Thoeries of Evolution
Evolution is the process by which living organisms originated on earth and have changed
their forms to adapt to the changing environment. The earliest known fossil organisms
are the single-celled forms resembling modern bacteria; they date from about 3.4 billion
years ago. Evolution has resulted in successive radiations of new types of organisms,
many of which have become extinct, but some of which have developed into the present
fauna and flora of the world (Wilson 17).
Evolution has been studied for nearly two centuries. One of the earliest evolutionists
was Jean Baptiste de Lamarck, who argued that the patterns of resemblance found in
various creatures arose through evolutionary modifications of a common lineage.
Naturalists had already established that different animals are adapted to different modes
of life and environmental conditions; Lamarck believed that environmental changes evoked
in individual animals direct adaptive responses that could be passed on to their
offspring as inheritable traits. This generalized hypothesis of evolution by acquired
characteristics was not tested scientifically during Lamarck's lifetime.
A successful explanation of evolutionary processes was proposed by Charles Darwin.
His most famous book, On the Origin of Species by Means of Natural Selection (1859), is a
landmark in human understanding of nature. Pointing to variability within species,
Darwin observed that while offspring inherit a resemblance to their parents, they are not
identical to them. He further noted that some of the differences between offspring and
parents were not due soley to the environment but were themselves often inheritable.
Animal breeders were often able to change the characteristics of domestic animals by
selecting for reproduction those individuals with the most desirable qualities. Darwin
reasoned that, in nature, individuals with qualities that made them better adjusted to
their environments or gave them higher reproductive capacities would tend to leave more
offspring; such individuals were said to have higher fitness. Because more individuals
are born than survive to breed, constant winnowing of the less fit-a natural
selection-should occur, leading to a population that is well adapted to the environment
it inhabits. When environmental conditions change, populations require new properties to
maintain their fitness. Either the survival of a sufficient number of individuals with
suitable traits leads to an eventual adaptation of the population as a whole, or the
population becomes extinct. Evolution proceeds by the natural selection of well-adapted
individuals over a span of many generations, according to Darwin's theory(Microsoft 96).
The parts of Darwin's theory that were the hardest to test scientifically were the
interferences about the heritability of traits because heredity was not understood at
that time. The basic rules of inheritance became known to science during the turn of the
century, when the earlier genetic works of Gregor Mendel came to light. Mendel had
discovered that characteristics are transmitted across generations in discrete units,
known as genes that are inherited in a statistically predictable fashion. The discovery
was then made that inheritable changes in genes could occur spontaneously and randomly
without regard to the environment. Since mutations were seen to be the only source of
genetic novelty, many geneticists believed that evolution was driven onward by the random
accumulationof favorable mutation changes. Natural selection was reduced to a minor
role by mutationist such as Vries. Morgan, and Bates.
While mutation was replacing Darwinism, the leading evolutionary theory, the science of
population genetics was being founded by Sewall Wright, J.B.S. Haldine, and several other
geneticists, all working independantly. They developed arguments to show that even when
a mutation that is immediately favored appears, its subsequent spread within a population
depends on such variables as the following:
the size of the population
the length of generations
the degree to which the mutation is favorable
the rate at which the same mutation reappears in descendants
Furthermore, a given gene is favorable only under certain environmental conditions. If
conditions change in space, then the gene may be favored only in a localized part of the
population; if conditions change over time, the gene may become generally unfavorable.
Because different individuals usually have different assortments of genes, the total
number of genes available for inheritance by the next generation can be large, forming a
vast store of genetic variability. This is called the gene pool. Sexual reproduction
ensures that the genes are rearranged in each generation, a process called recombonation.
Mutations provide the gene pool with a continuous supply of new genes; through the
process of natural selection the gene frequencies change so that advantageous genes occur
in greater proportions(Ardrey 24).
As the new evolutionary theory became enriched from such diverse sources, it became
known as the synthetic theory. Three American scientists made controbutions that were
especially important. The German-born Ernst Mayr, a zoologist, showed that new species
usually arise in geographic isolation, often following a genetic turn that quickly
changes the contents of their gene pools. George Simpson, a paleontologist, showed from
the fossil record that rates and modes of evolution are correlated. G. Ledyard
Stebbins, a botanist, showed that plants display evolutionary patterns similar to those
of animals, and especially that plant evolution has demonstrated diverse adaptive
responses to environmental pressures and opportunities. In additon, these biologists
reviewed a broad range of genetic, ecological, and systematic evidence to show that the
synthetic theory was strongly supported by observation and experiment.
During the establishment of the synthetic theory of evolution, the science of heredity
underwent another drastic change in 1953, when James Watson and Francis Crick
demonstrated the way genetic material is composed of two nucleic acids, deoxyribonucleic
acid (DNA) and ribonucleic acid (RNA). Nucleic acid molecules contain genetic codes that
dictate the manufacture of proteins, and the latter direct the biochemical pathways of
development and metabolism in an organism. Natural selection can then operate to favor
or supress a particular gene according to how strongly its protein product contributes to
the reproductive success of the organism.
