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Introduction To Evolution Essay Research Paper Introduction

Introduction To Evolution Essay, Research Paper


Introduction To Evolution


What is Evolution? Evolution is the process by which all living things


have developed from primitive organisms through changes occurring over billions


of years, a process that includes all animals and plants. Exactly how evolution


occurs is still a matter of debate, but there are many different theories and


that it occurs is a scientific fact. Biologists agree that all living things


come through a long history of changes shaped by physical and chemical processes


that are still taking place. It is possible that all organisms can be traced


back to the origin of Life from one celled organims.


The most direct proof of evolution is the science of Paleontology,


or the study of life in the past through fossil remains or impressions, usually


in rock. Changes occur in living organisms that serve to increase their


adaptability, for survival and reproduction, in changing environments. Evolution


apparently has no built-in direction purpose. A given kind of organism may


evolve only when it occurs in a variety of forms differing in hereditary traits,


that are passed from parent to offspring. By chance, some varieties prove to be


ill adapted to their current environment and thus disappear, whereas others


prove to be adaptive, and their numbers increase. The elimination of the unfit,


or the “survival of the fittest,” is known as Natural Selection because it is


nature that discards or favors a particular 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 to


propose a general theory of evolution. He said that hereditary material,


consisting of particles, was transmitted from parents to offspring. His opinion


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


naturalists.


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


was created separately, either through a supreme being or through spontaneous


generation the concept that organisms arose fully developed from soil or water.


The work of the Swedish naturalist Carolus Linnaeus in advancing the classifying


of biological organisms focused attention on the close similarity between


certain species. Speculation began as to the existence of a sort of blood


relationship between these species. These questions coupled with the emerging


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


of the day evolved from earlier forms through a process of change. Extremely


important was the realization that different layers of rock represented


different time periods and that 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 an


evolutionary theory based on the “use and disuse” of organs. Lamarck stated that


an individual acquires traits during its lifetime and that such traits are in


some way put into the hereditary material and passed to the next generation.


This was an attempt to explain how a species could change gradually over time.


According to Lamarck, giraffes, for example, have long necks because for many


generations individual giraffes stretched to reach the uppermost leaves of trees,


in each generation the giraffes added some length to their necks, and they


passed this on to their offspring. New organs arise from new needs and develop


in the extent that they are used, disuse of organs leads to their disappearance.


Later, the science of Genetics disproved Lamarck’s theory, it was found that


acquired traits cannot be inherited.


Malthus


Thomas Robert Malthus, an English clergyman, through his work An


Essay on the Principle of Population, had a great influence in directing


naturalists toward a theory of natural selection. Malthus proposed that


environmental factors such as famine and disease limited population growth.


Darwin


After more than 20 years of observation and experiment, Charles


Darwin proposed his theory of evolution through natural selection to the


Linnaean Society of London in 1858. He presented his discovery along with


another English naturalist, Alfred Russel Wallace, who independently discovered


natural selection at about 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 has


been the explanation of the inheritance in individuals of the same species.


Gregor Mendel discovered the basic principles of inheritance in 1865, but his


work was unknown to Darwin. Mendel’s work was “rediscovered” by other scientists


around 1900. From that time to 1925 the science of genetics developed rapidly,


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


incorrect. Only since 1925 has natural selection again been recognized as


essential in evolution. The modern theory of evolution combines the findings of


modern genetics with the basic framework supplied by Darwin and Wallace,


creating the basic principle of Population Genetics. Modern population genetics


was developed largely 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.


Ledyard Stebbins. According to the theory, variability among individuals in a


population of sexually reproducing organisms is produced by mutation and genetic


recombination. The resulting genetic variability is subject to natural selection


in the environment.


POPULATION GENETICS


The word population is used in a special sense to describe evolution.


The study of single individuals provides few clues as to the possible outcomes


of evolution because single individuals cannot evolve in their lifetime. An


individual represents a store of genes that participates in evolution only when


those genes are passed on to further generations, or populations. The gene is


the basic unit in the cell for transmitting hereditary characteristics to


offspring. Individuals are units upon which natural selection operates, but the


trend of evolution can be traced through time only for groups of interbreeding


individuals, populations can be analyzed statistically and their evolution


predicted in terms of average numbers.


The Hardy-Weinberg law, which was discovered independently in 1908


by a British mathematician, Godfrey H. Hardy, and a German physician, Wilhelm


Weinberg, provides a standard for quantitatively measuring the extent of


evolutionary change in a population. The law states that the gene frequencies,


or ratios of different genes in a population, will remain constant unless they


are changed by outside forces, such as selective reproduction and mutation. This


discovery reestablished natural selection as an evolutionary force. Comparing


the actual gene frequencies observed in a population with the frequencies


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


population deviates from a nonevolving state called the Hardy-Weinberg


equilibrium. Given a large, randomly breeding population, the Hardy-Weinberg


equilibrium will hold true, because it depends on the laws of probability.


Changes are produced in the gene pool through mutations, gene flow, genetic


drift, and natural selection.


