Untitled Essay, Research Paper


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.


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


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.


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.


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.


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.


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.


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


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



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


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 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).


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.


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


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 tracheophytes 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


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.


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


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


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


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.


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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