Untitled Essay, Research Paper
BODYINTRODUCTION TO EVOLUTION
What is Evolution? Evolution is the process by which all living thingshave developed from primitive organisms through changes occurring overbillions of years, a process that includes all animals and plants. Exactly howevolution occurs is still a matter of debate, but there are many differenttheories and that it occurs is a scientific fact. Biologists agree that all livingthings come through a long history of changes shaped by physical andchemical processes that are still taking place. It is possible that all organismscan be traced back to the origin of Life from one celled organims.The most direct proof of evolution is the science of Paleontology, orthe study of life in the past through fossil remains or impressions, usually inrock. Changes occur in living organisms that serve to increase theiradaptability, for survival and reproduction, in changing environments.Evolution apparently has no built-in direction purpose. A given kind oforganism may evolve only when it occurs in a variety of forms differing inhereditary traits, that are passed from parent to offspring. By chance, somevarieties prove to be ill adapted to their current environment and thusdisappear, whereas others prove to be adaptive, and their numbers increase.The elimination of the unfit, or the "survival of the fittest," is known asNatural Selection because it is nature that discards or favors aarticular being. Evolution takes place only when natural selection
operates on apopulation of organisms containing diverse inheritable forms.
HISTORY
Pierre Louis Moreau de Maupertuis (1698-1759) was the first
topropose a general theory of evolution. He said that hereditary material,consisting of particles, was transmitted from parents to offspring. His
opinionof the part played by natural selection had little influence on other
naturalists.
Until the mid-19th century, naturalists believed that each
species wascreated separately, either through a supreme being or through
spontaneousgeneration the concept that organisms arose fully developed from soil or
water. Thework of the Swedish naturalist Carolus Linnaeus in advancing the
classifying ofbiological organisms focused attention on the close similarity between
certainspecies. Speculation began as to the existence of a sort of blood
relationshipbetween these species. These questions coupled with the emerging
sciences ofgeology and paleontology gave rise to hypotheses that the life-forms of
the dayevolved from earlier forms through a process of change. Extremely
important wasthe realization that different layers of rock represented different time
periods andthat each layer had a distinctive set of fossils of life-forms that had
lived in the past.
Lamarckism
Jean Baptiste Lamarck was one of several theorists who
proposed anevolutionary theory based on the "use and disuse" of organs. Lamarck
stated thatan individual acquires traits during its lifetime and that such traits
are in some wayput into the hereditary material and passed to the next generation. Thiswas an attempt to explain how a species could change gradually over
time.According to Lamarck, giraffes, for example, have long necks because for
manygenerations individual giraffes stretched to reach the uppermost leaves
of trees, ineach generation the giraffes added some length to their necks, and they
passed thison to their offspring. New organs arise from new needs and develop inthe extent that they are used, disuse of organs leads totheir disappearance. Later, the science of Genetics disproved
Lamarck’s theory, itwas found that acquired traits cannot be inherited.
Malthus
Thomas Robert Malthus, an English clergyman, through his
work An Essayon the Principle of Population, had a great influence in directing
naturalists towarda theory of natural selection. Malthus proposed that environmental
factors such asfamine and disease limited population growth.
Darwin
After more than 20 years of observation and experiment,
Charles Darwinproposed his theory of evolution through natural selection to the
Linnaean Societyof London in 1858. He presented his discovery along with another Englishnaturalist, Alfred Russel Wallace, who independently discovered natural
selection atabout the same time. The following year Darwin published his full
theory,supported with enormous evidence, in On the Origin of Species.
