702 GENETICS AND EVOLUTION 



but closely related species. For some 16,000,000 years during the Tertiary 

 period there was no connection between North and South America and 

 animals could migrate freely between what is now the Gulf of Mexico 

 and the Pacific Ocean. When the Isthmus reemerged the closely related 

 groups of animals were separated, and the differences between the two 

 fauna today represent the mutations which have accumulated since. 



Geographic isolation is usually not permanent and two closely re- 

 lated but previously isolated groups may come into contact and inter- 

 breed unless genetic isolation, or intersterility, has arisen. The several 

 races of man arose by geographic isolation and the accumulation of 

 chance mutations, but since interracial sterility has not developed the 

 differences begin to disappear rapidly when geographic isolation breaks 

 down and interbreeding occurs. 



Genetic isolation results from one or more mutations which occur 

 by chance, independently of other mutations. The mutations for inter- 

 sterility may arise only after a long period of geographic isolation has 

 produced marked differences between two groups of organisms, or they 

 may arise within a single, otherwise homogeneous group. An example 

 of the latter is found in the fruit fly, DrosopJiila pseudoobscura, which 

 consists of two varieties, externally indistinguishable, yet completely 

 sterile when crossed. The two groups are isolated as effectively as if they 

 lived on different continents. In succeeding generations, as mutations 

 accumulate in the two groups by chance and selection, they will un- 

 doubtedly become visibly different. Biologists do not usually consider 

 two closely related but different groups of organisms to be separate 

 species unless genetic isolation has occurred. 



Two groups of closely related animals living in the same geographic 

 area may nevertheless be effectively isolated if they occupy different 

 habitats. This is called ecologic isolation; marine animals, for example, 

 that live in the intertidal zone are effectively isolated from others living 

 only a few feet away below the low-tide mark. Two groups of animals 

 that breed at different times of the year are also effectively prevented 

 from interbreeding; this might be called physiologic isolation. 



Ro/e of Natural Selection. Darwin assumed that the variation in a 

 particular character would continue to occur, so that natural selection 

 would operate indefinitely. From the facts of heredity in the previous 

 chapters it should be clear that selection can operate only until the 

 population becomes homozygous for all the genes for that trait— for large 

 body size, for example. After that condition has been reached, neither 

 artificial nor natural selection can affect the trait further until additional 

 mutations for large body size have occurred. 



It must be emphasized that natural selection can operate only upon 

 the organism as a whole, not upon its individual traits. One organism 

 may survive despite certain obviously disadvantageous characters, while 

 another may be eliminated despite traits which appear to be very helpful 

 for getting along in life. The animals and plants that win the struggle 

 for existence are usually not perfectly adapted to their environment, but 

 have qualities the sum of which renders them a little more capable of 

 surviving and reproducing than their competitors. Since the environment 



