The Theory Of Evolution


The Theory Of Evolution Essay, Research Paper



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

articular being. Evolution takes place only when natural selection

operates on a

population of organisms containing diverse inheritable forms.


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


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

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


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


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

species was

created separately, either through a supreme being or through


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


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


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.


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


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


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.


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.


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


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


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


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


Only since 1925 has natural selection again been recognized as essential

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


genetics with the basic framework supplied by Darwin and Wallace,

creating the

basic principle of Population Genetics. Modern population genetics was


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



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


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


through time only for groups of interbreeding individuals, populations

can be

analyzed statistically and their evolution predicted in terms of average


The Hardy-Weinberg law, which was discovered independently

in 1908 by

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


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.


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


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

Given a

large, randomly breeding population, the Hardy-Weinberg equilibrium will


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

produced in the

gene pool through mutations, gene flow, genetic drift, and natural



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


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


there 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 the

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


state, represented by the Hardy-Weinberg equilibrium.


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,


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


extensive enough to establish a new species. The evolutionary changes


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


Adaptive Radiation .

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


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

islands, and,

in the absence of competition, they occupied several ecological habitats


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


This is called convergent evolution and reflects the similar selective

pressure of

similar environments. Examples of convergent evolution are the eye in


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).


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


contemporary species.


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


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


The two major groups of plants are the bryophytes and the


the two groups most likely diverged from one common group 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


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



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


during periods of drought.


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


Paleozoic waters were dominated by arthropods called

trilobites and large

scorpionlike forms called eurypterids. Common in all Paleozoic periods


million years ago) were the nautiloid ,which are related to the modern

nautilus, and

the brachiopods, or lampshells. The odd graptolites,colonial animals


carbonaceous remains resemble pencil marks, attained the peak of their

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


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

burrows in

rocks of the Ordovician Period; these trace fossils indicate that


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


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


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


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


invertebrate animals. The fresh and salt waters also contained

cartilaginous and

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


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


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


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

tails, but

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


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

contained bony


The ancient amphibians never became completely adapted for

existence on

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


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


by the reptiles, represented a major evolutionary advance.

The Reptilian Age

Perhaps the most important factor contributing to the becoming of


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


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


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


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


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


among the widely differing kinds of vertebrates. This suggests that


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


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


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


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


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

activities (the

phenomenon of ectothermy). Most scientists regard dinosaurs as


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

fast metabolic

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


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


The reptilian dynasty collapsed before the close of the Mesozoic


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


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


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


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


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


unquestionable dominance during the Cenozoic Era (beginning 65 million



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


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


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

especially by

their appendages, which developed from common ancestors to become


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


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


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

Biologists are

careful to distinguish such homologous features from what they call


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


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


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


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

the modern

horse, which dates back about 1 million years.


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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