Genetics
The contribution of genetics to the understanding of
evolution hasbeen the explanation of the inheritance in individuals of the same
species. GregorMendel discovered the basic principles of inheritance in 1865, but his
work wasunknown to Darwin. Mendel’s work was "rediscovered" by other scientists
around1900. From that time to 1925 the science of genetics developed rapidly,
and manyof Darwin’s ideas about the inheritance of variations were found to be
incorrect.Only since 1925 has natural selection again been recognized as essentialin evolution. The modern theory of evolution combines the findings of
moderngenetics with the basic framework supplied by Darwin and Wallace,
creating thebasic principle of Population Genetics. Modern population genetics was
developedlargely during the 1930s and ’40s by the mathematicians J. B. S. Haldane
and R. A.Fisher and by the biologists Theodosius Dobzhansky , Julian Huxley,
Ernst Mayr ,George Gaylord SIMPSON, Sewall Wright, Berhard Rensch, and G. LedyardStebbins. According to the theory, variability among individuals in a
population ofsexually reproducing organisms is produced by mutation and geneticrecombination. The resulting genetic variability is subject to natural
selection in theenvironment.
POPULATION GENETICS
The word population is used in a special sense to describe
evolution. Thestudy of single individuals provides few clues as to the possible
outcomes ofevolution because single individuals cannot evolve in their lifetime. An
individualrepresents a store of genes that participates in evolution only when
those genes arepassed on to further generations, or populations. The gene is the basic
unit in thecell for transmitting hereditary characteristics to offspring.
Individuals are unitsupon which natural selection operates, but the trend of evolution can be
tracedthrough time only for groups of interbreeding individuals, populations
can beanalyzed statistically and their evolution predicted in terms of average
numbers.
The Hardy-Weinberg law, which was discovered independently
in 1908 bya British mathematician, Godfrey H. Hardy, and a German physician,
WilhelmWeinberg, provides a standard for quantitatively measuring the extent ofevolutionary change in a population. The law states that the gene
frequencies, orratios of different genes in a population, will remain constant unless
they arechanged by outside forces, such as selective reproduction and mutation.
Thisdiscovery reestablished natural selection as an evolutionary force.
Comparing theactual gene frequencies observed in a population with the frequencies
predicted, bythe Hardy-Weinberg law gives a numerical measure of how far the
populationdeviates from a nonevolving state called the Hardy-Weinberg equilibrium.
Given alarge, randomly breeding population, the Hardy-Weinberg equilibrium will
holdtrue, because it depends on the laws of probability. Changes are
produced in thegene pool through mutations, gene flow, genetic drift, and natural
selection.
Mutation
A mutation is an inheritable change in the character of a
gene. Mutationsmost often occur spontaneously, but they may be induced by some externalstimulus, such as irradiation or certain chemicals. The rate of mutation
in humans isextremely low; nevertheless, the number of genes in every sex cell, is
so large thatthe probability is high for at least one gene to carry a mutation.
Gene Flow
New genes can be introduced into a population through new
breedingorganisms or gametes from another population, as in plant pollen. Gene
flow canwork against the processes of natural selection.
Genetic Drift
A change in the gene pool due to chance is called genetic
drift. Thefrequency of loss is greater the smaller the population. Thus, in small
populationsthere is a tendency for less variation because mates are more similar
genetically.
Natural Selection
Over a period of time natural selection will result in
changes in thefrequency of alleles in the gene pool, or greater deviation from the
nonevolvingstate, represented by the Hardy-Weinberg equilibrium.
NEW SPECIES
New species may evolve either by the change of one species
to another orby the splitting of one species into two or more new species. Splitting,
thepredominant mode of species formation, results from the geographical
isolation ofpopulations of species. Isolated populations undergo different
mutations, andselection pressures and may evolve along different lines. If the
isolation is sufficientto prevent interbreeding with other populations, these differences may
becomeextensive enough to establish a new species. The evolutionary changes
broughtabout by isolation include differences in the reproductive systems of
the group.When a single group of organisms diversifies over time into several
subgroups byexpanding into the available niches of a new environment, it is said to
undergoAdaptive Radiation .
