THE GROUPS AND ANCESTRY OF ORGANISMS 



123 



resemble those of nearby land and probably evolved 

 from the continental forms, island creatures fre- 

 quently are dift'erent from their mainland ancestors. 

 The main reason for the divergence is believed to be a 

 general lack of competition on the islands, where 

 species frequently have no predators and/ or competi- 

 tors. Under such conditions, a phenomenon called 

 genelic drift may exist. Owing to the general reduction 

 of the normal unfavorable environmental conditions, 

 selection may be weak; and, rather than maintaining 

 and further developing the continental adaptations, 

 organisms may drift into a less adaptive evolution. 



Islands, however, can be extremely severe habitats. 

 Chance island immigrants might require much the 

 same food and shelter, interspecific competition may 

 develop, and one or more of the competing species 

 may become extinct. On the other hand, most com- 

 petitors might survive if chance mutations allow fairly 

 rapid rates of evolution. Hence, fairly recent island 

 derivatives of continental species can evolve so much 

 as to appear greatly divergent from their relatives. 



Endemhm. An endemic is a species with a fairly 

 restricted but continuous range. Any species limited 

 to one island is an endemic (unless the island is ex- 

 tremely large), and so is any species still restricted 

 to its center of origin. However, when a center of 

 dispersal is first formed, and even if this population 

 is of endemic size, it is not endemic because of the 

 parent population. The final endemism of any spe- 

 cies is found in the last, single, small population just 

 prior to extinction. 



Many species display the same endemism, if all 

 share an identical, small geographic range. The area 

 they occupy is called an endemic area. Examples of 

 such areas are found in some islands and high moun- 

 tains. If an endemic area represents the final popula- 

 tion of a formerly more wide-ranging species now on 

 the road to extinction, the locality is called a relict 

 area: however, a relict area can be any kind of living 

 historical community, not only an endemic one. 



SELECTED READINGS 



Andrews, H. N., Jr., 1961. Studies in Paleobotany. John 



Wiley & Sons, New York. 

 Bates, M., and P. S. Humphrey, 1957. The Danvin Reader. 



The Macmillan Co., New York. 

 Benson, L., 1957. Plant Clas.^jication. D. C. Heath, Boston. 



Bonner, D. M., 1961. Heredity. Foundations of Modern 



Biology Series. Prentice-Hall, Englewood Cliffs, N. J. 

 Bowen, R. N. C, 1958. The K\ploratinn of Time. Philosophi- 

 cal Library, New York. 

 Dobzhansky, T., 1955. Evolution, Genetics and .Man. John 



Wiley & Sons, New York. 

 Dodson, E. O., 1960. Evolution: Process and Product, rev. ed. 



Reinhold Publishing Corp., New York. 

 Dunbar, C. O., 1960. Historical Geology. 2nd ed. John 



Wiley & Sons, New York. 

 Fenton, C. L., and M. A. Fenton, 1958. The Fossil Book. 



Doubleday & Co., Garden City, N. Y. 

 Griffin, D. R., 1962. .Animal Structure and Function. Modern 

 Biology Series. Holt, Rinehart & Winston, New York. 



Hanson, E. D., 1961. Animal Diversity. Foundations of 

 Modern Biology Series. Prentice-Hall, Englewood 

 Cliffs, N.J. 



Hardin, Garrett, 1959. .Vature and .Man's Fate. Holt, Rine- 

 hart & Winston, New York. 



Huxley, J. S., 1942. Evolution: The .Modern Synthesis. Har- 

 per, New York. 



Hyman, L. H., 1940. The Invertebrates. Vol. 1: Protozoa 

 through Ctenophora. McGraw-Hill Book Co., New York. 



Levine, R. P., 1962. Genetics. Modern Biology Series. 

 Holt, Rinehart & Winston, New York. 



Life Editorial Staff and Ruth Moore, 1963. Evolution. 

 Time Inc., New York. 



Matthews, W. H., 1962. Fossils: An Introduction to Pre- 

 historic Life. Barnes & Noble, New York. 



Mayr, E., E. G. Linsley, and R. L. Usinger, 1953. .Methods 

 and Principles of .Systematic ^oology. McGraw-Hill Book 

 Co., New York. 



Merrel, D. J., 1962. The Modern Theory of Evolution, Evolu- 

 tion and Genetics. Holt, Rinehart & Winston, New 

 York. 



Romer, A. S., 1945. I'ertebrate Paleontology. 2nd ed. The 

 University of Chicago Press, Chicago. 



. 1959. The I'ertebrate Story. 4th ed. The University 



of Chicago Press, Chicago. 



Ross, H. H., 1962. .-1 Synthesis of Evolutionary Theory. 

 Prentice-Hall, Englewood Cliffs, N.J. 



Savage, J. M., 1963. Evolution. Modern Biology Series. 

 Holt, Rinehart & Winston, New York. 



Shrock, R. R., and W. H. Twenhofel, 1953. Pnnaples of 

 Invertebrate Paleontology. McGraw-Hill Book Co., New 

 York. 



Simpson, G. G., 1949. The Meaning of Evolution. Yale 

 University Press, New Haven, Conn. 



. 1953. Life of the Past. Yale University Press, New 



Haven, Conn. 



. 1953. Evolution and Geography, .-in Essay on Histori- 

 cal Biogeography with Special Reference to .Mammals. 

 Oregon State System of H igher Education. 



