THE GROUPS AND ANCESTRY OF ORGANISMS 



119 



gap could accomplish the changes that are seen in the 

 fossil record. In spite of the fact that no unique type 

 of macroevolution seems necessary to explain life, past 

 or present, some biologists believe that there is 

 evidence suggesting such a unique mechanism. 



EVOLUTION CHARACTERISTICS 



Many important aspects of speciation and evolu- 

 tion in general contribute to better understanding 

 of the principle. 



NATURAL SELECTION 



There is little doubt that individual traits can be 

 beneficial, neutral, or harmful to an organism. How- 

 ever, man's judgments of values too often are crude, 

 if not incorrect, owing to his misinterpretation of the 

 influence of traits, and/or his ignoring the total fea- 

 tures of the organism. An individual or a species is 

 more than the sum total of its parts; there is a syner- 

 gistic relationship. Although a single part can have 

 pronounced influence upon the success or failure of 

 an organism, the individual is most often affected by 

 the sum total of its characteristics. In spite of the 

 limitations of man's interpretation, however, there are 

 many features that are apparently beneficial (and so, 

 examples of natural selection) and that are called 

 adaptive characters or adaptations. When traits are be- 

 lieved neutral or harmful to the species, they are 

 called nonadaptwc characters. 



Industrial Melanism. Some city-dwelling populations 

 of insect species have darkened, presumably an adap- 

 tion to smoke in the atmosphere. Supposedly the 

 insects in such areas accumulated chance mutations 

 until their coloration was genetically darker and less 

 conspicuous in the darkened sky. This darkening 

 adaptation is interpreted either as enabling the in- 

 sect to blend into its environment and be less subject 

 to predation, or as being incidental pigment altera- 

 tions associated with internal functional changes. 



Adaptive Resemblance. Many fairly common struc- 

 tures are believed to be adaptive in nature. Cryptic 

 coloration involves colors or patterns that blend the 

 animal into its background. Warning coloration, found 

 in animals well suited to repel a potential predator, is 

 conspicuous and advertises the bearer. Mimicry in its 

 true form is found in a helpless and desirable prey 



animal that has the warning coloration of undesirable 

 prey. 



Nonadaptive Characters. The existence of traits 

 that are not of distinct benefit to organisms is a de- 

 batable subject among biologists. Some can conceive 

 of organisms having characteristics that are not neces- 

 sarily beneficial, if they are not actually harmful; 

 other biologists cannot. The difference in opinion is 

 possible because it is probably impossible to prove 

 that a particular character is nonadaptive; just be- 

 cause man cannot demonstrate the use of a structure 

 does not mean that the structure is useless. An addi- 

 tional problem in such proof stems from known ex- 

 amples of traits having definite value, but only during 

 a short part of the life cycle. Therefore, it is possible 

 to assemble strong evidence for the value of a par- 

 ticular feature, but impossible to prove one valueless. 



PATHS OF EVOLUTION 



The processes of evolution allow different paths of 

 evolution. Many of these paths often are contained in 

 a single line of development, either as separate events 

 through time or as simultaneous occurrences. 



Ancestors. The ancestors that give rise to new 

 species and even new paths of evolution likely are not 

 extremely specialized, so-called advanced creatures, 

 but are populations (never individuals) of species with 

 rather generalized features. Moreover, they do not 

 always evolve into more complex creatures. For ex- 

 ample, parasites usually are structurally more simple 

 than were their ancestors (but they might be con- 

 sidered functionally more complex). A case where 

 ancestors give rise to simpler creatures is called 

 regression. 



Orthogenesis. Orthogenesis is the well-docu- 

 mented phenomenon of "straight-line evolution." Its 

 apparent verification led to the erroneous belief that 

 there could be a basis for a predetermined direction of 

 evolution (Figure 7.10). Actually, the best-docu- 

 mented records of evolutionary history indicate that 

 strict straight-line evolution probably does not take 

 place, but there are well-known examples in which 

 certain structures became increasingly suited to en- 

 vironmental conditions — at least, that is often the 

 interpretation. Under such circumstances, the grad- 

 ual refinement of structures through the chance oc- 

 currence and fixation of mutations is somewhat 

 directional. 