Darwin’s Finches, in the Galapagos Islands, west of Ecuador,
illustrateadaptive radiation. They were probably the first land birds to reach the
islands, and,in the absence of competition, they occupied several ecological habitats
anddiverged along several different lines. Such patterns of divergence are
reflected inthe biologists’ scheme of classification of organisms, which groups
together animalsthat have common characteristics. An adaptive radiation followed the
first conquestof land by vertebrates.
Natural selection can also lead populations of different
species living insimilar environments or having similar ways of life to evolve similar
characteristics.This is called convergent evolution and reflects the similar selective
pressure ofsimilar environments. Examples of convergent evolution are the eye in
cephalodmollusks, such as the octopus, and in vertebrates; wings in insects,
extinct flyingreptiles, birds, and bats; and the flipperlike appendages of the sea
turtle (reptile),penguin (bird), and walrus (mammal).
MOLECULAR EVOLUTION
An outpouring of new evidence supporting evolution has come
in the 20thcentury from molecular biology, an unknown field in Darwin’s day. Thefundamental tenet of molecular biology is that genes are coded sequences
of theDNA molecule in the chromosome and that a gene codes for a precise
sequence ofamino acids in a protein. Mutations alter DNA chemically, leading to
modified ornew proteins. Over evolutionary time, proteins have had histories that
are astraceable as those of large-scale structures such as bones and teeth.
The further inthe past that some ancestral stock diverged into present-day species,
the moreevident are the changes in the amino-acid sequences of the proteins of
thecontemporary species.
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 isbased on modern green algae having in common with modern plants the samephotosynthetic pigments, cell walls of cellulose, and multicell forms
having a lifecycle characterized by Alternation Of Generations. Photosynthesis almost
certainlydeveloped first in bacteria. The green algae may have been preadapted to
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. Thebryophytes, which lack complex conducting systems, are small and are
found inmoist areas. The tracheophytes are plants with efficient conducting
systems; theydominate the landscape today. The seed is the major development in
tracheophytes,and it is most important for survival on land.
Fossil evidence indicates that land plants first appeared
during the SilurianPeriod of the Paleozoic Era (425-400 million years ago) and diversified
in theDevonian Period. Near the end of the Carboniferous Period, fernlike
plants hadseedlike structures. At the close of the Permian Period, when the land
became drierand colder, seed plants gained an evolutionary advantage and became the
dominantplants.
Plant leaves have a wide range of shapes and sizes, and some
variations ofleaves are adaptations to the environment; for example, small, leathery
leaves foundon plants in dry climates are able to conserve water and capture less
light. Also,early angiosperms adapted to seasonal water shortages by dropping their
leavesduring periods of drought.
EVIDENCE FOR EVOLUTION
The Fossil Record has important insights into the history of
life. The orderof fossils, starting at the bottom and rising upward in stratified rock,
corresponds totheir age, from oldest to youngest.
Deep Cambrian rocks, up to 570 million years old, contain
the remains ofvarious marine invertebrate animals, sponges, jellyfish, worms,
shellfish, starfish,and crustaceans. These invertebrates were already so well developed
that they musthave become differentiated during the long period preceding the
Cambrian. Somefossil-bearing rocks lying well below the oldest Cambrian strata contain
imprints ofjellyfish, tracks of worms, and traces of soft corals and other animals
of uncertainnature.
Paleozoic waters were dominated by arthropods called
trilobites and largescorpionlike forms called eurypterids. Common in all Paleozoic periods
(570-230million years ago) were the nautiloid ,which are related to the modern
nautilus, andthe brachiopods, or lampshells. The odd graptolites,colonial animals
whosecarbonaceous remains resemble pencil marks, attained the peak of theirdevelopment in the Ordovician Period (500-430 million years ago) and
thenabruptly declined. In the mid-1980s researchers found fossil animal
burrows inrocks of the Ordovician Period; these trace fossils indicate that
terrestrialecosystems may have evolved sooner than was once thought.
