Lamarck Evolution Laws
Jean Baptiste de Lamarck (1744 – 1829) is well known for his theory
of evolution. In 1809 he published his book titled ‘Philosophie Zoologique’.
This book contains his views on evolutionary mechanisms. Eventhough the
views of Lamarck are not fully accepted, he occupies a very important place
in the history of evolutionary thought.
The theory of evolution as proposed by Lamarck is popularly known
as the ‘theory of inheritance of acquired characters’. According to this theory
modifications or changes acquired during the life time of an organism can
automatically be transmitted to succeeding generations. While elaborating
this theory, Lamarck advanced four laws or propositions.
I Law (or) Proposition
‘In evolution, during course of time, organisms or their
component parts gradually tend to increase in size.
Lamarck cited the evolution of horses as an example to
explain this law. The modern horses, namely, Equus evolved from very small
ancestral forms called Hyracotherium or Eohippus. Such small forms
survived years ago. They gradually evolved into larger modern Equus.
Lamarck’s opinion was based on fossils of several intermediate ancestors of
horses. Whose fossils had already been discovered. However during recent
years various other fossils had been obtained. Of these fossils, some of them
are much smaller than their immediate ancestors. This finding is against the
view already expresed by Lamarck. Thus the first law of Lamarck lost its
significance.
II Law or Proposition
‘If an organism is ‘in need’ of an organ, sooner or later it will
arise.
This view of Lamarck emphasized the significance of mind and its
thinking being related to needs in an environment. Thus, according to Lamarck
a continuous thinking for several generations can lead to the origin of an
adaptive character. Lamarck elaborated his view citing the lengthening of
neck in giraffee over the years.
It is known through fossil records that the ancestors of modern
giraffe were small and they had short neck and forelimbs. They lived in the
grasslands of Africa. These ancestral animals were feeding on grasses and
the leaves of small trees nearby. Gradually, as the grasslands were
transformed into deserts, the animals became dependent on trees for food.
Due to competition for food they had to stretch their neck for more leaves.
They strained their neck for several generations with a very strong inner
feeling to have longer neck. This strong desire, in course of time, led to gradual
increase in the length of neck and forelimbs.
In this explanation Lamarck considered that mere ‘want’ or ‘inner
feeling’ to possess a particular character can lead to the
origin of such a character. This view of Lamarck is not accepted by modern
evolutionists.
III Law (or) Proposition – Law of use and disuse
According to this law, constant use of an organ changes its
efficiency and makes that organ to increase in size with better development.
Similarly if an organ is not used for a long time, it might lead to reduction in
efficiency and size of that organ. The development of hand muscles of a
blacksmith and thigh muscles in the legs of an experienced runner were
quoted as examples. Eventhough this view of Lamarck is correct and
acceptable, it is not relevant to evolution due to lack of inheritance.
IV Law (or) Proposition. Inheritance of Acquired Characters
‘Bodily changes or new charateristics obtained by an
organism during its life time will automatically get transferred to the
next generation’.
While proposing this law, Lamarck did not provide any
specific example. He simply believed that due to conditions prevalent in an
environment, an organism can use an organ extensively and such an usage
can lead to more efficient and perfect nature of that organ. Similarly, an organ
not used for a longer period would degenerate. These perfect or degenerate
characteristics will be inherited by subsequent generations resulting in new
types.
The IV Law of Lamarck had been subjected to severe
criticisms. Several experiments had been carried out, either to prove or
disprove this concept.
In 1890, the German Scientist, August Weismann performed some
experiments with the rats. He selected a set of healthy male and female rats.
He started cutting their tails continually for more than twenty generations.
This experiment was performed to verify inheritance of the acquired
character, namely the tailless condition. Interestingly such a condition was
never observed in any of the young rats born. This finding led to the
proposition of the theory, that any change to the body regions (somatoplasm) will not have influence over the reproductive cells (Germplasm). Thus
Weismann, for the first time segregated germplasm from the somatoplasm.
This lead to the formulation of the ‘Germplasm theory’ which states
that ‘any change to the somatoplam will not have an influence over
the germplasm’.
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