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Essay/Term paper: Introduction to evolution

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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 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.

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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