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Principles
of Evolution The modern theory of evolution explains
the diversity of life from the perspective of science. It provides a scientific explanation of how the multitude of
species was created. Advances
in science, especially genetics, have strengthened this view of life in
part by explaining the basis of variation and inheritance. As you will see, variation and inheritance are both very
important to the theory. All
aspects of modern biology are affected by the theory of evolution by means
of natural selection. It has
been said that ‘nothing makes sense in biology except in light of the
theory of evolution.’ Note
the often used phrase, “The Darwinian Revolution”—a scientific
revolution that has changed the focus of biology. Theories
of Evolution I.
The Theory of Inheritance of Acquired Characteristics.
(1809) by Jean
Lamarck (see p. 178) Modifications acquired during one’s
lifetime are inherited by the next generation, ex. giraffes acquired a
long neck slowly over time as each generation of giraffe stretched its
neck slightly longer in trying to reach leaves high in trees.
At fist glance this theory is deceptively close to Darwin’s
theory (both include the concept that evolution produces life forms
adapted to their environments) but the inheritance of acquired
characteristics implies that the organism itself in response to
environmental pressures can control the direction
of change. Unfortunately,
there have been no discoveries of any such mechanism of change. II.
The Theory of Evolution by means of Natural Selection A.
The founders:
Both men were excellent observers and
prolific collectors and both worked as taxonomists. B.
Seven Influential Factors for both Wallace and Darwin 1).
Geology (see also p. 178-179)– study of the earth’s structure,
origin, and history. The
book, Principles of Geology by Charles Lyell (1797-1875),
includes the “theory of
uniformity” or “uniformitarianism"
- Geological processes have been
uniform through time. Geological
changes of the past were caused by same, observable processes of today.
The process of change has been uniform through time.
Canyons and thick layers of sedimentary rock are the profound
results of accumulated gradual change over vast stretches of time. The significance
to biology is this, slight changes
over a long time have large-scale impact and one can look at the present
to see the processes of the past.
Three concepts from geology are important in shaping the theory of
evolution by means of natural selection: 2).
Fossils -
extinct forms similar yet distinct from extant forms.
See fig. 12.4 showing modern day South American armadillo and an extinct,
South American glyptodont (armadillo-like but much larger).
Perhaps a history of change (evolution) connects the living with
the extinct. 3).
Island Life. Isolated
island populations differ slightly from nearby mainland
populations. e.g. Galapagos
Islands (ca. 5 MYO, 600 mi. off W coast of S.A) the birds, lizards, huge tortoises, similar to ‘nearby’ mainland
species yet obviously different. Therefore
perhaps there has been Descent with
modification (i.e., the island life evolved from mainland ancestors). 4).
Overproduction of individuals -
Thomas
Malthus (British Economist, p. 181) in his “Essay on the Principle of
Populations”(1798) attempts to explain that much of human suffering,
hunger, sickness, homelessness, and war are due to competition between an
ever increasing number of individuals or groups of individuals for limited
resources. The number of
individuals (population size) tends to increase exponentially.
This exponential capacity can’t be sustained (if it were, earth
would be covered over many times with individual life forms).
Thus,
more individuals are born than can live to reproduce and this leads to a struggle for existence.
Darwin and Wallace saw the concept relative to all species not just
humans. Only some
individuals live to reproduce, many (in fact, most) will die before
reaching reproductive age (think of all the acorns lying under an oak
tree, few if any will become a mature tree). What
determines, in nature [not human society] who lives and who dies?
An answer lies in part in variations among individuals.
[Social Darwinism is a distortion of the Theory of Evolution in the
Darwinian sense, Social Darwinism was used by Hitler to justify the
extirpation of Jews and his quest to destroy other countries;
Antievolutionists use Social Darwinism as an attack on the Theory of
Evolution. Social Darwinism is the erroneous application of “survival
of the fittest” to human societies.] 5).
Individuals within a species vary extensively. How does one know variations exist?
Taxonomy! A taxonomist
observes and describes the variation within a species. You don’t have to
be a taxonomist to know that individual people vary.
People vary not only in morphological traits such as hair texture
and body shape but also in physiological traits as seen in the
susceptibility of American Indians to European diseases [measles, mumps].
A knowledge of the variation in species of wild animal be they
bird, beetle, or barracuda, requires the careful and detailed observation
of many specimens. Only a
scrutinizing taxonomist, whether professional or amateur, sees the
variation between individuals within a wild species. Both Darwin and
Wallace were exceptional taxonomists who worked many years studying in
detail the variations of a particular group (e.g. Darwin worked on
barnacles for years). 6).
Many of the variations are inheritable and
some variations may impart greater reproductive success, as in artificial
selection below. 7).
Artificial Selection – “selective breeding by
humans of another species” (Harcourt Dictionary).
Variation among crops & livestock may be favored (selected) or
disfavored (selected against), or neutral, i.e., neither favored nor
disfavored, by man's control over which individuals are allowed to
reproduce. Man selects the
best (according to his taste) and breeds these for future generations.
Over time, new and improved varieties are created.
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