Many of the Paleozoic marine invertebrate groups either
became extinct ordeclined sharply in numbers before the Mesozoic Era (230-65 million
years ago).During the Mesozoic, shelled ammonoids flourished in the seas, and
insects andreptiles were the predominant land animals. At the close of the Mesozoic
the once-successful marine ammonoids perished and the reptilian dynasty
collapsed, givingway to birds and mammals. Insects have continued to thrive and have
differentiatedinto a staggering number of species.
During the course of evolution plant and animal groups have
interacted toone another’s advantage. For example, as flowering plants have become
lessdependent on wind for pollination, a great variety of insects have
emerged asspecialists in transporting pollen. The colors and fragrances of flowers
have evolvedas adaptations to attract insects. Birds, which feed on seeds, fruits,
and buds, haveevolved rapidly in intimate association with the flowering plants. The
emergence ofherbivorous mammals has coincided with the widespread distribution of
grasses,and the herbivorous mammals in turn have contributed to the evolution ofcarnivorous mammals.
Fish and Amphibians
During the Devonian Period (390-340 million years ago) the vast
land areasof the Earth were largely populated by animal life, save for rare
creatures likescorpions and millipedes. The seas, however, were crowded with a variety
ofinvertebrate animals. The fresh and salt waters also contained
cartilaginous andbony Fish. From one of the many groups of fish inhabiting pools and
swampsemerged the first land vertebrates, starting the vertebrates on their
conquest of allavailable terrestrial habitats.
Among the numerous Devonian aquatic forms were the Crossopterygii,lobe-finned fish that possessed the ability to gulp air when they rose
to the surface.These ancient air- breathing fish represent the stock from which the
first landvertebrates, the amphibians, were derived. Scientists continue to
speculate aboutwhat led to venture onto land. The crossopterygians that migrated onto
land wereonly crudely adapted for terrestrial existence, but because they did not
encountercompetitors, they survived.
Lobe-finned fish did, however, possess certain characteristics
that servedthem well in their new environment, including primitive lungs and
internal nostrils,both of which are essential for breathing out of the water.Such characteristics, called preadaptations, did not develop because the
others werepreparing to migrate to the land; they were already present by accident
and becameselected traits only when they imparted an advantage to the fish on
land.
The early land-dwelling amphibians were slim-bodied with fishlike
tails, butthey had limbs capable of locomotion on land. These limbs probably
developedfrom the lateral fins, which contained fleshy lobes that in turn
contained bonyelements.
The ancient amphibians never became completely adapted for
existence onland, however. They spent much of their lives in the water, and their
moderndescendants, the salamanders, newts, frogs, and toads–still must return
to water todeposit their eggs. The elimination of a water-dwelling stage, which was
achievedby the reptiles, represented a major evolutionary advance.
The Reptilian Age Perhaps the most important factor contributing to the becoming of
reptilesfrom the amphibians was the development of a shell- covered egg that
could be laidon land. This development enabled the reptiles to spread throughout the
Earth’slandmasses in one of the most spectacular adaptive radiations in
biological history.
Like the eggs of birds, which developed later, reptile eggs
contain acomplex series of membranes that protect and nourish the embryo and help
itbreathe. The space between the embryo and the amnion is filled with an
amnioticfluid that resembles seawater; a similar fluid is found in the fetuses
of mammals,including humans. This fact has been interpreted as an indication that
life originatedin the sea and that the balance of salts in various body fluids did not
change verymuch in evolution. The membranes found in the human embryo are
essentiallysimilar to those in reptile and bird eggs. The human yolk sac remains
small andfunctionless, and the exhibits have no development in the human embryo.Nevertheless, the presence of a yolk sac and allantois in the human
embryo is oneof the strongest pieces of evidence documenting the evolutionary
relationshipsamong the widely differing kinds of vertebrates. This suggests that
mammals,including humans, are descended from animals that reproduced by means ofexternally laid eggs that were rich in yolk.
The reptiles, and in particular the dinosaurs, were the dominant
landanimals of the Earth for well over 100 million years. The Mesozoic Era,
duringwhich the reptiles thrived, is often referred to as the Age of Reptiles.
