230 I The Process of Evolution 



sucking blood where the clothes are in contact with the body. Body 

 lice attach their eggs to the clothing. 



The common human nematode parasite, Ascaris lumbricoides, is 

 morphologically and serologically indistinguishable from the pig 

 parasite, A. suum, but in most cases eggs from one will not develop 

 properly in the host of the other. 



DISCUSSION OF OBSERVED PATTERNS 



The basic reason for the diverse patterns of differentiation described 

 in the preceding section is that the physical environment is, and 

 always has been, heterogeneous. This heterogeneity has meant that 

 evolutionary forces, especially selection, have operated differentially. 

 In turn, this has produced a heterogeneous biotic environment and 

 further differentiation in the forces operating in any portion of that 

 environment. 



It is all too easy to fall into the mistake of assuming that differ- 

 entiation of populations into genetically isolated forms is somehow 

 the "goal" of evolution. Differentiation often takes place, but usually 

 those instances where incipient species become submerged again 

 are unrecorded. When morphological divergence of two populations 

 has progressed to a certain point, often we no longer say that they 

 interbreed, we say that they hybridize. This implies that they have 

 somehow or other made a "mistake." The fact that occasional genetic 

 interchange enriches the variability of both populations is forgotten. 

 Differentiation is not necessarily a step toward the formation of 

 isolated populations. It is merely one of the many things that hap- 

 pen to populations in nature. 



Geographic Variation in Selection Pressures 



Geographic variation, in most cases, may be attributed primarily to 

 the different selection pressures prevailing in different areas. Thus 

 Achillea growing at high altitude is under selection pressure that 

 favors dwarfed forms physiologically adjusted to rigorous mountain- 

 top conditions. Lowland Achillea are not subjected to the same 

 pressures but to others. Nafrix sipedon populations have adjusted 

 both to "normal" swamp habitats and to the special conditions pres- 

 ent on the Lake Erie islands. Cepaea populations face selection pres- 

 sures partially determined by local vegetation types, and Bistort 

 populations to pressures varying with the relative presence or ab- 

 sence of industrial pollution. The frequency of the so-called "sickle- 

 cell" gene in human populations varies geographically; it is high in 

 regions where malignant tertiary malaria is present, low elsewhere. 



