Theory of 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 selectionoperates on apopulation of organisms containing diverse inheritable forms. HISTORY Pierre Louis Moreau de Maupertuis (1698-1759) was the firsttopropose a general theory of evolution. He said that hereditary material,consisting of particles, was transmitted from parents to offspring. Hisopinionof the part played by natural selection had little influence on othernaturalists. Until the mid-19th century, naturalists believed that eachspecies wascreated separately, either through a supreme being or throughspontaneousgeneration the concept that organisms arose fully developed from soil orwater. Thework of the Swedish naturalist Carolus Linnaeus in advancing theclassifying ofbiological organisms focused attention on the close similarity betweencertainspecies. Speculation began as to the existence of a sort of bloodrelationshipbetween these species. These questions coupled with the emergingsciences ofgeology and paleontology gave rise to hypotheses that the life-forms ofthe dayevolved from earlier forms through a process of change. Extremelyimportant wasthe realization that different layers of rock represented different timeperiods andthat each layer had a distinctive set of fossils of life-forms that hadlived in the past. Lamarckism Jean Baptiste Lamarck was one of several theorists whoproposed anevolutionary theory based on the “use and disuse” of organs. Lamarckstated thatan individual acquires traits during its lifetime and that such traitsare in some wayput into the hereditary material and passed to the next generation. Thiswas an attempt to explain how a species could change gradually overtime.According to Lamarck, giraffes, for example, have long necks because formanygenerations individual giraffes stretched to reach the uppermost leavesof trees, ineach generation the giraffes added some length to their necks, and theypassed thison to their offspring. New organs arise from new needs and develop in the extent that they are used, disuse of organs leads totheir disappearance. Later, the science of Genetics disprovedLamarck’s theory, itwas found that acquired traits cannot be inherited. Malthus Thomas Robert Malthus, an English clergyman, through hiswork An Essayon the Principle of Population, had a great influence in directingnaturalists towarda theory of natural selection. Malthus proposed that environmentalfactors 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 theLinnaean Societyof London in 1858. He presented his discovery along with another Englishnaturalist, Alfred Russel Wallace, who independently discovered naturalselection atabout the same time. The following year Darwin published his fulltheory,supported with enormous evidence, in On the Origin of Species. Genetics The contribution of genetics to the understanding ofevolution hasbeen the explanation of the inheritance in individuals of the samespecies. GregorMendel discovered the basic principles of inheritance in 1865, but hiswork wasunknown to Darwin. Mendel’s work was “rediscovered” by other scientistsaround1900. From that time to 1925 the science of genetics developed rapidly,and manyof Darwin’s ideas about the inheritance of variations were found to beincorrect.Only since 1925 has natural selection again been recognized as essentialin evolution. The modern theory of evolution combines the findings ofmoderngenetics with the basic framework supplied by Darwin and Wallace,creating thebasic principle of Population Genetics. Modern population genetics wasdevelopedlargely during the 1930s and ’40s by the mathematicians J. B. S. Haldaneand 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 apopulation ofsexually reproducing organisms is produced by mutation and geneticrecombination. The resulting genetic variability is subject to naturalselection in theenvironment. POPULATION GENETICS The word population is used in a special sense to describeevolution. Thestudy of single individuals provides few clues as to the possibleoutcomes ofevolution because single individuals cannot evolve in their lifetime. Anindividualrepresents a store of genes that participates in evolution only whenthose genes arepassed on to further generations, or populations. The gene is the basicunit in thecell for transmitting hereditary characteristics to offspring. Individuals are unitsupon which natural selection operates, but the trend of evolution can betracedthrough time only for groups of interbreeding individuals, populationscan beanalyzed statistically and their evolution predicted in terms of averagenumbers. The Hardy-Weinberg law, which was discovered independentlyin 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 genefrequencies, orratios of different genes in a population, will remain constant unlessthey 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 frequenciespredicted, bythe Hardy-Weinberg law gives a numerical measure of how far thepopulationdeviates from a nonevolving state