists who


proposed anevolutionary theory based on the "use and disuse" of organs. Lamarck


stated thatan individual acquires traits during its lifetime and that such traits


are in some wayput into the hereditary material and passed to the next generation. Thiswas an attempt to explain how a species could change gradually over


time.According to Lamarck, giraffes, for example, have long necks because for


manygenerations individual giraffes stretched to reach the uppermost leaves


of trees, ineach generation the giraffes added some length to their necks, and they


passed thison to their offspring. New organs arise from new needs and develop inthe extent that they are used, disuse of organs leads totheir disappearance. Later, the science of Genetics disproved


Lamarck’s theory, itwas found that acquired traits cannot be inherited.


Malthus


Thomas Robert Malthus, an English clergyman, through his


work An Essayon the Principle of Population, had a great influence in directing


naturalists towarda theory of natural selection. Malthus proposed that environmental


factors such asfamine and disease limited population growth.


Darwin


After more than 20 years of observation and experiment,


Charles Darwinproposed his theory of evolution through natural selection to the


Linnaean Societyof London in 1858. He presented his discovery along with another Englishnaturalist, Alfred Russel Wallace, who independently discovered natural


selection atabout the same time. The following year Darwin published his full


theory,supported with enormous evidence, in On the Origin of Species.


Genetics


The contribution of genetics to the understanding of


evolution hasbeen the explanation of the inheritance in individuals of the same


species. GregorMendel discovered the basic principles of inheritance in 1865, but his


work wasunknown to Darwin. Mendel’s work was "rediscovered" by other scientists


around1900. From that time to 1925 the science of genetics developed rapidly,


and manyof Darwin’s ideas about the inheritance of variations were found to be


incorrect.Only since 1925 has natural selection again been recognized as essentialin evolution. The modern theory of evolution combines the findings of


moderngenetics with the basic framework supplied by Darwin and Wallace,


creating thebasic principle of Population Genetics. Modern population genetics was


developedlargely during the 1930s and ’40s by the mathematicians J. B. S. Haldane


and R. A.Fisher and by the biologists Theodosius Dobzhansky , Julian Huxley,


Ernst Mayr ,George Gaylord SIMPSON, Sewall Wright, Berhard Rensch, and G. LedyardStebbins. According to the theory, variability among individuals in a


population ofsexually reproducing organisms is produced by mutation and geneticrecombination. The resulting genetic variability is subject to natural


selection in theenvironment.


POPULATION GENETICS


The word population is used in a special sense to describe


evolution. Thestudy of single individuals provides few clues as to the possible


outcomes ofevolution because single individuals cannot evolve in their lifetime. An


individualrepresents a store of genes that participates in evolution only when


those genes arepassed on to further generations, or populations. The gene is the basic


unit in thecell for transmitting hereditary characteristics to offspring.


Individuals are unitsupon which natural selection operates, but the trend of evolution can be


tracedthrough time only for groups of interbreeding individuals, populations


can beanalyzed statistically and their evolution predicted in terms of average


numbers.


The Hardy-Weinberg law, which was discovered independently


in 1908 bya British mathematician, Godfrey H. Hardy, and a German physician,


WilhelmWeinberg, provides a standard for quantitatively measuring the extent ofevolutionary change in a population. The law states that the gene


frequencies, orratios of different genes in a population, will remain constant unless


they arechanged by outside forces, such as selective reproduction and mutation.


Thisdiscovery reestablished natural selection as an evolutionary force.


Comparing theactual gene frequencies observed in a population with the frequencies


predicted, bythe Hardy-Weinberg law gives a numerical measure of how far the


populationdeviates from a nonevolving state called the Hardy-Weinberg equilibrium.


Given alarge, randomly breeding population, the Hardy-Weinberg equilibrium will


holdtrue, because it depends on the laws of probability. Changes are


produced in thegene pool through mutations, gene flow, genetic drift, and natural


selection.


Mutation


A mutation is an inheritable change in the character of a


gene. Mutationsmost often occur spontaneously, but they may be induced by some externalstimulus, such as irradiation or certain chemicals. The rate of mutation


in humans isextremely low; nevertheless, the number of genes in every sex cell, is


so large thatthe probability is high for at least one gene to carry a mutation.


Gene Flow


New genes can be introduced into a population through new


breedingorganisms or gametes from another population, as in plant pollen. Gene


flow canwork against the processes of natural selection.


