This document contains the Theories to Explain the Diversity of Life. Two main theories of evolution i.e. Lamarckism and Darwinism are discussed.

1. Yousaf Rafique | Evolution | March 5, 2018
Theories to Explain the
Diversity of Life
Evolution and Basic Principles of Systematic Zoology

2. PAGE 1
Theories to Explain the Diversity of life
Theories for the explanation of diversity of life are given below:
Lamarckism (Inheritance of acquired characters)
 Lamarckism (or Lamarckian inheritance) is the hypothesis that an organism can
pass on characteristics that it has acquired during its lifetime to its offspring.
 It is also known as heritability of acquired characters or soft inheritance.
 This is the theory of organic evolution asserting that environmental changes cause
structural changes in animal and plants that are transmitted to their offspring.
 This theory explain that adaptation is concerned with hereditary changes caused by
environmental changes.
 The more uses of some organs according to their need increased in size, shape or
functions while less use of other organ led them to lost.
 According to Lamarck, animals move toward improved adaptation causing directional
changes.
 The long neck of the giraffe is often used as an example in popular explanations of
Lamarckism. However, this was only a small part of his theory of evolution towards
"perfection"; it was a hypothetical illustration; and he used it to discuss his theory of
heredity, not evolution
Objections on Lamarckism
 Objections were made on Lamarckism which are:
 Acquired characters cannot be inherited from parents to offspring.
 The phenomenon behind the adaptation was not explained in Lamarckism i.e.
evolution of adaptation was unexplained.
Jean-Baptiste Lamarck

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Darwinism (Theory of Natural Selection)
 Darwin’s theory of evolution is the widely held notion that all life is related and
has descendent from a common ancestor, the birds and the bananas, the
fishes and the flowers, all are related.
 Natural selection acts to preserve and accumulate minor advantageous
genetic mutations and is the most important fact of evolution.
 The elements or points of this theory are
Over production
In nature, plants and animals reproduce more and more to produce a lot
number of offspring which cause the struggle.
Struggle for existence
There is always competition among individuals and physical environment.
Struggle can be intraspecific, interspecific and environmental.
Variation
Darwin recognized that two individuals do not carry same requirements due
to which changes occur which causes variations. If there are no variations,
means no change so no evolution will occur. According to him, variation can
be continuous, cause evolution and discontinuous which are appeared
suddenly and do not cause evolution.
Natural Selection/Survival of the fittest
Natural selection is the differential survival and reproduction of individuals
due to differences in phenotype. It is a key mechanism of evolution, the
change in heritable traits characteristics of a population or generation. The
individuals provided with the favorable variations will lead a more successful
life and will survive. Survival variations lead to the speciation. Evolution
results by the cumulative effect of variations occur generation to generation.
Criticism on Darwin
Criticism made on Darwin are following
 All the work by Darwin lake experimental proofs.
 Darwinism does not explain the phenomenon behind the variations.
 Absence of connecting links between species cannot be explained on the
bases of this theory.
 The phenomenon of mimicry was not explained by Darwin.
 Natural selection does not explain the presence of vestigial organs.