All life is based on fundamental molecule Deoxyribose nucleic acid (DNA). This molecule carries genetic codes from all proteins that make up the structural and functional components of life and is passed from generation to generation. Study of DNA molecule is important to know different aspects of life. DNA molecule controls activities of life which bears hydrogen, oxygen, carbon, nitrogen and phosphorus; cell is the fundamental unit of life.
Animals are united at all levels because of their evolutionary origin and shared forces that influence their history. Evolutionary processes are remarkable for their relative simplicity, yet they have some effects on life forms. Evolutionary processes have resulted in an estimated 4 to 30 million species of organisms living today. Out of these 1.4 million species have been described, many existed in the past and have become extinct.
They of organic evolution are the concept that population of organisms change over time.
Charles Darwin (1809 – 1892) described evidence for his theory in the origin of species by means of Natural selection in 1859. By 1900 most biologists were convinced that evolution takes place through natural selection evidences.
A number of evidences in favour of evolution are available some of these are as follows:
(1) Biogeography: It is the study of geographical study of plants and animals. Biogeographers try to explain why organisms are distributed as they are. Biogeographic studies show that life forms in different parts of the world have distinctive evolutionary histories. One of the distribution patterns that biogeographers try to explain is how similar groups of organisms can live in places separated by seemingly impenetrable of most continents of the earth, yet they cannot cross wide open oceans. Similarities in morphology suggest common ancestry but similarly obvious differences result from millions of years of independent evolution. Geographers also try to explain why plants and animals separated by geographical barriers like ocean, sea, rivers, streams, mountains, deserts and forests etc are often very different inspite of similar environments e.g. animals living in Australia and Tasmania are very different from animals in any other part of the world. Major native herbivores of
and Australia are many species of Kangaroo. Tasmania wolf known as tiger now considered being extinct was predatory marsupial that was unlike any other large predator. Tasmania
(2) Paleontology: It is based on study of fossil record, provides direct evidences for evolution. Fossils are the evidences of plants and animals that existed in the past have become incorporated into earth’s crust in the form of rocks or minerals. Paleontologists estimate that the earth is about 4.6 billion years old. They have also used the fossil record to describe the history of life on the earth.
(3) Comparative anatomy: A structure in one animal may resemble a structure in another animal because of common evolutionary origin. Structures or organs derived from common ancestry are called homologous organs and its study is known as homology. Homologous organs have same or similar origin and perform same functions e.g. vertebrate appendages have common arrangements of similar bones even though the function of appendages may vary. This indicates that vertebrates have evolved from common ancestor convergent evolution occurs when two unrelated organisms adapt to similar conditions e.g. wing of a bird and wing of an insect are both adapted for flight but they are not homologous. Being homologous these structures are analogous that is having similar function but dissimilar origin.
Organisms often retain structures that have lost their usefulness. These structures are poorly developed and are called vestigial structures e.g. boa constrictors have minute remnants of hind limbs bones left over from appendages of their reptilian ancestors. Such remnants of once useful structures are clear indications of evolution.
(4) Molecular biology: It has provided important information on evolutionary relationships. Structure and function are based on genetic blue print found in all living animals in the form of DNA molecule related animals have similar DNA derived from common ancestor. Because DNA carries the codes for proteins that make up each animal, related animals have similar proteins. Evolutionary theory has impressed biologist to believe with fundamental unity of all of biologists.