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Evolution
The Origin of the Species
Before Darwin’s Evolutionary
Theory
In the 1700’s the study of fossils proved the existence of old organisms
different from those found before then. This suggested that living species
had undergone changes over time.
The naturalist Georges Louis Buffon ( 1707-1788) thought that living
forms had originated from a small number of ancestors.
Erasmus Darwin
1731-1802
Prominent
English
physician,
grandfather of the naturalist Charles
Darwin. He believed that species
modified themselves by adapting to
their environment in a purposive
way. It is as a transitional figure that
Erasmus Darwin is primarily
important. He embodied the attitudes
and
values
of
18th-century
materialism, but his conclusions
concerning evolution were drawn
from simple observation and were
rejected by the more sophisticated of
the 19th-century scientists, his
grandson Charles foremost among
them.
Jean-Baptiste Lamarck
1744-1829
Jean-Baptiste-Pierre-Antoine
de Monet, Chevalier de
Lamarck
pioneer
French
biologist who is best known for
his idea that acquired traits
are inheritable, an idea known
as Lamarckism, which is
controverted by
Darwinian
theory.
Cuvier
By rejecting the 18th-century method of
arranging animals in a continuous series
in favour of classifying them in four
separate groups, he raised the key
question of why animals were
anatomically
different.
Although
Cuvier's doctrine of catastrophism did
not last, he did set the science of
palaeontology on a firm, empirical
foundation. He did this by introducing
fossils into zoological classification,
showing the progressive relation
between rock strata and their fossil
remains, and by demonstrating, in his
comparative
anatomy
and
his
reconstructions of fossil skeletons, the
importance
of
functional
and
anatomical relationships
Charles Darwin
Charles Robert Darwin
(12 February 1809 – 19 April 1882)
was an English naturalist who
achieved lasting fame by producing
considerable evidence that species
originated through evolutionary
change, at the same time proposing
the scientific theory that natural
selection is the mechanism by which
such change occurs. This theory is
now considered a cornerstone of
biology.
Charles Darwin
Darwin developed an interest in
natural history while studying first
medicine, then theology, at university.
Darwin's observations on his fiveyear voyage on the Beagle brought
him eminence as a geologist and fame
as a popular author. His biological
finds led him to study the
transmutation of species and in 1838
he conceived his theory of natural
selection. Fully aware that others had
been severely punished for such
"heretical" ideas, he confided only in
his closest friends and continued his
research
to
meet
anticipated
objections. However, in 1858 the
information that Alfred Russel
Wallace had developed a similar
theory forced an early joint
publication of the theory.
Darwin’s Evolutionary Theory
Lyell was a Scottish geologist largely
responsible for the general acceptance
of the view that all features of the
Earth's surface are produced by
physical, chemical, and biological
processes through long periods of
geological time. The concept was
called uniformitarianism (initially set
forth by James Hutton). Lyell's
achievements laid the foundations for
evolutionary biology as well as for
an understanding of the Earth's
development.
Darwin’s Evolutionary Theory
Darwin's theory of evolution has four main parts:
a. Organisms have changed over time, and the ones living today are different
from those that lived in the past. Furthermore, many organisms that once lived
are now extinct. The world is not constant, but changing. The fossil record
provided ample evidence for this view.
b. All organisms are derived from common ancestors by a process of branching.
Over time, populations split into different species, which are related because
they are descended from a common ancestor. Thus, if one goes far enough back
in time, any pair of organisms has a common ancestor. This explained the
similarities of organisms that were classified together -- they were similar
because of shared traits inherited from their common ancestor. It also explained
why similar species tended to occur in the same geographic region.
c. Change is gradual and slow, taking place over a long time. This was supported
by the fossil record, and was consistent with the fact that no naturalist had
observed the sudden appearance of a new species.
d. The mechanism of evolutionary change was natural selection. This was the
most important and revolutionary part of Darwin's theory, and it deserves to be
considered in greater detail.
The Voyage on the Beagle
Charles Darwin sailed from
Plymouth on the 27
December 1831 on a
journey that was to take him
many thousands of miles.
He sailed in a ship called the
Beagle. The Beagle was not
a big ship, it was only about
28 metres long and had to
be home to 74 people for the
whole voyage.