In terms of evolutionary success, the larger the animal, the
greater thelikelihood that the animal will maintain a constant Body Temperature
independentof the environmental temperature. Birds and mammals, for example,
produce andcontrol their own body heat through internal metabolic activities (a
state known asendothermy, or warm-bloodedness), whereas today’s reptiles are thermally
unstable(cold-blooded), regulating their body temperatures by behavioral
activities (thephenomenon of ectothermy). Most scientists regard dinosaurs as
lumbering,oversized, cold-blooded lizards, rather than large, lively, animals with
fast metabolicrates; some biologists, however–notably Robert T. Bakker of The Johns
HopkinsUniversity–assert that a huge dinosaur could not possibly have warmed
up everymorning on a sunny rock and must have relied on internal heat
production.
The reptilian dynasty collapsed before the close of the Mesozoic
Era.Relatively few of the Mesozoic reptiles have survived to modern times;
thoseremaining include the Crocodile,Lizard,snake, and turtle. The cause of
the declineand death of the large array of reptiles is unknown, but their
disappearance isusually attributed to some radical change in environmental conditions.
Like the giant reptiles, most lineages of organisms have
eventually becomeextinct, although some have not changed appreciably in millions of
years. Theopossum, for example, has survived almost unchanged since the late
CretaceousPeriod (more than 65 million years ago), and the Horseshoe Crab,
Limulus, is notvery different from fossils 500 million years old. We have no
explanation for theunexpected stability of such organisms; perhaps they have achieved an
almostperfect adjustment to a unchanging environment. Such stable forms,
however, arenot at all dominant in the world today. The human species, one of the
dominantmodern life forms, has evolved rapidly in a very short time.
The Rise of Mammals
The decline of the reptiles provided evolutionary opportunities
for birds andmammals. Small and inconspicuous during the Mesozoic Era, mammals rose
tounquestionable dominance during the Cenozoic Era (beginning 65 million
yearsago).
The mammals diversified into marine forms, such as the whale,
dolphin,seal, and walrus; fossorial (adapted to digging) forms living
underground, such asthe mole; flying and gliding animals, such as the bat and flying
squirrel; andcursorial animals (adapted for running), such as the horse. These
variousmammalian groups are well adapted to their different modes of life,
especially bytheir appendages, which developed from common ancestors to become
specializedfor swimming, flight, and movement on land.
Although there is little superficial resemblance among the arm of
a person,the flipper of a whale, and the wing of a bat, a closer comparison of
their skeletalelements shows that, bone for bone, they are structurally similar.
Biologists regardsuch structural similarities, or homologies, as evidence of evolutionary
relationships.The homologous limb bones of all four-legged vertebrates, for example,
areassumed to be derived from the limb bones of a common ancestor.
Biologists arecareful to distinguish such homologous features from what they call
analogousfeatures, which perform similar functions but are structurally
different. Forexample, the wing of a bird and the wing of a butterfly are analogous;
both areused for flight, but they are entirely different structurally. Analogous
structures donot indicate evolutionary relationships.
Closely related fossils preserved in continuous successions of
rock stratahave allowed evolutionists to trace in detail the evolution of many
species as it hasoccurred over several million years. The ancestry of the horse can be
tracedthrough thousands of fossil remains to a small terrier-sized animal with
four toes onthe front feet and three toes on the hind feet. This ancestor lived in
the EoceneEpoch, about 54 million years ago. From fossils in the higher layers of
stratifiedrock, the horse is found to have gradually acquired its modern form by
eventuallyevolving to a one-toed horse almost like modern horses and finally to
the modernhorse, which dates back about 1 million years.
CONCLUSION TO EVOLUTION
Although we are not totally certain that evolution is how we got
the way weare now, it is a strong belief among many people today, and scientist
are findingmore and more evidence to back up the evolutionary theory.