called the Hardy-Weinberg equilibrium. Given alarge, randomly breeding population, the Hardy-Weinberg equilibrium willholdtrue, because it depends on the laws of probability. Changes areproduced in thegene pool through mutations, gene flow, genetic drift, and naturalselection. Mutation A mutation is an inheritable change in the character of agene. Mutationsmost often occur spontaneously, but they may be induced by some externalstimulus, such as irradiation or certain chemicals. The rate of mutationin humans isextremely low; nevertheless, the number of genes in every sex cell, isso 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 newbreedingorganisms or gametes from another population, as in plant pollen. Geneflow canwork against the processes of natural selection. Genetic Drift A change in the gene pool due to chance is called geneticdrift. Thefrequency of loss is greater the smaller the population. Thus, in smallpopulationsthere is a tendency for less variation because mates are more similargenetically. Natural Selection Over a period of time natural selection will result inchanges in thefrequency of alleles in the gene pool, or greater deviation from thenonevolvingstate, represented by the Hardy-Weinberg equilibrium. NEW SPECIES New species may evolve either by the change of one speciesto another orby the splitting of one species into two or more new species. Splitting,thepredominant mode of species formation, results from the geographicalisolation ofpopulations of species. Isolated populations undergo differentmutations, andselection pressures and may evolve along different lines. If theisolation is sufficientto prevent interbreeding with other populations, these differences maybecomeextensive enough to establish a new species. The evolutionary changesbroughtabout by isolation include differences in the reproductive systems ofthe group.When a single group of organisms diversifies over time into severalsubgroups byexpanding into the available niches of a new environment, it is said toundergoAdaptive Radiation . Darwin’s Finches, in the Galapagos Islands, west of Ecuador,illustrateadaptive radiation. They were probably the first land birds to reach theislands, and,in the absence of competition, they occupied several ecological habitatsanddiverged along several different lines. Such patterns of divergence arereflected inthe biologists’ scheme of classification of organisms, which groupstogether animalsthat have common characteristics. An adaptive radiation followed thefirst conquestof land by vertebrates. Natural selection can also lead populations of differentspecies living insimilar environments or having similar ways of life to evolve similarcharacteristics.This is called convergent evolution and reflects the similar selectivepressure ofsimilar environments. Examples of convergent evolution are the eye incephalodmollusks, such as the octopus, and in vertebrates; wings in insects,extinct flyingreptiles, birds, and bats; and the flipperlike appendages of the seaturtle (reptile),penguin (bird), and walrus (mammal). MOLECULAR EVOLUTION An outpouring of new evidence supporting evolution has comein the 20thcentury from molecular biology, an unknown field in Darwin’s day. Thefundamental tenet of molecular biology is that genes are coded sequencesof theDNA molecule in the chromosome and that a gene codes for a precisesequence ofamino acids in a protein. Mutations alter DNA chemically, leading tomodified ornew proteins. Over evolutionary time, proteins have had histories thatare 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 ofthecontemporary species. PLANT EVOLUTION Biologists believe that plants arose from the multicellulargreen 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 formshaving a life cycle characterized by Alternation Of Generations. Photosynthesis almostcertainlydeveloped first in bacteria. The green algae may have been preadapted toland. The two major groups of plants are the bryophytes and thetracheophytes;the two groups most likely diverged from one common group of plants. Thebryophytes, which lack complex conducting systems, are small and arefound inmoist areas. The tracheophytes are plants with efficient conductingsystems; theydominate the landscape today. The seed is the major development intracheophytes,and it is most important for survival on land. Fossil evidence indicates that land plants first appearedduring the SilurianPeriod of the Paleozoic Era (425-400 million years ago) and diversifiedin theDevonian Period. Near the end of the Carboniferous Period, fernlikeplants hadseedlike structures. At the close of the Permian Period, when the landbecame drierand colder, seed plants gained an evolutionary advantage and became thedominantplants. Plant leaves have a wide range of shapes and sizes, and somevariations ofleaves are adaptations to the environment; for example, small, leatheryleaves foundon plants in dry climates are able to conserve water and capture lesslight. Also,early angiosperms adapted to seasonal water shortages by dropping theirleavesduring periods of drought. EVIDENCE FOR EVOLUTION The Fossil Record has important insights into the history oflife. 