626 



ECOLOGY AND EVOLUTION 



pression. Inherited physiologic, ecologic, or 

 behavioristic characters are as important 

 as morphologic in the concept of the spe- 

 cies. Morphology, being a visual expression 

 of physiologic development, will continue 

 to be used by taxonomists and students of 

 phylogeny for the practical arrangement of 

 most species. Through the use of the term 

 "natural population," our definition of a 

 species and Turesson's definition of the 

 ecospecies, to be given shortly, are in essen- 

 tial agreement. 



A number of recent writers (Dobzhan- 

 sky, 1941; Mayr, 1942; Clausen, Keck, and 

 Hiesey, 1945) also incorporate permanent 

 isolation, through psychological separation, 

 inviabihty of hybrids, hybrid sterility, or 

 infertility, in their definitions of a species, 

 but extrinsic isolating agents have closely 

 similar evolutionary consequences. Repro- 

 ductive isolation, regardless of what factor 

 keeps the genes from moving across the 

 species boundary, is the more inclusive and 

 more practical characteristic of the major- 

 ity of recognized species. If the various 

 potentialities of the species under domesti- 

 cation, as contrasted with the natural en- 

 vironment, need to be incorporated in the 

 species concept, Turesson's term cenospe- 

 cies may be used. Cenospecies has also 

 been used for groups of species in nature 

 that are separated only by extrinsic fac- 

 tors, but siiperspecies is a better term for 

 such closely related species groups. 



A number of terms for the subdivision 

 of species and species types have resulted 

 from ecologic data, particularly reproduc- 

 tive isolation. 



Turesson (1922) defined an ecotype as 

 "the product arising as a result of the 

 genotypical response of an ecospecies to a 

 particular habitat" (see also Gregor, 1944). 

 The ecospecies was defined as "the Lin- 

 nean species or genotype compounds as 

 they are realised in nature" (see Turesson, 

 1931; Axelrod, 1941). 



Turesson separated the genetically difi^er- 

 ent populations responding to their habitat 

 (ecotypes) from the physiologic (non gene- 

 tic) response to habitat (see Turill, 1946). 

 He used the term "ecophene" for the latter 

 reaction type, produced, for example, by 

 the modificatory influences of extreme habi- 

 tat factors, such as shade, tree-line, and 

 others. 



The term geotype may be used for geno- 



type populations in habitats partially iso- 

 lated by topographic barriers with modifi- 

 cations not necessarily under the influence 

 of natural selection (equivalent to "geo- 

 ecotype"; Gregor, 1932). This category in- 

 cludes the majority of geographic subspe- 

 cies and races. 



Huxley (1939) suggested the term 

 ecocline for quantitative gradation through 

 successive ecologic zones of the habitat 

 (Fig. 230). He also used the term geo- 

 cline for quantitative gradations based 

 upon topographic or spatial separation, 

 chronocline for temporal gradations found 

 in paleontological sequences, and taxocline 

 for gradations involving hybridization. 

 Phenotypic characters appearing at diflFer- 

 ent times in the life cycle (ontoclines) may 

 be correlated with ecoclines. In African 

 buflFalos there is an ecocline ranging from 

 pale red forest adults to black plains adults, 

 and at the same time there is an increase 

 of the slope of the graph of the red-black 

 ontocline from the forest forms to the forms 

 of the open plains. 



Some chnes appear to be adaptively cor- 

 related, either directly or indirectly, with 

 environmental factors; others are caused by 

 the localized appearance and subsequent 

 spread of a mutation having selective ad- 

 vantage; and still others are correlated with 

 emigration or dispersal (Huxley, 1939). 



According to Huxley, ecoclines would in- 

 clude a number of ecologic gradations, 

 some of which have been named. Berg- 

 mann's rule concerning absolute size in re- 

 lation to temperature, Allen's rule concern- 

 ing relative size of exposed parts of the 

 body in relation to temperature, and Glo- 

 ger's rule or rules concerning different types 

 of pigmentation in relation to temperature 

 and humidity, jointly embody the best- 

 known cases in warm-blooded animals. Ap- 

 parently adaptive geographical gradients 

 are shown by many other characters as 

 well, such as clutch size (in birds) with 

 latitude (see p. 701), size with salinity in 

 marine organisms, number of fin rays and 

 vertebrae with salinity in fishes, shell thick- 

 ness with aridity in landsnails, relative 

 heart weight with temperature in warm- 

 blooded animals, relative gut size with 

 temperature in cold-blooded animals (but 

 with more complex relations than in warm- 

 blooded forms), tongue length with lati- 

 tude in bees, and temperature resistance in 