Life originated more than 3.4 billion years ago, when the earth's environment was much
different than that of today. Especially important was the lack of significant amounts
of free oxygen in the atmosphere. Experiments have shown that rather complicated organic
molecules, including amino acids, can arise spontaneously under conditions that are
believed to simulate the earth's primitive environment.
The earliest organisms that still exists would be cells, resembling modern bacteria.
These simple unicellular forms(procaryotes) were at first anaerobic, but they diversified
into and array of adaptive types from which blue-green algae descended, including aerobic
photosynthesizers. Advnced cells (eucaryotes) may have evolved through the amalgamation
of a number of distinct simple cell types. A large ingesting cell may have incorporated
as symbionts some small blue-green algal cells that evolved into chloroplast and some
tiny aerobic bacteria that evolved into mitochondria(Reader 45).
In order for complex animal communities to develop, plants must first become established
to support herbivore populations, which in turn may support predators and scavengers.
Land plants appeared about 400 million years ago, spreading from lowland swamps as
expanding greenbelts(Gribbon 208).
Dinosaurs and mammals shared the terrestrial environment for 135 million years.
Dinosaurs may well have been more active, and certainly were larger, than their mamalian
contemporaries, which were small and pssibly nocturnal. The mammals, however, survived a
wave of extinction that eliminated dinosaurs about 65 million years ago, and subsequently
diversified into many of the habitats and modes of life that formerly had been
dinosaurian(Gribbon 211).
Humans belong to an order of mammals, the primates, which existed before the dinosaurs
became extinct. Early primates seem to have been tree dwelling and may have resembled
squirrels in their habitats. Many of the primate attributes, the short face, overlapping
visual fields, grasping hands, large brains, and even alertness and curiosity, must have
been acquired as arboreal adaptations. Descent from tree habitats to forest floors and
eventually to more open country, however, was associated with the development of many of
the unique features of the human primate, including erect posture and reduced canine
teeth, which suggest new habitats of feeding(Schwartz 78).
The history of life as inferred from the fossil record displays a wide variety of trends
and patterns. Lineages may evolve slowly at one time and rapidly at another time, they
may follow one pathway of change for sometime only to switch to another pathway, and
they may diversify rapidly at one time and then shrink under widespread extinctions.
The key to many of these patterns is the rate and nature of environmental change.
Species become adapted to the environmental conditions that exist at a given time, and
when change leads to new conditions, they must evolve new adaptations or become extinct.
When the environment undergoes a particularly rapid or extensive change, waves of
extinction occur. These are followed by waves of development of new species. The times
of mass extinction are not yet well understood. Although the most famous one is that of
the dinosaurs, about 65 million years ago, such events appear in the fossil record as far
back as Precambrian time, when life first arose. In fact, five mass extinctions on the
scale of that at the end of the age of dinosaurs are known over the past 600 million
years. Some scientists also claim to have demonstrated a definite periodicity to smaller
periods of mass extinction, and in particular a 26-million-year cycle of eight
extinctions over the past 250 million years(Wilson 34).
Controversy has arisen over the proposal made by some geologists that mass extinctions
are related to periodic catastrophes such as the striking of the earth's surface by a
large asteroid or comet. Many paleontologists and evolutionary theorists reject such
hypotheses as unjustified. The feel that periods of mass extinctions can be accounted
for by less spectacular evolutionary processes and by more earthbound events such as
cycles of climatic change and volcanic activity. Whatever proposals may eventually
prove true, however, it seems fairly certain that periodic waves of mass extinction do
occur.
Species adapted to live in environments that are changeable in the short term have broad
tolerances, which may better enable them to survive extensive changes. Human beings are
uniquely adapted in that they make and use tools and devices and invent and propogate
procedures that give them extended control over their environments. Humans are
significantly changing the environment itself. The effects are most complex and cannot
be predicted, and yet like the likelihood is that evolutionary patterns in the future
will reflect the influence of the human species(Microsoft96).
Works Cited
Ardrey, Robert. The Hunting Hypothesis: A Personal Conclusion Concerning the
Evolutionary Nature of Man. New York: Antheneum, 1976.
Encarta 96. Computer Software. Microsoft, 1995.
Gribbon, John and Cherfas, Jeremy. The Monkey Puzzle: Reshaping the Evolutionary Tree.
Philly: Pantheon, 1982.
Reader, John. Missing links: The Hunt for Earliest Man. Boston: Little, 1981
Schwartz, Jeffery H. The Red Ape: Orang-Utans and Human Origins.
San Francisco: Houghton, 1987.
Wilson, Peter J. The Domestication of the Human Species. Oxford:
Yale, 1991.
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