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, containthe remains ofvarious marine invertebrate animals, sponges, jellyfish, worms,shellfish, starfish,and crustaceans. These invertebrates were already so well developedthat they musthave become differentiated during the long period preceding theCambrian. Somefossil-bearing rocks lying well below the oldest Cambrian strata containimprints ofjellyfish, tracks of worms, and traces of soft corals and other animalsof uncertainnature. Paleozoic waters were dominated by arthropods calledtrilobites and largescorpionlike forms called eurypterids. Common in all Paleozoic periods(570-230million years ago) were the nautiloid ,which are related to the modernnautilus, andthe brachiopods, or lampshells. The odd graptolites,colonial animalswhosecarbonaceous remains resemble pencil marks, attained the peak of theirdevelopment in the Ordovician Period (500-430 million years ago) andthenabruptly declined. In the mid-1980s researchers found fossil animalburrows inrocks of the Ordovician Period; these trace fossils indicate thatterrestrialecosystems may have evolved sooner than was once thought. Many of the Paleozoic marine invertebrate groups eitherbecame extinct ordeclined sharply in numbers before the Mesozoic Era (230-65 millionyears ago).During the Mesozoic, shelled ammonoids flourished in the seas, andinsects andreptiles were the predominant land animals. At the close of the Mesozoicthe once-successful marine ammonoids perished and the reptilian dynastycollapsed, givingway to birds and mammals. Insects have continued to thrive and havedifferentiatedinto a staggering number of species. During the course of evolution plant and animal groups haveinteracted toone another’s advantage. For example, as flowering plants have becomelessdependent on wind for pollination, a great variety of insects haveemerged asspecialists in transporting pollen. The colors and fragrances of flowershave evolvedas adaptations to attract insects. Birds, which feed on seeds, fruits,and buds, haveevolved rapidly in intimate association with the flowering plants. Theemergence ofherbivorous mammals has coincided with the widespread distribution ofgrasses,and the herbivorous mammals in turn have contributed to the evolution ofcarnivorous mammals. Fish and AmphibiansDuring the Devonian Period (390-340 million years ago) the vastland areasof the Earth were largely populated by animal life, save for rarecreatures likescorpions and millipedes. The seas, however, were crowded with a varietyofinvertebrate animals. The fresh and salt waters also containedcartilaginous andbony Fish. From one of the many groups of fish inhabiting pools andswampsemerged the first land vertebrates, starting the vertebrates on theirconquest 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 roseto the surface.These ancient air- breathing fish represent the stock from which thefirst landvertebrates, the amphibians, were derived. Scientists continue tospeculate aboutwhat led to venture onto land. The crossopterygians that migrated ontoland wereonly crudely adapted for terrestrial existence, but because they did notencountercompetitors, they survived.Lobe-finned fish did, however, possess certain characteristicsthat servedthem well in their new environment, including primitive lungs andinternal nostrils,both of which are essential for breathing out of the water.Such characteristics, called preadaptations, did not develop because theothers werepreparing to migrate to the land; they were already present by accidentand becameselected traits only when they imparted an advantage to the fish onland.The early land-dwelling amphibians were slim-bodied with fishliketails, butthey had limbs capable of locomotion on land. These limbs probablydevelopedfrom the lateral fins, which contained fleshy lobes that in turncontained bonyelements.The ancient amphibians never became completely adapted forexistence onland, however. They spent much of their lives in the water, and theirmoderndescendants, the salamanders, newts, frogs, and toads–still must returnto water todeposit their eggs. The elimination of a water-dwelling stage, which wasachievedby the reptiles, represented a major evolutionary advance. The Reptilian AgePerhaps the most important factor contributing to the becoming ofreptilesfrom the amphibians was the development of a shell- covered egg thatcould be laidon land. This development enabled the reptiles to spread throughout theEarth’slandmasses in one of the most spectacular adaptive radiations inbiological history.Like the eggs of birds, which developed later, reptile eggscontain acomplex series of membranes that protect and nourish the embryo and helpitbreathe. The space between the embryo and the amnion is filled with anamnioticfluid