Fossils are the only direct evidences to suggest that evolution has actually taken place and that the species are not fixed but ever changing entities. Unfortunately fossil history of almost every species is incomplete; thereby the evidence of fossils cannot be entirely relied upon in evolutionary studies. Therefore, other evidences, albeit circumstantial, have to be taken into consideration while studying evolution. Some of such evidences are outlined below.
MORPHOLOGICAL AND ANATOMICAL EVIDENCES
While comparing different organs of animal body of various groups, some interesting facts emerge that can be used to demonstrate that gradual change has actually taken place in species.
Homologous organs:When organs of similar origin and ancestry are found to perform different functions they are called homologous organs. For example, limbs of all tetrapods have evolved from the same basic structure found in amphibians but they have modified to perform varied functions in different groups, such as fore limb of man, horse, frog, bat’s and bird’s wing, flipper of dolphin and fossorial leg of mole, all have the same basic arrangement of bones.
They all have humerus, radius and ulna, carpals, metacarpals and phalanges in the fore leg and femur, tibia and fibula, tarsals, metatarsals and phalanges in the hind leg. Only their shape and size has modified to suit the specific need of the animal. Serial homology in the limbs of different groups of arthropods from the basic crustacean plan is another striking example of evolutionary modifications.
Analogous organs:Analogous organs are those which perform the same function but have not evolved from the identical ancestral form but have independent origin. For example, wings of insects, bats and birds are used for flight but have evolved independently and thus structurally different. Insect wing is made of chitinous venation and membrane, wing of bat is made of skin called patagium stretched between fingers while bird wing is made of feathers but they are all used for flying. Bones that support the wing of bat and bird are homologous but the patagium and feathers are not.
Adaptations: Environment and ecological conditions change continuously, more so over a long period of time. This puts pressure on the animals and plants to either change accordingly or perish. We, therefore, find animals belonging to the same group, living in different environmental conditions and undergoing different modifications not only in their morphology but also in habits. This may lead to convergent, divergent or parallel evolution in species.
Convergent evolution: Whales and dolphins having lived in aquatic situations for such a long time have changed into fish-like form because the environmental conditions demanded it but they still continue to carry mammalian features such as suckling the young and breathing air. This is called convergent evolution in which unrelated organisms start resembling each other due to environmental influence. Pectoral fins of fish and flipper-like fore limbs in dolphin, sea turtle and penguins demonstrate similarity due to convergence.
Parallel evolution: Sometimes related animals evolve side by side and resemble each other due to evolving in the similar environmental conditions, e.g. ungulates: deer, antelopes, goats, sheep, cow, bison etc. offer identical adaptations for grazing and fast running. All carnivores belonging to cat and dog family show parallel evolution in modifications that help in hunting down the prey. In Australia, marsupials exhibit parallel evolution to the placentals of the world because they occupy identical ecological niches.
Adaptive radiation: Populations of widely distributed species often encounter various types of environmental conditions to which they must adapt gradually. Such populations diverge from each other and in the long run become different species or genera. This phenomenon is called adaptive radiation and was noticed by Charles Darwin in different species of finches found in Galapagos Islands. All these finches had evolved from a single population that was accidentally blown off the coast of South America and upon finding all ecological niches vacant, adapted to feeding on different kinds of food.
Coevolution: An interesting case of evolution can be seen in coevolution, in which two or more species depend so heavily on each other for their survival and propagation that they constantly evolve together. Predator-prey, host-parasite and symbiotic species offer such reciprocal adaptations. While the prey constantly looks for new methods of escaping from predation, the predator manages to acquire equally efficient techniques to catch the prey and the coevolution goes on. The most remarkable coevolution is often seen in flowering plants and their insect pollinators. The nectaries of flowers are often so concealed as to be found only by a particular species of insect pollinator.
Mimicry: Mimicry is a phenomenon in which two different animals resemble each other for gaining protection from the predators. In Batesian mimicry the model is protected and the mimic gets protection by resembling morphologically with the model. So the predators are confused and let the mimic go too. For example, the poisonous Danaus butterfly is mimicked by Hypolimnas female and is avoided by the birds.
In Mullerian mimicry both the model as well as the mimic are protected but still resemble each other so that damage done to the population by the learning predators is shared by the two species. Several tailed amphibians having poisonous glands resemble each other and so do the wasps and unpalatable moths.
Vestigial organs: They are those organs which were functional in the ancestors but have lost their significance due to changing circumstances. When organs lose their utility they start to atrophy and eventually become rudimentary. In man 180 such organs are present, e.g. vermiform appendix, ear muscles, plica semilunaris in the eye which is a rudiment of nictitating membrane, rudimentary tail vertebrae called coccyx, small canines, third molar, body hairs etc.
These organs were in use in prehistoric man who lived in caves and fed on raw diet but lost their utility when he started to cook food and live in protected situations. Similarly, ostriches and other flightless birds have functionless wings, which lost their value when these birds started depending on their running speed to escape from predators.