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Theories Of Organic Evolution


From very early times, scientists have been trying to explain the diversity of life forms through various theories. In the 19th century, however, the idea that complex animals and plants developed by gradual change from simpler forms was taken seriously. The mechanism of the origin of new species from the existing species was explained first by Jean Baptise Lamarack and then by Charles Darwin.



Having accepted the fact that new species have arisen from pre-existing species with modification, a number of scientists have explained their opinion about the mechanism by which this might have occurred. The first scientific theory concerning this came from Lamarck. His ideas, written in his book, Philosophy of Zoology (1809), are known asLamarckism.

  • Based on his observation, Lamarck proposed that variations among organisms originate because of response to the needs of the environment. Moreover, this ability to respond in a particular direction guides a trait’s adaptation. Thus, Lamarck placed fossils in the evolutionary context and stressed on adaptation as means for evolutionary modification. His theory is often called the theory of inheritance of acquired characters or the theory of use and disuse of organ.
  • Lamarck tried to explain the origin of long neck and high shoulders of giraffe on the basis of this principle. According to Lamarck, as the giraffes continually strained to stretch their necks for browsing leaves of higher levels of plants, their necks grew longer and shoulders grew higher in response to their needs.
  • But Lamarckism was very strongly criticised by August Weismann and he discarded the idea of inheritance of acquired characters. He showed that even after cutting the tails of mice continuously for 21 generations, a tail-less mouse was never born. Lamarck’s theory of evolution is now considered as an erroneous assumption since the acquired characters are not inherited.


  • Charles Robert Darwin clearly and convincingly set forth the concept of natural selection as the mechanism of evolution. Darwin gave the biological world a master key that unlocked the previous intricacies about evolution. In 1836, Darwin returned to England after 5 years on the Beagle and became established as one of the foremost naturalist of his time.
  • Darwin explained that despite having the enormous potential of fertility, the population size of any kind of organism remains within a limit. It is due to struggle between members of same species and different species for food, space and mate. The struggle eliminates the unfit individuals.
  • In other words, the fit organisms possess some variations, which are favourable, and they can leave the progeny to continue the favourable variations.
  • This, he called it as natural selection. These variations, when accumulated for a long time, lead to the origin of a new species.



With the development of new branches of biology, scientists have formulated modern concept about organic evolution. According to this modern concept, evolution is due to natural selection as explained by Darwin and due to ‘heritable variations in characteristics’.

Sources of variation

  • Genetic variation arises due to mutation, and it can account for the creation of a new species. Mutation is the process by which a gene or a chromosome set undergoes a structural change or a change in the amount of DNA it contains. Mutation may lead to a change in the expression and behaviour of a gene. It often produces lethal phenotypic effects.
  • Another source of variation is genetic recombination. It is a natural process due to which the arrangement of genes in the offspring is in a combination that differs from that of the parents. This is because the offspring receives genes from both parents and this ensures a certain amount of genetic variability from parents to offsprings.
  • Both mutations and genetic recombination may give rise to new characters. Natural selection may start working on these newly shown characters by favouring some of them and eliminating others. In this way, natural selection may lead to the evolution of a new group.
  • The process of formation of one or more new species from the existing one by evolutionary means is known as speciation or origin of species. It is now believed that sudden changes in genes or chromosomes also play an important role in speciation. Thus, all the above forces lead to the origin of new species or speciation. Figure 13.1 represents the types of speciation.


Difference between Darwinism and Neo-Darwinism is clearly shown in Table

Darwinism Neo-Darwinism
It is the original theory postulated by Darwin to explain
evolution and speciation.
This is the modern concept which is a modification of Darwinism in the light of new branches of biology such as genetics, cell biology, molecular biology etc.
Causes of variation are not explained.
It considers all favourable variations as heritable, which
form raw materials for evolution.
Causes of variations are explained.
It considers only genetic variations (mutation) as heritable,
which form raw materials for evolution.


Darwinism Neo-Darwinism
An individual is the unit of evolution in this theory.
Natural selection refers to survival of the fittest and
weeding out of unfit individuals in this theory.
Reproductive isolation is not considered as a factor in
Population is the unit of evolution in this theory
Natural selection refers to differential reproduction,
leading to change in gene frequency.
Reproductive isolation is an essential factor in speciation

Tracing evolutionary relationships

Tracing evolutionary relationships Classification is the method to group or categorise organisms by biological type, such as genus and species based on similarities and dissimilarities. Characteristics are physical, physiological and behaviouristic features of organisms. They are of two types, ancestral and derived.