The Voyage on the Beagle
The Development of a Theory
During the voyage Darwin
discovered that the animals on
the Galapagos Islands were not
quite like the animals he had
seen in South America. In some
ways the lizards were the same
as those in South America but
were different in others. Darwin
also observed this to be the same
with the many different types of
finches on the islands. They
were different from the finches
he had seen in South America.
The Development of a Theory
Darwin then found out something
even more important. A tortoise or
finch on one island was not quite
like a tortoise or a finch on another
island. By looking at the shell of a
tortoise or the beak of a finch you
could tell which island it had come
from. All the finches had dull
coloured feathers and short tails.
They all laid four pink and white
eggs in nests with small roofs. The
difference was they all had different
beaks. On one island they had
strong thick beaks for cracking open
nuts and seeds. On another island
eighty kilometres away they had
long thin beaks for catching insects.
The Origin of the Species
First published on November 24, 1859, The
Origin of Species (full title On the Origin of
Species by Means of Natural Selection, or
the Preservation of Favoured Races in the
Struggle for Life) by English naturalist
Charles Darwin is one of the pivotal works
in scientific history, and arguably the preeminent work in biology. In it, Darwin
makes "one long argument" for his theory
that "groups" of organisms, rather than
individual organisms, gradually evolve
through the process of natural selection—a
mechanism effectively introduced to the
public at large by the book. The work
presents detailed scientific evidence he had
accumulated both on the Voyage of the
Beagle in the 1830s and since his return.
Natural Selection
Natural selection is the process that results in the adaptation of an organism
to its environment by means of selectively reproducing changes in its
genotype, or genetic constitution.
In natural selection, those variations in the genotype that increase an
organism's chances of survival and procreation are preserved and
multiplied from generation to generation at the expense of less
advantageous ones. Evolution often occurs as a consequence of this
process. Natural selection may arise from differences in survival, in
fertility, in rate of development, in mating success, or in any other aspect of
the life cycle. All such differences result in natural selection to the extent
that they affect the number of progeny an organism leaves.
Artificial Selection
Close and constant observation of selective breeding of
animals and plants gave Darwin substantial proof to his
theory.
According to Darwin species evolved from a common
ancient ancestor, through modification.
Natural selection was the process which brought about new
species.
Types of Selection
A. Stabilizing selection
Natural selection can be studied by analyzing its effects on
changing gene frequencies; but it can also be explored by
examining its effects on the observable characteristics—or
phenotypes—of individuals in a population. Distribution scales
of phenotypic traits such as height, weight, number of progeny,
or longevity typically show greater numbers of individuals with
intermediate values and fewer and fewer toward the extremes
(the so-called normal distribution). When individuals with
intermediate phenotypes are favoured and extreme phenotypes
are selected against, the selection is said to be stabilizing.
B. Directional selection
The distribution of phenotypes in a population sometimes changes
systematically in a particular direction. The physical and biological aspects
of the environment are continuously changing, and over long periods of
time the changes may be substantial. The climate and even the
configuration of the land or waters vary incessantly. Changes also take
place in the biotic conditions; that is, in the other organisms present,
whether predators, prey, parasites, or competitors. Genetic changes occur as
a consequence, because the genotypic fitnesses may be shifted so that
different sets of alleles are favoured. The opportunity for directional
selection also arises when organisms colonize new environments where the
conditions are different from those of their original habitat. In addition, the
appearance of a new favourable allele or a new genetic combination may
prompt directional changes, as the new genetic constitution replaces the
preexisting one.
C. Diversifying Natural Selection
Diversifying, or dynamic, natural selection is one of the
important and basic processes by which evolutionary changes
occur. Under conditions of changing environmental pressure,
advantageous genotypes will be assimilated into the gene pool,
and those individuals within the population who have superior
fitness will leave more numerous offspring than those without.
Sexual Selection
Mutual attraction between the sexes is an important factor in reproduction.
The males and females of many animal species are similar in size and
shape except for the sexual organs and secondary sexual characteristics
such as the breasts of female mammals. There are, however, species in
which the sexes exhibit striking dimorphism. Particularly in birds and
mammals, the males are often larger and stronger, more brightly coloured,
or endowed with conspicuous adornments. But bright colours make
animals more visible to predators; the long plumage of male peacocks and
birds of paradise and the enormous antlers of aged male deer are
cumbersome loads in the best of cases. Darwin knew that natural selection
could not be expected to favour the evolution of disadvantageous traits, and
he was able to offer a solution to this problem. He proposed that such traits
arise by “sexual selection,” which “depends not on a struggle for existence
in relation to other organic beings or to external conditions, but on a
struggle between the individuals of one sex, generally the males, for the
possession of the other sex.”
