^Miocene 



10 15 



MILLIONS OF 



FIG. 19-5 Age and time of origin of present-day species of reef 

 corals and mollusks as shown by their percentage of occurrence 

 in fossil faunas of the East Indies (after Umbgrove 1946). 



value, but be closely linked genetically with selec- 

 tively preferred favorable characters and thus con- 

 tinue in the organism. 



Before a population can occupy a new region or 

 even expand its niche it must show at least some pre- 

 adaptation for it (Allee, Emerson, et al. 1949, Simp- 

 son 1953). Preadaptation may take the form of a 

 wide range of tolerance that can encompass the con- 

 ditions of the new habitat as well as the old : or it 

 may take the form of a new use for a structure, dif- 

 ferent from its original function (Bock 1959). The 

 European rabbit certainly showed a good deal of 

 preadaptation to the Australian environment; it be- 

 came a local pest within three years of the intro- 

 duction of 24 individuals in 1859. We have earlier 

 described why preadaptation must have been neces- 

 sary for the origin of parasitism. Preadaptation per- 

 mits individuals to exist in new habitats or to perform 

 new functions, but subsequent perfection of an adap- 

 tive trait depends on the accumulation and selection 

 of additional favorable genetic variations over many 

 subsequent generations. 



ADAPTIVE RADIATION 



When a species bypasses or surmounts a 

 dispersal barrier, it may penetrate an area having a 

 variety of niches novel to the species. A plastic spe- 

 cies may quickly differentiate adaptively into a num- 

 ber of new species, each becoming established in an 

 unoccupied niche or, if sufficiently aggressive, displac- 

 ing an original but less adaptive occupant. Such adap- 

 tive radiation is known to have occurred in the case of 



ancient marsupials, which crossed from Asia to Aus- 

 tralia and differentiated into the variety of species 

 now found there. 



The invasion and occupancy of the Hawaiian 

 Islands by snails, insects, and birds is of special in- 

 terest. There are some 3722 insect species endemic 

 to the islands. All of these species appear to be de- 

 rived from some 250 ancestral forms that arrived in 

 14 separate invasions since Pliocene time (Zimmer- 

 man 1948). 



The ancestral prototype of the honeycreeper birds 

 reached the Hawaiian Islands sometime within the 

 last five million years (Baldwin 1953). Different 

 populations became isolated on different islands, as a 

 result of which there arose the so-called red and 

 black nectar-eating species that are grouped in the 

 subfamily Drepaniinae. As the nectar-feeding niches 

 became fully occupied, a population diverged in 

 its behavior, feeding more heavily on insects than 

 on nectar. The new niche allowed redispersion of the 

 population through the various islands, and there 

 ensued a second burst of speciation yielding the so- 

 called green insect-eating forms belonging to the 

 subfamily Psittirostrinae. Additional speciation pro- 

 duced short- and long-billed species of insect-eaters. 

 Somewhere in the lineage of the latter group, the 

 birds acquired seed- and fruit-eating habits, and the 

 long bill also became a thick bill. Rapid evolution in 

 this family still appears to be in process. 



RATE OF EVOLUTION 



There is evidence that, under natural con- 

 ditions, variations in local populations may sometimes 

 be manifested within a surprisingly few generations 

 (Huxley 1943). Melanistic forms of butterflies now 

 occur in industrial areas of England where vegetation 

 has become coated with dark-colored debris, although 

 100 years ago such butterflies and the industrial soot 

 as well were virtually absent (Kettlewell 1956). 

 House mice isolated on a sandy island have become 

 within 100 years a distinctly paler population than 

 the one on the adjacent mainland (Jameson 1898). 

 The period of time required for evolution from 

 one taxonomic level to another varies enormously 

 between different kinds of organisms. It has been 

 estimated that the rate of change in characteristics of 

 several lineages of mammals since Pleistocene time, 

 when it can be measured quantitatively, for instance 

 in the length or breadth of the skull, is of the order 

 of 0.2 per cent per 1000 years (Kurten 1958). Ap- 

 parently, subspecies commonly require 10,000 years 

 to become well defined, and may continue to evolve 

 for 500,000 years before rising to the species level. 

 The evolution of a fully defined species usually re- 

 quires at least 50,000 years and frequently a very 



266 Geographic distribution of communities 