  • Ancestral characteristic is a modified character shared between two groups and is present in their common ancestor(s) in an unmodified form. The more common characteristics the two species have, the more closely they are related. The more closely they are related, the more recent will be their common ancestor. Dog and wolf are closely related species. They evolved from a common ancestor. They are closely related to leopards. Therefore, leopard and dog have a distant common ancestor.
  • Derived characteristics develop due to evolutionary changes in ancestral or basic characteristics. Further evolutionary changes in the derived characters produce new features that give rise to smaller subgroups.

Evidences of Evolution

A number of common features of different kinds of organisms provide evidences in favour of evolution.

Evidences from homologous and analogous organs

  • The organs which are structurally similar but functionally dissimilar are called homologous organs. Homologous structures in different organisms are inherited from a common ancestor.
  • Vertebrate forelimbs contain the same sets of bones organised in similar ways despite their dissimilar functions. One can observe that the forearms of human, the wings of bat, the flippers of whales and the forelimbs of other vertebrates all are formed of the same basic skeletal elements. Even, they have a common structural plan. All of them contain bones named as humerus, radius, ulna, carpals, metacarpals and phalanges. Also, all the bones are derived from the same part of the body.
  • The converse of homologous organs is analogous organs. They are similar in function but are structurally different and unrelated. For example, the wings of birds and the wings of butterfly, both of which are used for flying, are completely different in their anatomical framework. Neither do they have similar origin nor do they have evolved from the same organ in a common ancestor. The flippers of penguin (bird) and dolphin (mammal), which perform similar functions in these aquatic animals, have originated from different structures of two different lineages.

Evidence from embryological studies

  • The embryology of different vertebrates provides strong evidence favouring organic evolution. The early embryos of different vertebrates show remarkable similarities. This indicates common origin and ancestry of the different vertebrates. This observation led Ernst Haeckel to propose a law, which is popularly known as the biogenetic law. This law states that ontogeny recapitulates phylogeny, which means that during the embryonic development of any organism, its complete evolutionary history is repeated.
  • Vestigial organs are remains of a structure that were functional in some ancestor but are no longer functional in the organism which have descended from the same ancestors. In humans, many vestigial structures show a relationship to other mammals, including the primates. For example, muscles of the external ear and scalp are rudimentary and often non-functional. But these are common to many mammals where they are functional. The reduced tailbones and nictitating membrane of the eye, the appendix of the caecum, rudimentary body hair and wisdom teeth, all are examples of vestigial organs. The presence of vestigial organs is explained by the common descent hypothesis.

Evidence from Fossils

  • Fossils are the remains and/or impressions of organisms that lived in the past. Fossils include not only the bones, teeth and other hard parts of the animal or plant body, but also any impression or imprint left by the past organisms in the soft soil that got hardened and retained these as impressions or imprints.
  • The fossils also provide evidences for evolution. For example, Archaeopteryx is a fossil bird that lived during upper Jurassic period of Mesozoic era. The primitive bird is considered to be a connecting link between reptiles and birds, as it possesses both reptilian and avian characters. This observation provides a clue that the birds have evolved from reptiles.


Age of Appearance Million Years Ago


Brain Capacity (cm3)








Large canines, incisors, molars

Soft fruit, leaves

Kruckle walker

Earliest fossil ape, persisted until 10 million years ago



Deeper jaw

Small canines, flattened molars, thick enamel

Seeds and nuts

Partially upright

Earliest hominid ground dwelling in savanna



Large jaws


Small canines, small incisors


Fully erect

Still at home in trees, but Savanna dwellers

Homo habilus


Lighter jaws




Fully erect

Earliest stone tools, began hunting, increase in brain

Homo eretus


Thick low forehead, brow ridges


Small carnivorous


5–6 feet tall

Beginning of cultural evolution, stone tools, hunting in hands, rudimentary language, used fire

Homo sapiens



Heavy jaw, long face, brow ridges, enlarged nasal cavity.


carnivorous wisdom teeth


5–6 feet

Cave dweller, buried their dead, flint flake tools

Homo sapiens



Vaulted cranium shorter skull, reduced jaws


Teeth close together, 
wisdom teeth


5–6 feet

Giving rise to geological races, cave painting

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