Evolution in biology
can simply be defined as descent with modification. The central idea behind
evolution is that all life on Earth shares a common ancestor. Evolution occurs
if there is a change in the heritable characteristics of a population over successive
generations. Evolutionary processes bring about biodiversity
Theories
of Evolution:
Lamarckism: Lamarckism which is also
known as “use and disuse” or “The inheritance of the acquired characteristics” was
laid down by a French biologist, Jean-Baptiste de Lamarck. Lamarck published
this theory in his book “Philosophie zoologique”. According to Lamarck:
1.
The ecological conditions in which the organisms
live influence the life processes of the organisms.
2.
The structure and development of an organ
depends of the use and disuse of that organ.
3.
By continual use of an organ to adapt to a
particular environment enhances the size and efficiency of that organ and
continuous disuse of an organ may lead to reduced efficiency and degeneration
of that organ and may become lost at times.
4.
The change in character of an organism due to
environment during the life of an organism become heritable and can be
transmitted to the next generation.
Lamarck’s theory may
sound conceivable, but later the validity of the theory was seriously
questioned by the scientific community.
Darwinism: Darwinism is the “Theory
of natural selection”. Darwin explained the process of evolution in his book “On
the origin of species by means of natural selection” or simply “The origin of
species”. Darwinism is one of the most convincing and impressive explanation of
evolution. Darwin’s theory is based on the following:
1.
Prodigality of reproduction: An individual have natural tendency to
overproduce offspring. If all the offspring survive and reproduce then these
will overcrowd the world within a few years. But the number of offspring of
species is maintained at more or less constant level under natural conditions.
2.
Struggle for survival: Overproduction
of offspring leads to struggle for survival among the individuals as food and
space remain almost constant. There is always competition for food and space
among the organisms of a species. The struggle for survival can be classified
as:
1.
Intraspecific competition: Competition
among the individuals of the same species.
2.
Interspecific competition:
Competition among the individuals of different species.
3.
Struggle against Environment:
Individual of a species struggle against the environmental conditions such as
excessive heat or cold, drought, flood, earthquakes etc. for survival.
3.
Variation among organisms: Variation is the difference between the
organisms of the same species. Darwin believed that some of the variations
among the organisms of the same species let them adapt to a certain environmental
condition. Sometime a variation is beneficial while sometime it is not.
4.
Survival of the fittest: The organism with advantageous variation
survive in greater number while the unfit are eliminated.
5.
Natural selection: The organism with advantageous variation that survived will
produce offspring and tend to preserve the variation and pass it down to the
next generation.
6.
Origin of Species: Continuous natural selection preserve and accumulate
particular types of variations which may lead to a new species similar to its
ancestors.
Evidence
of evolution:
1.
Homologous organs: These are structurally and ontogenetically similar
organs due to common ancestry but they have different functions.
For example: flippers of whale, wings of bats, forelimbs of horse and the hands
of human being.
2.
Analogous organs: These are structurally and ontogenetically different
but they have similar functions. For example: Wings of butterfly,
the wings of birds and the wings of bat.
3.
Vestigial organs: These are one of the most convincing evidence of evolution.
Vestigial organs are rudimentary and functionless organs which may be well
developed in another organisms. Such as, vermiform appendix in human, caudal
vertebrae of human etc.
4.
Embryological evidence: The “Theory of recapitulation” or the “Biogenetic
law” which is formulated by E. Haeckel explains that an organism during its
development briefs the ancestral history of the organism i.e. “ontogeny recapitulates
phylogeny”.
Pattern of Evolution:
1.
Convergent Evolution: Convergent evolution produces analogous structure. Convergent
evolution is a process in which two or more species become more and more similar.
2.
Divergent Evolution: Divergent evolution produces homologous structure. Homologous
structures have common origin. Divergent evolution is the process in which two
or more related species become more and more dissimilar.
3.
Coevolution:
Coevolution is the process where two or more species reciprocally affect
each other’s evolution. Coevolution occurs when different species have close
ecological interactions with each other. These ecological interactions include:
1.
Predator/prey and parasite/host
2.
Competitive species
3.
Mutualistic species
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