(Wilson and Brown 1953; Syst. Zool. 3, 1954: 97- 

 126, 133). The taxonomic differences between sub- 

 species are usually less pronounced than those be- 

 tween even closely related species, but they are genet- 

 ically fostered differences nonetheless (Sumner 1924, 

 Huxley 1943). Race and variety are terms some- 

 times used as synonyms for subspecies, or for popu- 

 lations even less well defined than a subspecies. It is 

 difficult to determine whether populations that differ 

 phenotypically are subspecies or species when their 

 ranges do not verge, for in the absence of opportunity 

 for them to commingle, it remains uncertain whether 

 interbreeding would or could occur. The assignment 

 of taxonomic rank under these circumstances is 

 dependent upon considerable subjective inference. 

 Boundaries between adjacent subspecies are fre- 

 quently arbitrary in that they fit differences between 

 the populations for some characters but not others. 

 Differences between subspecies or populations are 

 often correlated with differences in topography, soil, 

 climate, or vegetation, and, at least in some instances, 

 appear to be adaptive to these differences in the envi- 

 ronment. 



If a subspecies becomes isolated by a barrier so 

 that it is prevented from interbreeding with the rest 

 of the species, variations in taxonomic characters may 

 accumulate and the population evolve so distinctively 

 as to pass beyond the rank of subspecies into that of 

 species. However, not all or even most subspecies 

 change into species, a process which depends on effec- 

 tive, permanent reproductive separation and on the 

 forces of natural selection. Speciation, more pre- 

 cisely, is the process of differentiation between popu- 

 lations of the same species in consequence of repro- 

 ductive isolation (Simpson 1953). 



Populations that do not differ by clearly defined 

 or conspicuous taxonomic characteristics, but never- 

 theless do not interbreed because of physiological or 

 behavioral differences, are described as sibling species. 

 Sibling species have been noted especially among Dip- 

 tera (Drosophila. Anopheles), Hymenoptera (ants), 

 Lepidoptera (especially moths), and Protozoa {Para- 

 mecium) (Mayr et al. 1953). It is apparent that the 

 evolution of physiological and behavioral differences 

 often precedes the differentiation of recognizable 

 taxonomic distinctions (Krumbiegel 1932, Thorpe 

 1930). 



Two species the distributional ranges of which do 

 not overlap (i.e., there is geographic isolation be- 

 tween them) are said to be allopatric. Two species 

 are said to be sympatric when their ranges overlap, 

 even though they may locally be ecologically segre- 

 gated into different habitats ; for example, the situ- 

 ation in which one species of snail is limited to flood- 

 plain and another species to upland habitats, or one 

 species of rodent confined to the foothills and a closely 

 related species to a higher zone on mountains only a 



short distance away. Populations should be con- 

 sidered sympatric if they occur within the common 

 dispersal ranges of their young, so that there is at 

 least possible a continuous and appreciable inter- 

 breeding and flow of genes among them. 



ISOLATION OF POPULATIONS 



Sympatric species do not interbreed be- 

 cause one or more isolating mechanisms keep them 

 separated. We will proceed to examine what iso- 

 lating mechanisms are, and how geographic isolation 

 permits them to arise. 



Isolating mechanism 



Isolating mechanisms that prevent sympatric 

 species from freely interbreeding are largely or en- 

 tirely biotic factors (Mayr 1942, Huxley 1943, Allee, 

 Emerson, et al. 1949, Dobzhansky 1951, Patterson 

 and Stone 1952). They are the following types: 



Ecological: segregation of species into different 

 habitats, communities, or niches by reason of 

 structural adaptations, physiological adjust- 

 ments, or behavior responses. 



Ethological: difference in sign stimuli or be- 

 havior patterns required for successful spe- 

 cies and sex recognition and for mating. 



Mechanical: lack of physical conformity of sex- 

 ual organs, chemical incompatibility of sperm 

 and egg. 

 Genetic: hybrid sterility or decrease of fertility. 



Ecological isolation was described and illustrated 

 in the discussion of niches (Chapter 18). 



Animals may fail to find breeding partners ; i.e., 

 they may remain ethologically isolated. Careful stud- 

 ies in all vertebrate groups and in such invertebrates 

 as insects, spiders, crustaceans, and snails indicate 

 that animals identify the sex of individuals of their 

 kind only by actively recognizing special clues or sign 

 stimuli (Tinbergen 1951). These clues may be spe- 

 cial color markings, shape or outline, scent, song or 

 call-notes, touch, behavior patterns, or some combi- 

 nation of these. Courtship leading to copulation is 

 often complex and involves a number of steps, each 

 step in the behavior serving as a releaser for the next 

 (Fig. 2-7). If a step is not performed properly and in 

 its sequence, the courtship performance ceases forth- 

 with, and there is no sexual consummation. Different 

 species may be effectively isolated from interbreeding 

 simply because the sexes of one possess different sign 

 stimuli than the sexes of the other, and the sexes of 

 one kind characteristically pursue patterns of pairing 



258 Ecological processes and dynamics 



