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Species Genus Family Order - Domain Figure pat{:erm be fossils of transitional forms between them and there are. Transitional species are defined as those that show a mixture of features from both their ancestors and their descendants. There are transitional fossils between fish and amphibians, reptiles and birds, hippos and whales, and apes and humans. ' fl,ogical inf,erence. Modern evolutionary theory evolution to occur, only two processes are required. One is mutation a change in the states that for genetic material - of an organism. The other is natural selection, defined as a change in the genetic composition of a population of organisms caused by differences in reproduction between individuals. Both mutation and natural selection can be directly observed, so evolution is inevitable. F{ierarchical classification. If all organisms are related by descent, it should be possible to classify them into a nested pattern of groups within groups. Each group is defined by an agreed set of similar characteristics. The application of this hierarchical scheme for the human species is illustrated in 3 of - Homo sapiens Horzo Hominidae Great apes Primates Aoes. monkevs, lemurs - Chordata Animals that have a notochord Animatia Multicellular eukaryotes that eat other organisms .- Eukaffa Organisms with cells that contain a nucleus Qr.garTislals qaul 8le aflassi$ieci im a gTnou,rgls Humans Mammalia Chordates that have nrammary gland Phylum Kingdom - - - hierarthicel fashiotl fim a ruesteol trulthim glreug'rs. ' Vestigial structures. Vestigial structures are defined as structures that are reduced in complexity and function compared with similar structures in other organisms. These structures occur at both the anatomical level in the form of vestigial organs and at the molecular level in the form of pseudogenes. There are many examples of vestigial anatomical structures that have no known functions, such as reduced pelvises and hind limb bones inside the skins of pythons and whales, and the wisdom teeth, appendix and coccyx of humans. Pseudogenes are genes that have lost their original protein-coding ability. About 600/o of the 1000 smell receptor genes in humans are pseudogenes - we rely far less on smell and more on sight than our ancestors did. Dobzhansky was correct: evolution does make sense ofbiology. Figure 3. Eiogeography. Location provides a better index of biological similarity than does similarity of climate because geography reflects descent from common ancestors. For example, both the North and South American continents share the same range of humid forests, arid mountains, grassy plains, marshes, lakes and rivers as does the European continent. But there are few species of animals or plants that occur naturally in both locations. No',,emher 2C't2 o Evolution is the change in the genetic composition of organisms between generations. o Natural selection is the change in the genetic composition of organisms between generations caused by differences in reproduction. o A scientific theory is a conceptual model that explains whole sets of facts and withstands falsifiable predictions. /1