that resembles seawater; a similar fluid is found in the fetusesof mammals,including humans. This fact has been interpreted as an indication thatlife originatedin the sea and that the balance of salts in various body fluids did notchange verymuch in evolution. The membranes found in the human embryo areessentiallysimilar to those in reptile and bird eggs. The human yolk sac remainssmall andfunctionless, and the exhibits have no development in the human embryo.Nevertheless, the presence of a yolk sac and allantois in the humanembryo is oneof the strongest pieces of evidence documenting the evolutionaryrelationshipsamong the widely differing kinds of vertebrates. This suggests thatmammals,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 dominantlandanimals 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, thegreater thelikelihood that the animal will maintain a constant Body Temperatureindependentof the environmental temperature. Birds and mammals, for example,produce andcontrol their own body heat through internal metabolic activities (astate known asendothermy, or warm-bloodedness), whereas today’s reptiles are thermallyunstable(cold-blooded), regulating their body temperatures by behavioralactivities (thephenomenon of ectothermy). Most scientists regard dinosaurs aslumbering,oversized, cold-blooded lizards, rather than large, lively, animals withfast metabolicrates; some biologists, however–notably Robert T. Bakker of The JohnsHopkinsUniversity–assert that a huge dinosaur could not possibly have warmedup everymorning on a sunny rock and must have relied on internal heatproduction.The reptilian dynasty collapsed before the close of the MesozoicEra.Relatively few of the Mesozoic reptiles have survived to modern times;thoseremaining include the Crocodile,Lizard,snake, and turtle. The cause ofthe declineand death of the large array of reptiles is unknown, but theirdisappearance isusually attributed to some radical change in environmental conditions.Like the giant reptiles, most lineages of organisms haveeventually becomeextinct, although some have not changed appreciably in millions ofyears. Theopossum, for example, has survived almost unchanged since the lateCretaceousPeriod (more than 65 million years ago), and the Horseshoe Crab,Limulus, is notvery different from fossils 500 million years old. We have noexplanation for theunexpected stability of such organisms; perhaps they have achieved analmostperfect adjustment to a unchanging environment. Such stable forms,however, arenot at all dominant in the world today. The human species, one of thedominantmodern life forms, has evolved rapidly in a very short time. The Rise of MammalsThe decline of the reptiles provided evolutionary opportunitiesfor birds andmammals. Small and inconspicuous during the Mesozoic Era, mammals rosetounquestionable dominance during the Cenozoic Era (beginning 65 millionyearsago).The mammals diversified into marine forms, such as the whale,dolphin,seal, and walrus; fossorial (adapted to digging) forms livingunderground, such asthe mole; flying and gliding animals, such as the bat and flyingsquirrel; andcursorial animals (adapted for running), such as the horse. Thesevariousmammalian groups are well adapted to their different modes of life,especially bytheir appendages, which developed from common ancestors to becomespecializedfor swimming, flight, and movement on land.Although there is little superficial resemblance among the arm ofa person,the flipper of a whale, and the wing of a bat, a closer comparison oftheir skeletalelements shows that, bone for bone, they are structurally similar. Biologists regardsuch structural similarities, or homologies, as evidence of evolutionaryrelationships.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 callanalogousfeatures, which perform similar functions but are structurallydifferent. Forexample, the wing of a bird and the wing of a butterfly are analogous;both areused for flight, but they are entirely different structurally. Analogousstructures donot indicate evolutionary relationships.Closely related fossils preserved in continuous successions ofrock stratahave allowed evolutionists to trace in detail the evolution of manyspecies as it hasoccurred over several million years. The ancestry of the horse can betracedthrough thousands of fossil remains to a small terrier-sized animal withfour toes onthe front feet and three toes on the hind feet. This ancestor lived inthe EoceneEpoch, about 54 million years ago. From fossils in the higher layers ofstratifiedrock, the horse is found to have gradually acquired its modern form byeventuallyevolving to a one-toed horse almost like modern horses and finally tothe modernhorse, which dates back about 1 million years.CONCLUSION TO EVOLUTIONAlthough we are not totally certain that evolution is how we gotthe way weare now, it is a strong belief among many people today, and scientistare findingmore and more evidence to back up the evolutionary theory.