7. EVOLUTION
THEORIES OF BIOLOGICAL EVOLUTION
It is proposed by
Lamarck. It states that evolution of life forms occurred by the inheritance of
acquired characters.
Acquired
characters are developed by use & disuse of organs.
o Evolution by use of
organs: E.g. Long neck of giraffe is due to
continuous elongation to forage leaves on trees. This acquired character was
inherited to succeeding generations.
o Evolution
by disuse: E.g. Disappearance
of limbs in snakes.
This
theory was eliminated out because it is proved that the characters are
inherited only through genes.
- Proposed
by Charles Darwin.
- It
was based on observations during a sea voyage in a sail ship called H.M.S.
Beagle.
- Alfred
Wallace (a naturalist worked in Malay
Archepelago) had also come to similar conclusions.
-
Work
of Thomas Malthus on populations
influenced Darwin.
Darwinism
is based on 2 key concepts:
o Branching descent: It explains that all organisms are modified
descendants of previous life forms.
o Natural selection: Consider
a bacterial colony A growing on a
given medium. If the medium composition is changed, only a part of the
population can survive under new condition. This variant population (B)
outgrows the others and appears as new species, i.e. B is better than A under new condition. Thus, nature selects for
fitness.
Natural selection is
based on the following facts:
·
Heritable minor
variations: It is either beneficial or harmful
to the organisms.
·
Overproduction:
Population size grows exponentially due to maximum reproduction (E.g. bacterial
population).
·
Limited natural
resources: Resources are not increased in
accordance with the population size.
·
Struggle for existence: It
is the competition among organisms for resources so that population size is
limited.
·
Survival of the fittest: In
struggle for existence, organisms with beneficial variations can utilize
resources better. Hence, they survive and reproduce. This is called Survival of the fittest. It leads to a
change in population characteristics and new forms appear.
Darwin
ignored about origin of variation and mechanism of evolution or speciation.
- Hugo de Vries
proposed Mutation Theory of
evolution.
- He
conducted experiments on Oenothera
lamarckiana
(evening
primrose) and believed that evolution takes place through mutation and not by minor
variation.
- Darwinian
variation is minor, slow and directional. It
results in gradual evolution.
- Mutational variation is sudden, random &
directionless. Here, speciation is by saltation
(single step, large mutation).
- Mutation is the origin of variation for evolution.