Theory Of Evolution
What is evolution?
The Origin Of Species
The tumult in the middle of the 19° century was
accompanied by the publication of Darwin’s
Origin of species. The principal topic of this
work was that the life forms we see today are
the result of millions of years of evolution by a
process Darwin called “natural selection”. The
work had a revolutionary impact on Western
thought which has still not been fully
assimilated.
A Process Of Continuous Change
Biological evolution is a genetic change in a
population from one generation to another.
The speed and direction of change is variable
with different species lines and at different
times. Continuous evolution over many
generations can result in the development of
new varieties and species. Likewise, failure to
evolve in response to environmental changes
can, and often does, lead to extinction. All life
forms, including people, evolved from earlier
species. Furthermore, all still living species of
organisms continue to evolve today.
The Fossil Record
The remains of animals and plants found in
sedimentary deposits provide a record of past
changes through time. The existence of fossils
confirms that species are not fixed but can
evolve into other species over time. The
evidence also shows that what has appeared to
be gaps in the fossil record is due to incomplete
specific species lines. The most of these so-called
“missing links in the chain of evolution” are
filled with transitional fossil specimens.
Geographic Distribution Of
Related Species
Another clue of evolution is found in the natural
geographic distribution of related species. It is clear that
major isolated land areas and island groups often
evolved their own distinct plant and animal
communities. For instance, before humans arrived 6040000 years ago, Australia had more than 100 species of
kangaroos, koalas and other marsupials but didn’t have
none of the most advanced terrestrial placental
mammals such as dogs, cats, bears, horses. Land
mammals were entirely absent from the isolated island
that make up Hawaii and New Zealand. Each of these
places had a great number of plant, insect and bird
species that were found nowhere else in the world. The
most likely explanation for the existence of Australia’s,
New Zealand’s, and Hawaii’s biotic environments is that
the life forms in these areas have been evolving in
isolation from the rest of the world for millions of years.
Pre-Darwinian Theories
Carolus Linneous
The Swedish botanist Karl von Linné(1707-1778)
was the first class scientist. His most
important contribution to science was is
logical classification system for all living
beings he proposed in is book “Systema
Naturae”(1735). He described plants and
animals on the basis of physical appearance
and classified them relatively to each other
according to the degree of their similarities. He
used a binomial nomenclature in naming
them. Organisms were given two Latin names:
genus (usually written with a capital letter)
and species. Each genus could have many
related species and it was also part of larger
categories of living beings.
Jean Baptiste De Monet ,
Chevalier De Lamarck
The first evolutionist who started his ideas about the
processes leading by the logical change was a French
aristocrat, J. B. Lamarck( 1744-1829). Unfortunately, his
theory about these processes was incorrect. Lamarck
believed that microscopic organisms appeared
spontaneously from inanimate materials and than
evolved gradually and progressively into more complex
forms through a constant tension to the perfection.
Lamarck thought that humans were the ultimate
product of this evolution. He believed that evolution was
mostly due to the inheritance of acquired characteristics
as creatures adapted to their environments. He thought
that giraffes evolved their long necks by each generation
stretching further to get leaves in trees and that this
change in body shape was then inherited. Lamarck also
believed that creatures could develop new organs or
change the structure and function of old ones as a result
of their use or disuse.
George Cuvier
A French scientist, George Cuvier(1769-1832),
discredited Lamarck’s theory. Cuvier did not
reject the idea that there had been earlier life
forms. He was the first scientist to describe
extinctions of ancient animals. However, he
rejected the idea that their existence implied
that evolution had occurred: he maintained
the “fixity” of species. Cuvier advocated the
theory of catastrophism. This held that there
have been violent and sudden natural
catastrophes such as great floods and the rapid
formation of major mountain chains. Plants
and animals living in those parts of the world
where such events occurred, were often killed
off according to Cuvier. Then new life forms
moved in from other areas. As a result, the
fossil record for a region show, abrupt changes
in species.
Charles
Darwin
Darwin had observed the wide
variety of finches and the unusual
types of tortoises and iguanas,
found on the Galapagos islands. He
also had a knowledge of
comparative anatomy and an
understanding of previously
circulating theories of evolution,
which were suggestive but flawed.
It was from these elements that
Darwin created his powerful new
According to Darwin’s theory, initially slight
variations in populations of organisms may
give certain individuals an advantage in
adapting to their environment. Any such
advantage may be passed on from parents to
offsprings, causing the advantageous traits to
proliferate in subsequent generations.
Meanwhile, the subsequent generations of
organisms may eventually come from their
own species. Species are caused by the struggle
for existence in specific environmental
conditions. For example, it’s clear that Bengal
tigers, Siberian tigers and African lions are
probably descended from the same predecessor,
but they now belong to different species of the
genus felis.
This work has been produced by:
Rosa Scanniffio
Flavia Ndriollari
Orsolinda Perrucci
Rosanna Lopedota