The five basic modes of locomotion are: running (in deer, cheetah, etc.), burrowing (in moles, rodents and rabbits), tree-climbing
(in squirrel and sloth), flying (in bats and flying squirrel) and swimming (in seals, whales, etc.).
Significance : Adaptive radiation helps in macro-evolution (at genus or species level) which divides a group into many new
groups.
— Adaptive convergence (Convergent evolution)
- Development of similar adaptive functional structures in unrelated groups of organisms is called adaptive convergence or
convergent evolution. - Examples : (i) Wings of insect, bird and bat show marked convergent evolution. (ii) Australian marsupials and placental
mammals show convergent evolution, e.g., Placental wolf and Tasmanian wolf-marsupial. - Anteaters such as spiny anteaters and scaly anteaters belong to different Orders of Class Mammalia, not closely related but
have acquired similar adaptations for diet of ants, termites and other insects.
Parallel evolution- When convergent evolution is found in closely related species, it is called parallel evolution. Example: development of
running habit in deer (2-toed) and horse (1-toed) with two vestigial splint bones. Tasmanian wolf is a marsupial while
wolf is a placental mammal. This also shows parallelism.
D. Palaeontological Evidences
- The fossils are the petrified remains and/or impressions of hard parts of the ancient organisms that are generally preserved
in the sedimentary rocks (formed at the sea floor by gradual deposition of soil particles in layer after layer and are richest
in fossils) or other media like volcanic ash, ice, peat bogs, sand, mud, etc. These conditions cut off oxygen and prevent
decay. These may also be formed from an impression of body part (e.g., footprint) or organic molecules (e.g., oil) or even a
coprolite (preserved excreta), etc. So, a fossil is an organic relic of the past life. - Paleontology is the study of past life based on the fossil records.
- Age of the fossils is determined by dating the rocks in which the fossils occur. It is done by carbon-14 dating technique
(carbon-14, a radioisotope produced by cosmic rays, is present in small proportions in air and is assimilated by the plants
in the process of photosynthesis. Half life period of^14 C is about 5,600 years. If the fossil animal has radioactivity one-eighth
of that of living animal, the fossil is about 16,800 years old (three half lives). This method is called absolute dating.
Missing links
- These are those extinct organisms that had the characters of two different groups of animals and confirm the path of
evolution between these groups. - E.g., Archaeopteryx (also called lizard-bird) is a missing link between reptiles and birds. Some of its reptilian and avian characters
are summarised below :
Reptilian characters Avian characters
(i) Presence of similar teeth in jaws. Presence of feathers on the body.
(ii) Each finger ending into a claw. Rounded cranium with intimate fusion of bones.
(iii) A long tail with free caudal vertebrae. Forelimbs modified into wings and have only three fingers.
(iv) Presence of keelless sternum. Presence of 4-toes in each foot and adapted for perching as in the birds.
(v) Non-pneumatic bones. Presence of furcula or wish-bone, jaws modified into beak.- Pteridosperms: These are missing links between ferns and gymnosperms (showed secondary growth and seed-habit).
- Distribution of fossils in different strata of rocks: It showed that there is a geological succession of fossils from
earliest to the recent rocks. Different aged rock sediments contain different life forms who probably died during the formation
of the particular sediment. Different classes of vertebrates appear chronologically in the fossil records. Fossil fishes predate
all other vertebrates, with amphibians next, followed by reptiles and then birds and mammals. The fossils showed following
changes in this geological succession: