102 I The Process of Evolution 



Table 6.5 | Probability of Extinction of a Mutation 

 Appearing in a Single Individual 



From The Genetical Theory of Natural Selection, Second Rev. Ed., by Ronald 

 A. Fisher, 1958. Published by Dover Publications, Inc., New York, N.Y., and 

 reprinted through permission of the publisher. 



SELECTION 



Selection is the nonrandom (differential) reproduction of genotypes. 

 One might regard the streams of hfe of a population as made up of 

 continually varying, dividing, fusing, and disappearing particles 

 flowing from the past to the present through a series of immensely 

 complex screening sieves. Selection can be said to have occurred 

 when the stream at a lower point differs from the stream at a higher 

 point to such a degree that it is highly improbable that the observed 

 difference is due to sampling error (drift) or mutation. In looking 

 for selection, one must be sure that the stream neither branches 

 (emigration) nor receives a tributary (immigration) in the stretch 

 observed. 



If a population is genetically heterogeneous, the probability of 

 success of some genotypes will be higher (with possible rare excep- 

 tions) than the probability of success of others. Thus certain kinds 

 of genetic information will become more and more common in the 

 gene pool of the population and other kinds will become less and 

 less common. The gene frequencies p and q will change with time 

 rather than remaining constant, as would be expected under the 

 conditions of Hardy-Weinberg equilibrium. 



It is popular to speak of selection as a great "creative force" in 

 evolution, the "cause" of observed trends. In fact, it is a phenomenon 

 observable only a posteriori— a description of occurrences. When a 



