ECOLOGY AND ISOLATION 



625 



unfit, such genetically distinctive popula- 

 tions may interbreed and produce inter- 

 mediate forms variable in a number of 

 characters (Hubbs and Miller, 1942). Hux- 

 ley (1942) refers to the results of such 

 fluctuations of isolating factors as reticulate 

 differentiation. 



Tf the species have been isolated for a 

 long time, divergent evolution may have 

 developed varying degrees of inability to 

 cross successfully (p. 676). If there is a 

 sharp psychological, physiological, or other 

 intrinsic isolation, merging of the popula- 

 tions will not occur even though the ex- 

 trinsic barriers have broken down. In- 

 stances are reported in which hybrid popu- 

 lations have become established, and such 

 hybrids may possibly give rise to new spe- 

 cies (Miller, 1938, 1939; Sweadner, 1937; 

 Dice, 1940a; Muller, 1942; Emsweller and 

 Stewart, 1944; White, 1945). On occasion, 

 the branching twigs of the phylogenetic 

 tree may fuse at their tips. 



These principles of evolution have re- 

 cently been applied to the production of 

 new cultivated plants. Natural species that 

 are reproductively isolated through chro- 

 mosome unbalance, but can still produce 

 sterile offspring, are crossed, and the drug 

 colchicine is applied, which sometimes 

 causes a doubling of the chromosomes 

 (amphidiploids). These artificial autopoly- 

 ploids (doubling of chromosomes in non- 

 hybrid organisms) from initially allopoly- 

 ploid plants (hybrids of two species) are 

 thus rendered self-fertile, and a hybrid 

 population representing a new form may 

 be established and later artificially selected 

 for characteristics desired by man. Poly- 

 ploids (multiplication of the chromosome 

 complement) may be artificially produced 

 for hybridization with known polyploid 

 plants, qualitative as well as quantitative 

 characters may be introduced, and plants 

 mav then be selected for new or wider 

 ecologic adaptation or other desired charac- 

 teristics (Dermen, 1940; Huxley, 1942, p. 

 346). 



Gene or chromosome combinations may 

 produce new characters not present in 

 either parent (Haldane, 1932, p. 96; Ir- 

 win and Cole, 1936, 1936a; McGibbon, 

 1944). Hvbrid characters may be more 

 favorable than those of either parents and 

 mav thus be subject to selection (Huskins, 

 1931; Buchholz, 1945). 



The wild hempnettle, Galeopsis tetrahit, 

 is a tetraploid species that probably arose 

 in nature from the hybridization of two 

 diploid species, G. pubescens and G. spe- 

 ciosa, with subsequent doubhng of the 

 chromosomes. In this instance an almost 

 identical species has been produced artifi- 

 cially by following the supposed manner of 

 origin of the wild allotetraploid (Miintzing, 

 1937). New species of willows with eco- 

 logic peculiarities have also arisen through 

 hybridization in nature. Huxley (1942, p. 

 339) discusses these and other cases under 

 the term convergent species-formation. 



Under natural conditions, completely iso- 

 lated species probably do not often produce 

 hybrid species through the breakdown of 

 important isolating factors. Because of 

 vegetative reproduction and self-fertiliza- 

 tion, many more examples of hybrid origin 

 of species are to be expected among 

 plants than among animals. However, in- 

 completely isolated subspecies may more 

 often reverse the process of divergence 

 in both plants and animals (see p. 626). 

 Probably the best illustration of such fusion 

 of once partially separated populations is 

 to be found among the racial intermixtures 

 of man." There is no reason to doubt that 

 numerous instances have also occurred 

 among other animals and in plants. 



With the advent of transportation by 

 modem boats, trains, automobiles, and air- 

 planes, much greater mixing of the natu- 

 rally isolated species of the world faunas 

 and floras is to be expected. Species per- 

 manently isolated by intrinsic differences 

 may be expected generally to maintain 

 their distinctions, while species isolated by 

 extrinsic barriers alone may often merge 

 with closelv allied species (Wiegand, 

 1935) . 



ECOLOGY AND 

 TAXONOMIC CATEGORIES 



A species may be defined as an evolved 

 (and probably evolving), genetically dis- 

 tinctive, reproductively isolated, natural 

 population (Emerson, 1945). Some scien- 

 tists emphasize morphologic characters of 

 species (Hoare, 1943; Sturtevant, 1944a). 

 It would seem better to emphasize genetic 

 characters, regardless of the type of ex- 



* Subspecies of man may have been more 

 sharply defined before the development of 

 "primitive" transportation. 