Genetic Drift


A change in the gene pool due to chance is called genetic


drift. Thefrequency of loss is greater the smaller the population. Thus, in small


populationsthere is a tendency for less variation because mates are more similar


genetically.


Natural Selection


Over a period of time natural selection will result in


changes in thefrequency of alleles in the gene pool, or greater deviation from the


nonevolvingstate, represented by the Hardy-Weinberg equilibrium.


NEW SPECIES


New species may evolve either by the change of one species


to another orby the splitting of one species into two or more new species. Splitting,


thepredominant mode of species formation, results from the geographical


isolation ofpopulations of species. Isolated populations undergo different


mutations, andselection pressures and may evolve along different lines. If the


isolation is sufficientto prevent interbreeding with other populations, these differences may


becomeextensive enough to establish a new species. The evolutionary changes


broughtabout by isolation include differences in the reproductive systems of


the group.When a single group of organisms diversifies over time into several


subgroups byexpanding into the available niches of a new environment, it is said to


undergoAdaptive Radiation .


Darwin’s Finches, in the Galapagos Islands, west of Ecuador,


illustrateadaptive radiation. They were probably the first land birds to reach the


islands, and,in the absence of competition, they occupied several ecological habitats


anddiverged along several different lines. Such patterns of divergence are


reflected inthe biologists’ scheme of classification of organisms, which groups


together animalsthat have common characteristics. An adaptive radiation followed the


first conquestof land by vertebrates.


Natural selection can also lead populations of different


species living insimilar environments or having similar ways of life to evolve similar


characteristics.This is called convergent evolution and reflects the similar selective


pressure ofsimilar environments. Examples of convergent evolution are the eye in


cephalodmollusks, such as the octopus, and in vertebrates; wings in insects,


extinct flyingreptiles, birds, and bats; and the flipperlike appendages of the sea


turtle (reptile),penguin (bird), and walrus (mammal).


MOLECULAR EVOLUTION


An outpouring of new evidence supporting evolution has come


in the 20thcentury from molecular biology, an unknown field in Darwin’s day. Thefundamental tenet of molecular biology is that genes are coded sequences


of theDNA molecule in the chromosome and that a gene codes for a precise


sequence ofamino acids in a protein. Mutations alter DNA chemically, leading to


modified ornew proteins. Over evolutionary time, proteins have had histories that


are astraceable as those of large-scale structures such as bones and teeth.


The further inthe past that some ancestral stock diverged into present-day species,


the moreevident are the changes in the amino-acid sequences of the proteins of


thecontemporary species.


PLANT EVOLUTION


Biologists believe that plants arose from the multicellular


green algae(phylum Chlorophyta) that invaded the land about 1.2 billion years ago.


Evidence isbased on modern green algae having in common with modern plants the samephotosynthetic pigments, cell walls of cellulose, and multicell forms


having a lifecycle characterized by Alternation Of Generations. Photosynthesis almost


certainlydeveloped first in bacteria. The green algae may have been preadapted to


land.


The two major groups of plants are the bryophytes and the


tracheophytes;the two groups most likely diverged from one common group of plants. Thebryophytes, which lack complex conducting systems, are small and are


found inmoist areas. The tracheophyte

s are plants with efficient conducting


systems; theydominate the landscape today. The seed is the major development in


tracheophytes,and it is most important for survival on land.


Fossil evidence indicates that land plants first appeared


during the SilurianPeriod of the Paleozoic Era (425-400 million years ago) and diversified


in theDevonian Period. Near the end of the Carboniferous Period, fernlike


plants hadseedlike structures. At the close of the Permian Period, when the land


became drierand colder, seed plants gained an evolutionary advantage and became the


dominantplants.


Plant leaves have a wide range of shapes and sizes, and some


variations ofleaves are adaptations to the environment; for example, small, leathery


leaves foundon plants in dry climates are able to conserve water and capture less


light. Also,early angiosperms adapted to seasonal water shortages by dropping their


leavesduring periods of drought.


EVIDENCE FOR EVOLUTION


The Fossil Record has important insights into the history of


life. The orderof fossils, starting at the bottom and rising upward in stratified rock,


corresponds totheir age, from oldest to youngest.


Deep Cambrian rocks, up to 570 million years old, contain


the remains ofvarious marine invertebrate animals, sponges, jellyfish, worms,


shellfish, starfish,and crustaceans. These invertebrates were already so well developed


that they musthave become differentiated during the long period preceding the


Cambrian. Somefossil-bearing rocks lying well below the oldest Cambrian strata contain


imprints ofjellyfish, tracks of worms, and traces of soft corals and other animals


of uncertainnature.