Mutation


A mutation is an inheritable change in the character of a gene.


Mutations most often occur spontaneously, but they may be induced by some


external stimulus, such as irradiation or certain chemicals. The rate of


mutation in humans is extremely low; nevertheless, the number of genes in every


sex cell, is so large that the probability is high for at least one gene to


carry a mutation.


Gene Flow


New genes can be introduced into a population through new breeding


organisms or gametes from another population, as in plant pollen. Gene flow can


work against the processes of natural selection.


Genetic Drift


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


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


populations there 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


the frequency of alleles in the gene pool, or greater deviation from the


nonevolving state, represented by the Hardy-Weinberg equilibrium.


NEW SPECIES


New species may evolve either by the change of one species to


another or by the splitting of one species into two or more new species.


Splitting, the predominant mode of species formation, results from the


geographical isolation of populations of species. Isolated populations undergo


different mutations, and selection pressures and may evolve along different


lines. If the isolation is sufficient to prevent interbreeding with other


populations, these differences may become extensive enough to establish a new


species. The evolutionary changes brought about by isolation include differences


in the reproductive systems of the group. When a single group of organisms


diversifies over time into several subgroups by expanding into the available


niches of a new environment, it is said to undergo Adaptive Radiation .


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


illustrate adaptive radiation. They were probably the first land birds to reach


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


ecological habitats and diverged along several different lines. Such patterns of


divergence are reflected in the biologists’ scheme of classification of


organisms, which groups together animals that have common characteristics. An


adaptive radiation followed the first conquest of land by vertebrates.


Natural selection can also lead populations of different species


living in similar environments or having similar ways of life to evolve similar


characteristics. This is called convergent evolution and reflects the similar


selective pressure of similar environments. Examples of convergent evolution are


the eye in cephalod mollusks, such as the octopus, and in vertebrates; wings in


insects, extinct flying reptiles, 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


20th century from molecular biology, an unknown field in Darwin’s day. The


fundamental tenet of molecular biology is that genes are coded sequences of the


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


amino acids in a protein. Mutations alter DNA chemically, leading to modified or


new proteins. Over evolutionary time, proteins have had histories that are as


traceable as those of large-scale structures such as bones and teeth. The


further in the past that some ancestral stock diverged into present-day species,


the more evident are the changes in the amino-acid sequences of the proteins of


the contemporary 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 is based on modern green algae having in common with modern plants the


same photosynthetic pigments, cell walls of cellulose, and multicell forms


having a life cycle characterized by Alternation Of Generations. Photosynthesis


almost certainly developed 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 grou

p of


plants. The bryophytes, which lack complex conducting systems, are small and are


found in moist areas. The tracheophytes are plants with efficient conducting


systems; they dominate 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


Silurian Period of the Paleozoic Era (425-400 million years ago) and diversified


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


plants had seedlike structures. At the close of the Permian Period, when the


land became drier and colder, seed plants gained an evolutionary advantage and


became the dominant plants.


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


variations of leaves are adaptations to the environment; for example, small,


leathery leaves found on plants in dry climates are able to conserve water and


capture less light. Also, early angiosperms adapted to seasonal water shortages


by dropping their leaves during periods of drought.


EVIDENCE FOR EVOLUTION


The Fossil Record has important insights into the history of life.


The order of fossils, starting at the bottom and rising upward in stratified


rock, corresponds to their age, from oldest to youngest.


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


remains of various marine invertebrate animals, sponges, jellyfish, worms,


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


developed that they must have become differentiated during the long period


preceding the Cambrian. Some fossil-bearing rocks lying well below the oldest


Cambrian strata contain imprints of jellyfish, tracks of worms, and traces of


soft corals and other animals of uncertain nature.


Paleozoic waters were dominated by arthropods called trilobites and


large scorpionlike forms called eurypterids. Common in all Paleozoic periods


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


nautilus, and the brachiopods, or lampshells. The odd graptolites,colonial


animals whose carbonaceous remains resemble pencil marks, attained the peak of


their development in the Ordovician Period (500-430 million years ago) and then


abruptly declined. In the mid-1980s researchers found fossil animal burrows in


rocks of the Ordovician Period; these trace fossils indicate that terrestrial


ecosystems may have evolved sooner than was once thought.


Many of the Paleozoic marine invertebrate groups either became


extinct or declined sharply in numbers before the Mesozoic Era (230-65 million


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


insects and reptiles were the predominant land animals. At the close of the


Mesozoic the once-successful marine ammonoids perished and the reptilian dynasty


collapsed, giving way to birds and mammals. Insects have continued to thrive and


have differentiated into a staggering number of species.


During the course of evolution plant and animal groups have


interacted to one another’s advantage. For example, as flowering plants have


become less dependent on wind for pollination, a great variety of insects have


emerged as specialists in transporting pollen. The colors and fragrances of


flowers have evolved as adaptations to attract insects. Birds, which feed on


seeds, fruits, and buds, have evolved rapidly in intimate association with the


flowering plants. The emergence of herbivorous mammals has coincided with the


widespread distribution of grasses, and the herbivorous mammals in turn have


contributed to the evolution of carnivorous mammals.


