EVIDENCES FOR EVOLUTION
Fossils are remained of hard parts of life-forms found in rocks. Rocks form sediments and a cross-section of earth's crust indicates the arrangement of sediments one over the other during the long history of earth. Different-aged rock sediments contain fossils of different life-forms who probably died during the formation of the particular sediments. Some of them appear similar to modern organisms. They represent extinct organisms (e.g., Dionsaurs). All this is called paleontological evidence.
Age of Fossils
The age of the fossils or rocks can be determined by 'Clock of the rock method' or 'Radioactive clock'. 'Clock of the rock' method is based on conversion of unstable radioactive nuclei into stable nuclei over a fixed period. The common radioactive elements which lose their radioactivity and change into their non-radioactive isotopes at a fixed rate are :
Potassium 40 ® Argon 40
Carbon 14 ® Nitrogen 14
Uranium 238 ® Lead 207
Rubidium 87 ® Strontium 87
Thorium 232 ® Lead 206
Comparative anatomy and morphology shows similarities and differences among organisms of today and those that existed many years ago.
Homologous : For example whales, bats cheetah and human (all mammals) share similarities in the pattern of bones of forelimbs. Though these forelimbs perform different functions in these animals, they have similar anatomical structure. Hence, in these animals, the same structure developed along different directions due to adaptations to different needs. This is divergent evolution and these structures are homologous. Homology indicates common ancestry. Other example are vertebrate hearts or brains. In plants also, the thorn and tendrils of Bougainvillea and Cucurbita represent homology. Homology is based on divergent evolution whereas analogy refers to a situation exactly opposite.
Analogy : Wings of butterfly and of birds look alike. They are not anatomically similar structures though they perform similar functions. Hence, analogous structures are a result of convergent evolution – different structures evolving for the same function and hence having similarity.
Other examples of analogy are the eye of the octopus and of mammals or the flippers of Penguins and Dolphins. One can say that it is the similar habitat that has resulted in selection of similar adaptive features in different group of organisms but towards the same function. Sweet potato (root modification) and potato (stem modification) is another example for analogy become insectivorous and vegetarian finches. Another example is Australian marsupial. A number of marsupials, each different from the other evolved from an ancestral stock, but all within the Australian island continent.
When more than one adaptive radiation appeared to have occurred in an isolated geographical area (representing different habitats), one can call this convergent evolution. Placental mammals in Australia also exhibit adaptive radiation in evolving into varieties of such placental mammals each of which appears to be ‘similar’ to a corresponding marsupial (e.g., Placental wolf and Tasmanian wolf marsupial).