Convergent Evolution

Convergent evolution is a common phenomenon, in which unrelated plants or animals come to resemble each other because they have adapted to similar ways of life. The similarity of some species of American cacti and certain desert living spurges in Africa is a good example. Seals and Penguins are apparently very different animals, but both are superbly adapted for swimming. Their limbs and general body shapes appear similar when observed in the water.
Few animals have a clear fossil record as the horse. The oldest known fossils of this family, from rocks some 55 million years old, are of a fox sized creature called Hyracotherium. Its teeth indicate that it ate leaves, and it probably lived in the forests. Fossils from successively younger rock layers show that the descendants of Hyracotheium gradually became larger and were represented by several distinct genera before producing the modern horse about two million years ago. Their toes were reduced to one on each foot and their teeth became larger and stronger to deal with tough grasses.

Taxonomy and Evolution

The sequence of bases along the DNA molecule may be compared between species. If the base sequences are similar then the species are closely related. For example, there is a 1.2 percent difference between the DNA of humans and that of chimpanzees, and a 1.4 percent differences between humans and gorillas, making the chimpanzee a closer relative to humans than the gorilla. Humans and orangutans share 97.6 percent of their DNA, but this does not match the 98.8 percent shared by chimpanzee and human.

Evolution is the process of gradual change by which the characteristics of living organization are altered from generation to generation the changes in given organism eventually become so great that it can no longer be regarded as the same species. Fossil records show that such replacement of one species by a more advanced one has been taking place for many millions of years, and that most species have survived for only about ten million years. The idea that living things have all evolved from simple ancestors was first put forward by the ancient Greeks, but until Charles Darwin and Alfred Russel Wallace published their work on natural selection no one could give satisfactory explanation.

Darwin and Wallace noticed that plants and animals produce large numbers of offspring and that there is a struggle for existence among the progeny- competition to find food and avoid predators as well as a struggle against adverse environmental factors. They also realized that the progeny vary, and reasoned that only those most suited to their surroundings would survive. The progressive improvement shown throughout evolution is thus brought about by natural selection of the best in each generation.