Paleozoic waters were dominated by arthropods called


trilobites and largescorpionlike forms called eurypterids. Common in all Paleozoic periods


(570-230million years ago) were the nautiloid ,which are related to the modern


nautilus, andthe brachiopods, or lampshells. The odd graptolites,colonial animals


whosecarbonaceous remains resemble pencil marks, attained the peak of theirdevelopment in the Ordovician Period (500-430 million years ago) and


thenabruptly declined. In the mid-1980s researchers found fossil animal


burrows inrocks of the Ordovician Period; these trace fossils indicate that


terrestrialecosystems may have evolved sooner than was once thought.


Many of the Paleozoic marine invertebrate groups either


became extinct ordeclined sharply in numbers before the Mesozoic Era (230-65 million


years ago).During the Mesozoic, shelled ammonoids flourished in the seas, and


insects andreptiles were the predominant land animals. At the close of the Mesozoic


the once-successful marine ammonoids perished and the reptilian dynasty


collapsed, givingway to birds and mammals. Insects have continued to thrive and have


differentiatedinto a staggering number of species.


During the course of evolution plant and animal groups have


interacted toone another’s advantage. For example, as flowering plants have become


lessdependent on wind for pollination, a great variety of insects have


emerged asspecialists in transporting pollen. The colors and fragrances of flowers


have evolvedas adaptations to attract insects. Birds, which feed on seeds, fruits,


and buds, haveevolved rapidly in intimate association with the flowering plants. The


emergence ofherbivorous mammals has coincided with the widespread distribution of


grasses,and the herbivorous mammals in turn have contributed to the evolution ofcarnivorous mammals.


Fish and Amphibians


During the Devonian Period (390-340 million years ago) the vast


land areasof the Earth were largely populated by animal life, save for rare


creatures likescorpions and millipedes. The seas, however, were crowded with a variety


ofinvertebrate animals. The fresh and salt waters also contained


cartilaginous andbony Fish. From one of the many groups of fish inhabiting pools and


swampsemerged the first land vertebrates, starting the vertebrates on their


conquest of allavailable terrestrial habitats.


Among the numerous Devonian aquatic forms were the Crossopterygii,lobe-finned fish that possessed the ability to gulp air when they rose


to the surface.These ancient air- breathing fish represent the stock from which the


first landvertebrates, the amphibians, were derived. Scientists continue to


speculate aboutwhat led to venture onto land. The crossopterygians that migrated onto


land wereonly crudely adapted for terrestrial existence, but because they did not


encountercompetitors, they survived.


Lobe-finned fish did, however, possess certain characteristics


that servedthem well in their new environment, including primitive lungs and


internal nostrils,both of which are essential for breathing out of the water.Such characteristics, called preadaptations, did not develop because the


others werepreparing to migrate to the land; they were already present by accident


and becameselected traits only when they imparted an advantage to the fish on


land.


The early land-dwelling amphibians were slim-bodied with fishlike


tails, butthey had limbs capable of locomotion on land. These limbs probably


developedfrom the lateral fins, which contained fleshy lobes that in turn


contained bonyelements.


The ancient amphibians never became completely adapted for


existence onland, however. They spent much of their lives in the water, and their


moderndescendants, the salamanders, newts, frogs, and toads–still must return


to water todeposit their eggs. The elimination of a water-dwelling stage, which was


achievedby the reptiles, represented a major evolutionary advance.


The Reptilian Age Perhaps the most important factor contributing to the becoming of


reptilesfrom the amphibians was the development of a shell- covered egg that


could be laidon land. This development enabled the reptiles to spread throughout the


Earth’slandmasses in one of the most spectacular adaptive radiations in


biological history.


Like the eggs of birds, which developed later, reptile eggs


contain acomplex series of membranes that protect and nourish the embryo and help


itbreathe. The space between the embryo and the amnion is filled with an


amnioticfluid that resembles seawater; a similar fluid is found in the fetuses


of mammals,including humans. This fact has been interpreted as an indication that


life originatedin the sea and that the balance of salts in various body fluids did not


change verymuch in evolution. The membranes found in the human embryo are


essentiallysimilar to those in reptile and bird eggs. The human yolk sac remains


small andfunctionless, and the exhibits have no development in the human embryo.Nevertheless, the presence of a yolk sac and allantois in the human


embryo is oneof the strongest pieces of evidence documenting the evolutionary


relationshipsamong the widely differing kinds of vertebrates. This suggests that


mammals,including humans, are descended from animals that reproduced by means ofexternally laid eggs that were rich in yolk.


