NATURAL SELECTION: I 453 



beneficial under others. If the climate should change, so that Drosophila 

 flies in a certain region must exist, for example, under more elevated tem- 

 peratures than prevail today, some of the mutations and gene combina- 

 tions which are now deleterious might then become beneficial and might 

 consequently be favored by natural selection. In the course of time the mu- 

 tation conferring increased viability at high temperatures might be incor- 

 porated into the population as "standard equipment." 



Or suppose the presently prevailing temperature conditions do not 

 change. Individuals possessing a chromosomal structure conveying in- 

 creased viability at high temperatures are preadapted (see p. 13) for inva- 

 sion of environments having higher temperatures than the general popula- 

 tion could tolerate. Some such preadaptive mutations must have been 

 involved, for example, in the successful invasion of hot springs, such as 

 those in Yellowstone National Park, by animals and plants now found 

 living in them. Mutations conferring increased viability at low tempera- 

 tures would correspondingly preadapt their possessors for invasion of 

 colder environmental niches than the niche to which the general popula- 

 tion is adapted. In correspondence with this thought, Timofeeff-Ressovsky 

 ( 1940) found that flies of Drosophila jimebris from southern Europe have 

 greater viability at high temperatures and lower viability at low tempera- 

 tures than do flies from northwestern Europe. Equally interesting were 

 flies from eastern Europe and neighboring sections of Asia; these flies 

 have greater viability at both high and low temperatures than do flies 

 from northwestern Europe, viability at intermediate temperatures being 

 the same (Fig. 20.1 ). This observation correlates well with the fact that 

 eastern Europe and adjoining portions of Asia have higher summer tem- 

 peratures and lower winter temperatures than does northwestern Europe. 

 Thus flies inhabiting the eastern regions must be adapted to withstand 

 both extremes instead of only one. 



We mentioned previously (p. 351) that many, if not most, mutations 

 have more than one effect on the organism. One of these efl'ects is fre- 

 quently a change in viability, and it is often of more importance to the 

 orsanism than are eflfects more noticeable to observers. Thus the success 

 of a mutation in becoming established in a population may depend upon 

 the mutation's efl'cct on viability, the visible bodily changes involved be- 

 ing more or less incidental accompaniments. We mention this matter be- 

 cause it helps to explain how characteristics of no evident significance to 

 an animal may nevertheless become established in the species. To give 

 a hypothetical example: If a certain mutation changed the color of an in- 

 sect's eyes from red to black and increased viability at low temperatures, 



