Introduction To Evolution Essay Research Paper Introduction


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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.


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


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.


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.


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 theory, 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

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.


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.


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 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).


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.


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 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

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.


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


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


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


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


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



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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