The reptiles, and in particular the dinosaurs, were the dominant


landanimals of the Earth for well over 100 million years. The Mesozoic Era,


duringwhich the reptiles thrived, is often referred to as the Age of Reptiles.


In terms of evolutionary success, the larger the animal, the


greater thelikelihood that the animal will maintain a constant Body Temperature


independentof the environmental temperature. Birds and mammals, for example,


produce andcontrol their own body heat through internal metabolic activities (a


state known asendothermy, or warm-bloodedness), whereas today’s reptiles are thermally


unstable(cold-blooded), regulating their body temperatures by behavioral


activities (thephenomenon of ectothermy). Most scientists regard dinosaurs as


lumbering,oversized, cold-blooded lizards, rather than large, lively, animals with


fast metabolicrates; some biologists, however–notably Robert T. Bakker of The Johns


HopkinsUniversity–assert that a huge dinosaur could not possibly have warmed


up everymorning on a sunny rock and must have relied on internal heat


production.


The reptilian dynasty collapsed before the close of the Mesozoic


Era.Relatively few of the Mesozoic reptiles have survived to modern times;


thoseremaining include the Crocodile,Lizard,snake, and turtle. The cause of


the declineand death of the large array of reptiles is unknown, but their


disappearance isusually attributed to some radical change in environmental conditions.


Like the giant reptiles, most lineages of organisms have


eventually becomeextinct, although some have not changed appreciably in millions of


years. Theopossum, for example, has survived almost unchanged since the late


CretaceousPeriod (more than 65 million years ago), and the Horseshoe Crab,


Limulus, is notvery different from fossils 500 million years old. We have no


explanation for theunexpected stability of such organisms; perhaps they have achieved an


almostperfect adjustment to a unchanging environment. Such stable forms,


however, arenot at all dominant in the world today. The human species, one of the


dominantmodern life forms, has evolved rapidly in a very short time.


The Rise of Mammals


The decline of the reptiles provided evolutionary opportunities


for birds andmammals. Small and inconspicuous during the Mesozoic Era, mammals rose


tounquestionable dominance during the Cenozoic Era (beginning 65 million


yearsago).


The mammals diversified into marine forms, such as the whale,


dolphin,seal, and walrus; fossorial (adapted to digging) forms living


underground, such asthe mole; flying and gliding animals, such as the bat and flying


squirrel; andcursorial animals (adapted for running), such as the horse. These


variousmammalian groups are well adapted to their different modes of life,


especially bytheir appendages, which developed from common ancestors to become


specializedfor swimming, flight, and movement on land.


Although there is little superficial resemblance among the arm of


a person,the flipper of a whale, and the wing of a bat, a closer comparison of


their skeletalelements shows that, bone for bone, they are structurally similar.


Biologists regardsuch structural similarities, or homologies, as evidence of evolutionary


relationships.The homologous limb bones of all four-legged vertebrates, for example,


areassumed to be derived from the limb bones of a common ancestor.


Biologists arecareful to distinguish such homologous features from what they call


analogousfeatures, which perform similar functions but are structurally


different. Forexample, the wing of a bird and the wing of a butterfly are analogous;


both areused for flight, but they are entirely different structurally. Analogous


structures donot indicate evolutionary relationships.


Closely related fossils preserved in continuous successions of


rock stratahave allowed evolutionists to trace in detail the evolution of many


species as it hasoccurred over several million years. The ancestry of the horse can be


tracedthrough thousands of fossil remains to a small terrier-sized animal with


four toes onthe front feet and three toes on the hind feet. This ancestor lived in


the EoceneEpoch, about 54 million years ago. From fossils in the higher layers of


stratifiedrock, the horse is found to have gradually acquired its modern form by


eventuallyevolving to a one-toed horse almost like modern horses and finally to


the modernhorse, which dates back about 1 million years.


CONCLUSION TO EVOLUTION


Although we are not totally certain that evolution is how we got


the way weare now, it is a strong belief among many people today, and scientist


are findingmore and more evidence to back up the evolutionary theory.

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