Fish and Amphibians


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


areas of the Earth were largely populated by animal life, save for rare


creatures like scorpions and millipedes. The seas, however, were crowded with a


variety of invertebrate animals. The fresh and salt waters also contained


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


pools and swamps emerged the first land vertebrates, starting the vertebrates on


their conquest of all available 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 land


vertebrates, the amphibians, were derived. Scientists continue to speculate


about what led to venture onto land. The crossopterygians that migrated onto


land were only crudely adapted for terrestrial existence, but because they did


not encounter competitors, they survived.


Lobe-finned fish did, however, possess certain characteristics that


served them 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


were preparing to migrate to the land; they were already present by accident and


became selected traits only when they imparted an advantage to the fish on land.


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


but they had limbs capable of locomotion on land. These limbs probably developed


from the lateral fins, which contained fleshy lobes that in turn contained bony


elements.


The ancient amphibians never became completely adapted for existence on


land, however. They spent much of their lives in the water, and their modern


descendants, the salamanders, newts, frogs, and toads–still must return to


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


was achieved by the reptiles, represented a major evolutionary advance.


The Reptilian Age


Perhaps the most important factor contributing to the becoming of


reptiles from the amphibians was the development of a shell- covered egg that


could be laid on land. This development enabled the reptiles to spread


throughout the Earth’s landmasses in one of the most spectacular adaptive


radiations in biological history.


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


complex series of membranes that protect and nourish the embryo and help it


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


fluid 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 originated in the sea and that the balance of salts in various body fluids


did not change very much in evolution. The membranes found in the human embryo


are essentially similar to those in reptile and bird eggs. The human yolk sac


remains small and functionless, and the exhibits have no development in the


human embryo. Nevertheless, the presence of a yolk sac and allantois in the


human embryo is one of the strongest pieces of evidence documenting the


evolutionary relationships among the widely differing kinds of vertebrates. This


suggests that mammals, including humans, are descended from animals that


reproduced by means of externally laid eggs that were rich in yolk.


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


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


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


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


likelihood that the animal will maintain a constant Body Temperature independent


of the environmental temperature. Birds and mammals, for example, produce and


control their own body heat through internal metabolic activities (a state known


as endothermy, or warm-bloodedness), whereas today’s reptiles are thermally


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


activities (the phenomenon of ectothermy). Most scientists regard dinosaurs as


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


with fast metabolic rates; some biologists, however–notably Robert T. Bakker of


The Johns Hopkins University–assert that a huge dinosaur could not possibly


have warmed up every morning 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; those


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


decline and death of the large array of reptiles is unknown, but their


disappearance is usually attributed to some radical change in environmental


conditions.


Like the giant reptiles, most lineages of organisms have eventually


become extinct, although some have not changed appreciably in millions of years.


The opossum, for example, has survived almost unchanged since the late


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


Limulus, is not very different from fossils 500 million years old. We have no


explanation for the unexpected stability of such organisms; perhaps they have


achieved an almost perfect adjustment to a unchanging environment. Such stable


forms, however, are not at all dominant in the world today. The human species,


one of the dominant modern life forms, has evolved rapidly in a very short time.


The Rise of Mammals


The decline of the reptiles provided evolutionary opportunities for


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


to unquestionable dominance during the Cenozoic Era (beginning 65 million years


ago).


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


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


as the mole; flying and gliding animals, such as the bat and flying squirrel;


and cursorial animals (adapted for running), such as the horse. These various


mammalian groups are well adapted to their different modes of life, especially


by their appendages, which developed from common ancestors to become specialized


for 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 skeletal elements shows that, bone for bone, they are structurally similar.


Biologists regard such structural similarities, or homologies, as evidence of


evolutionary relationships. The homologous limb bones of all four-legged


vertebrates, for example, are assumed to be derived from the limb bones of a


common ancestor. Biologists are careful to distinguish such homologous features


from what they call analogous features, which perform similar functions but are


structurally different. For example, the wing of a bird and the wing of a


butterfly are analogous; both are used for flight, but they are entirely


different structurally. Analogous structures do not indicate evolutionary


relationships.


Closely related fossils preserved in continuous successions of rock


strata have allowed evolutionists to trace in detail the evolution of many


species as it has occurred over several million years. The ancestry of the horse


can be traced through thousands of fossil remains to a small terrier-sized


animal with four toes on the front feet and three toes on the hind feet. This


ancestor lived in the Eocene Epoch, about 54 million years ago. From fossils in


the higher layers of stratified rock, the horse is found to have gradually


acquired its modern form by eventually evolving to a one-toed horse almost like


modern horses and finally to the modern horse, which dates back about 1 million


years.


CONCLUSION TO EVOLUTION


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


we are now, it is a strong belief among many people today, and scientist are


finding more and more evidence to back up the evolutionary theory.


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