It seems reasonable to assume that the selective forces invoKed in 

 the evolution of the genetic mechanism of early organisms must have 

 been concerned with stabilization of what was at first an almost 

 infinitely variable system. The mechanisms for replicating genetic 

 material generally ensure that it will b'> exacth' duplicated and 

 that, in the offspring, proteins similar to those of the parents will 

 develop. In more complex multicellular organisms, self-rcgulatorv 

 developmental mechanisms are combined with the nuclear and 

 extranuclear genetic material; together they provide a svstem that 

 usually results in what is thought of as a normal, functioning, wild- 

 type organism. As stated before, the basic phenomenon of genetics 

 is that "like begets like." 



VARIATION AND MENDELIAN GENETICS 



Nevertheless, errors in replication occur; they result in the variability 

 that permits selection. In general, analysis of the nature and trans- 

 mission of variability, from generation to generation, is the onlv 

 means of studying the mechanism of inheritance. If the patterns of 

 variation in organisms are examined, it will be seen that some organ- 

 isms appear to be more variable than others. Furthermore, the type 

 of variation pattern diflFers with respect to organisms and the traits 

 studied. In some instances, variation occurs in discrete steps and 

 may be termed discontinuous. In other cases, variation appears to 

 be continuous, individual organisms not falling into easily character- 

 ized discrete classes. Galton attempted to study continuous variation 

 when he made his classic investigations of the inheritance of intel- 

 ligence and other traits in human beings. Other workers, even before 

 and including Linnaeus, had studied continuous variation by mak- 

 ing crosses between varying plants and animals. The science of 

 genetics was not really born, however, until the inheritance of char- 

 acteristics that varied discontinuously was studied. Organisms hav- 

 ing these characteristics could be classified as one or another of a 

 very few distinct types. By observing the distribution in these classes 

 of offspring of an experimental cross of parents with different char- 

 acteristics, Mendel was able to describe the basic rules of behavior 

 of nuclear hereditary units. 



The importance of Mendel's studies was not generally appreci- 

 ated; indeed, Mendel was urged to suppress his results by other 

 scientists who felt that he was considerably off the beaten jxith of 

 scientific research. In 1900 Mendel's papers were discovered by three 



37 



