Main aspects
for assignment on concept of evolution are
→ Theories
of evolution
→ Homologue
and analogue
→ Evidances
of evolution
→ Hardy Weinberg
Theorm
→ Fossil and
fossilization
→Types of
frog and their geological scale
°THEORIES OF EVOLUTIONt:
“The
formation of complex organisms from simple one ,with the passage of time is
known as the process of evolution”
Many
scientists made many hypothesis for the concept of evolution,some historical
aspects about evolution are given below
1-ARISTOTLE (322-384)B.C
He described
concepts of change in living organisms over time.
2-Georges-Louis Buffon(1707-1788)
He spent many years studying
comparative anatomy. His observations of structural variations in particular
organs of related animals convinced him that change must have occurred during
the history of life on earth. Buffon attributed change in organisms to the
action of the environment. He believed in a special creation of species and
considered change as being degenerate. For example, he described apes as
degenerate humans.
3-Eramus Darwin(1731-1802)
A Physician and the grandfather of
Charles Darwin, was intensely interested in questions of origin and change. He
believed in the common ancestry of all organisms.
4- Jean Baptiste Lamarck (1744-1892)
He was a distinguished French
zoologist. His contributions to zoology include important studies of animal
classification. Lamarck published a set of invertebrate zoology books. His
theory was based on a widely accepted theory of inheritance that organisms
develop new organs, or modify existing organs. Lamarck believed that “ need “
was dictated by environmental change and that change involved movement toward
perfection. The idea that change in a species is directed by need logically led
Lamarck to the conclusion that species could not become extinct, they
simply evolved into different species.
Lamarck illustrated his ideas of
change with the often-quoted example of giraffe. He contended that ancestral
giraffes had short necks, much like those of any other mammal. Straining to
reach higher branches during browsing resulted in their acquiring higher
shoulders and longer necks. These modifications, produced in one generation,
were passed on to the next generation. Lamarck published his theory in 1802 and
included it in one of his invertebrate zoology books, Philosophie Zoo (1809).
He defended his ideas in spite of intense social criticism.
Lamarck’s acceptance of a theory of
inheritance that we now know is not correct led him to erroneous conclusions
about how evolution occurs. There is no evidence that changes in the
environment can initiate changes in organisms that can be passed on to future
generations. Instead, change originates in the process of gamete formation.
Homology
and Analogy:
Structures and processes of organisms
may be alike. There are two reasons for similarities, and both cases provide
evidence of evolution. Resemblance may occur when two unrelated organisms adapt
to similar conditions. For example, adaption for flight have produced flat,
gliding surfaces in the wings of birds and insects. These similarities indicate
that independent evolution in these two groups of animals to exploit a common
aerial environment. The evolution of superficially similar structures in
unrelated organisms is called convergent evolution, and the similar structures
are said to be analogous.
Resemblances may also occur because
two organisms share a common ancestry. Structures and processes in two kinds of
organisms that are derived from common ancestry are said to be homologous.
Homology can involve aspects of an organism’s structure, and these homologies
are studied in the discipline called comparative anatomy. Homology can also
involve aspects of animal development and function and homologous processes are
studied using techniques of molecular biology.
Evidences
of evolution:
Biogeography:
It was the geographical distribution
of species---- biogeography---- that first suggested the idea of evolution to
Darwin. Islands have many species of plants and neighboring island. Consider
armadillos, the armored mammals that live only in America. The evolutionary
view of biogeography predicts that contemporary armadillos record confirms that
such ancestors existed.
The Fossil Record:
The succession of fossil forms is a
strong evidences in favour of evolution. It provides a visual record in a
complete series showing the evolution of an organism. For instance, evidence
from biochemistry, molecular biology and cell biology places prokaryotes as the
ancestors of all life and predicts and bacteria should precede all eukaryotic
life in the fossil record. Indeed, the oldest known fossils are prokaryotes.
Comparative Anatomy:
Anatomical similarities between
species grouped in the same taxonomic category bring another support to the theory of the Descent with
modification. For example, the same skeletal elements make up the forelimbs of
human, cats, whales, bats, and all other mammals, although these appendages
have very different functions. The basic similarity of these forelimbs is the
consequence of the descent of all mammals from the common ancestor. The arms,
wings, flippers, and forelegs of different mammals are variations on a common
anatomical theme that has been modified for divergent functions. Similarities
in characteristics resulting from common ancestry is known as homology, and
such anatomical signs of evolution are called homologous structures. Common
anatomy supports that evolution is a remodeling process in which ancestral
structures that functioned in one capacity become modified as they take on new
functions. The flower parts of a flowering plant are homologous. They are
considered to have evolved from leaves to form sepals, petals, stamens and
carpels.
The oldest homologous structures are
vestigial organs, rudimentary structures of marginal, if any use to the
organism. Vestigial organs are historical remnants of structures that had
important functions in ancestors but are no longer essential presently.
Comparative Embroyology:
Closely related organisms go through
similar stages in their embryonic development. For example, all vertebrate
embryos go through a stage in which they have gill pouches on the slides of
their throats. At embryonic stage of development, similarities between fishes,
frogs, snakes, birds, humans, and all other vertebrates are much more apparent
than differences. As development progresses, the various vertebrates diverge
more and more, taking on the distinctive characteristics of their classes.
Molecular biology:
Evolutionary relationships among
species are reflected in their DNA and proteins--- in their genes and gene
products. If two species have genes and copied from a common ancestor. For
example, a common genetic code brings evidence that all life is related.
Molecular biology thus provided strong evidence in support of evolution as the
basis for the unity and diversity of life.
References:
Stephen A. Miller and
John P. Harley 8th edition
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