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
articular being. Evolution takes place only when natural selection
operates on a
population 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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