Sexual Selection
The concept of sexual selection as a special form of natural
selection is easily explained. Other things being equal,
organisms more proficient in securing mates have higher
fitness. There are two general circumstances leading to sexual
selection. One is the preference of one sex (often the females)
for individuals of the other sex that exhibit certain traits; the
other is increased strength (usually among the males) that
yields greater success in securing mates.
Sexual Selection
The presence of a particular trait
among the members of one sex
can make them somehow more
attractive to the opposite sex. This
type of “sex appeal” has been
experimentally demonstrated in
all sorts of animals, from vinegar
flies to pigeons, mice, dogs, and
rhesus monkeys. When, for
example, Drosophila flies, some
with yellow bodies and others
with the normal yellowish-gray
pigmentation, are placed together,
normal males are preferred over
yellow males by females with
either body colour.
Proofs of Evolution
Fossils are considered proofs of the theory of
evolution.
The findings of various forms of fossils show that evolution
takes place in a chronological order.
Through fossils it is possible to link the species to their extinct
ancestors.
Proofs confirming the Evolutionary
Theory:
I. Biogeography
Biogeography is the study of the patterns of distribution of
animals and plants. It looks for general principles to explain
how and why organisms are distributed the way they are
locally or regionally as well as worldwide. Biogeography uses
information from ecology and evolutionary biology as well as
from geology (study of origin and structure of the Earth),
paleontology (study of fossils), and climatology (study of the
world's climates).
II. Comparative
Anatomy
Comparative anatomy is the study of similarities and
differences in the anatomy of organisms. Two major
concepts of comparative anatomy are:
Homologous structures - structures (body parts/anatomy)
which are similar in different species because the species
have common descent. They may or may not perform the
same function. An example is the forelimb structure shared
by humans, cats, whales and bats.
III. Comparative Embryology:
One of the marvels of nature is the
way in which a complex organism
develops from a single cell. The
fully formed organism, however, is
not produced in an instant. It is the
outcome
of
a
number
of
increasingly
intricate
changes
occurring over a protracted period
of time. This sequence takes place
in the very young organism—the
embryo. The study of the embryo's
formation and development is a
branch
of
biology
called
embryology.
IV. Comparative Anatomy
Analogous structures structures which are similar
in
different
organisms
because they evolved in a
similar environment, rather
than were inherited from a
recent common ancestor.
They usually serve the same
or similar purposes. An
example is the torpedo body
shape of porpoises and
sharks. It evolved in a
water environment, but the
animals
have
different
ancestors.
Many vertebrates share the same structure at the embryonic
stage (as seen in the picture: chicken embryo and human
embryo)
Coda
V: Evolutionary Biology
Evolutionary biology seeks to answer questions about the origin and
the genetic relationships of plants and animals, including the origin
of humans. Evolutionary biologists examine relationships between
organisms by means of comparative studies within any of the
previously named fields of biology or through paleontology.
Comparative studies reveal differences and similarities between
species or higher taxonomic units.
VI. Molecular Biology
Molecular Biology is the field of science concerned with
studying the chemical structures and processes of biological
phenomena at the molecular level. The discipline is particularly
concerned with the study of proteins, nucleic acids, and enzymes—
i.e., the macromolecules that are essential to life processes. The
discipline seeks to understand the molecular basis of genetic
processes; molecular biologists map the location of genes on specific
chromosomes, associate these genes with particular characters of an
organism, and use recombinant-DNA technology to isolate and
modify specific genes.
Extinction
Extinction occurs when a species
can no longer reproduce at
replacement
levels.
Most
extinctions are thought to have
resulted from environmental
changes that affected the species
in either of two ways. The
doomed species might not have
been able to adapt to the changed
environment and thus perished
without descendants; or it may
have adapted but, in the process,
may have evolved into a distinctly
new species. The effect of humans
on the environment, through
hunting, collecting, and habitat
destruction, has become a
significant factor in plant and
animal extinctions.
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