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THE CHANGING WORLD OF LIFE 



Genes are known to undergo a sudden change at times that causes an en- 

 tirely different expression of the character which they control. For in- 

 stance, the gene for white coat color in guinea pigs may suddenly mutate 

 to black and a black guinea pig will be born in a race of pure white ones. 

 This black gene will continue to show in the offspring of this black 

 guinea pig, showing that the mutant gene is stable and remains in its 

 new condition through the generations. Such gene changes offer new 

 characteristics for selection, whether it be natural, sexual, or artificial. 



Courtesy Walter Landauer, Storrs Exp. Sta. 



Fig. 34.8. Mutation in domestic sheep. The individual on the left is an Ancon sheep 

 that arose by mutation from sheep like the one on the right. Note the shorter legs 



on the Ancon. 



Of course, all mutations will not be desirable changes ; in fact, many of 

 them will be definitely undesirable ; but some will be of advantage, and 

 these may be selected for leaving the disadvantageous ones to be 

 eliminated. 



Under natural conditions, mutations occur at a rather slow rate. 

 For instance, Haldane has estimated that a single human gene will mu- 

 tate naturally about once every 2.500,000 years, on the average. This 

 rate may seem so small as to make it appear as if mutations could have 

 little effect on evolution. However, when we consider the fact that there 

 are about 20,000 genes in each cell of man's body and take 30 years as 

 the average age of parents when children are born, we find that about 

 one child out of every five will have received some sort of mutated gene. 

 Many of these will be very minor in nature, but it is easy to see how 

 such a rate of mutation can have important evolutionary significance. 



