Evidence of Evolution – There are number of evidences which support the theory of evolution. These evidences are based on data obtained from many sources and in all cases the interpretation shows that evolution does occur.
Evidence for evolution comes from many fields as for example geology, biogeography, comparative anatomy, embryology, artificial selection and molecular biology. However we shall discuss only the evidences from comparative anatomy and artificial selection in this book.
The theory of evolution as proposed by Darwin has been modified in the light of modern evidence from genetics, molecular biology, palaeontology, ecology and ethnology (the study of behavior) and is known as neo-Darwinism or synthetic theory of evolution. This may be defined as the theory of organic evolution by the natural selection of inherited characteristics.
Comparative Anatomy
Evidence from comparative anatomy consists of two major types. One is the underlying similarity of comparable structure of pentadactyl limbs (penta: five and dactyl:digits) in different species (fig. 18.3). The arm of a man, the forelimb of horse, the flipper of a whale, the wings of bird and the
In human body, certain structures are considered vestigial e.g., human appendix, although not concerned with digestion is homologous with functional appendix of herbivorous mammals. The vertebrae of human coccyx are thought to represent vestigial structure of the tail possessed by our ancestors and embryos. Likewise non-functional bones in snakes and whales are thought to be homologous with hipbones and hindimbs of vertebrates.
Table. 18.2 Comparison of Lamarckism and Darwinism.
| Lamarckism | Darwinism |
| Ancestral giraffes probably had short necks and by continuous stretching enabled the giraffes to reach the leaves of trees.
| Ancestral giraffes probably had necks of various lengths. The variations were hereditary.
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| He offsprings had longer necks that were also stretched frequently in search of food.
| Competition and selection led to survival of longer necked offsprings at the expense of shorter necked giraffes.
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| Eventually the continued stretching of the neck gave rise to modern giraffes.
| Eventually only long necked giraffes survived the competition.
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Forelimb of a bat, all contain similar bones and muscles. Since these structures have different functions but their structural resemblance indicates that they were modifications over a time of some common ancestral structure. The term homology is applied to this phenomenon and such organs which are structurally alike but functionally different are known as homologous organs. The evolutionary process producing these organs is called divergent evolution.
The other type is shown by the development of structure with similar function in very different evolutionary lines. This is called convergent evolution and is seen in the close resemblance between the legs of insects and mammals and wings of butterflies and birds. These organs are called analogous organs and they serve similar functions but have different anatomical origin.
Fig. 18.3 pentadactyl limb (center) and its modifications depending on life style of various animals.
Another source of evidence for evolution in comparative anatomy is the existence of vestigial organs. These are the body parts that were presumably functional at an early time in the ancestral form but have since become remnants. The presence of vestigial organs is an evidence in favour of evolution because it would be much harder to account for non-functional vestiges without having been inherited from some ancestor where these were functional.
Artificial selection
Evidence of Evolution – One of Darwin’s most difficult tasks was to convince biologists that selection can produce change within a species.
He supported his argument by pointing out that farmers and breeders have developed many varieties of animals and plants through artificial selection. N addition Darwin himself bred pigeons and got variety in color, size, form and behavior (figure 18.4) through artificial selection which gave him a clue to the concept of artificial selection but he was unable to appreciate how this could operate in nature . Darwin used this observation to argue that species can change and that its characters are not fixed. He said if change is possible artificially, a similar process of change of selection can also occur in nature.
Fig 18.4 many types of pigeons produced by artificial selection.
Now a days biologists have produced completely new varieties of animals and plants by artificial selection. It is the process in which organisms with the ‘best’ or ‘desired’ traits/ characters are selected and bred with one another so that the traits/characters would be passed on to the offspring. Numerous varieties of animals (e.g., sheep, goats, cows, hens) have been produced by artificial selection to increase production of meat, milk, wool etc. similarly many plant varieties (fig. 18.5) for example cereals, fruits, vegetables etc. are produced for better quality in production and taste by artificial selection.
Evidence of Evolution
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Fig 18.5 many varieties of plants produced by artificial selection.