446 INTRODUCTION TO EVOLUTION 



territory within which an individual is likely to secure a mate is still fur- 

 ther restricted. 



We should also note that many species of animals pass through "botde- 

 necks" in their yearly life cycles. Among insects, especially, winter is likely 

 to constitute a bottleneck. In a given region there may be thousands of 

 individuals of a certain species throughout the summer, yet only a rela- 

 tively few may survive the winter to become progenitors of the next sum- 

 mer's population. If the few which survive the winter happen to possess 

 some genetic characteristic in greater frequency than the general popula- 

 tion of the preceding summer did, the population of the second summer 

 may differ considerably from that of the first summer. Thus Spencer (1947) 

 found that the fruit flies, Drosophila, of a certain small community pos- 

 sessed an unusual frequency of a recessive mutation named "stubble," af- 

 fecting the length of bristles. In these flies great seasonal fluctuations in 

 number of individuals occur, the parents of a given summer population 

 consisting of a few individuals who succeed in surviving the winter indoors. 

 Since the "stubble" gene seems not to be of significance in the lives of the 

 flies, the explanation for its having attained a considerable frequency of 

 occurrence in this particular locality seems to be genetic drift. By chance, 

 an unusual proportion of the few individuals surviving a winter possessed 

 the "stubble" gene; hence the gene occurred with increased frequency 

 among their offspring, the next summer's population. 



In somewhat similar manner an "error of sampling" may occur when, 

 through migration, a small group from a large population establishes a sub- 

 population in a new locality. The "founders" of the new population may 

 not be entirely typical of the large population from which they came. Sup- 

 pose, for example, that a large population has in its gene pool equal num- 

 bers of genes M and m (i.e., p — 0.5 and ^ = 0.5; p. 432). As we noted 

 previously, such a population would be expected to consist of y^MM in- 

 dividuals, y^Mm individuals, and y^mm individuals. Now if ten members 

 of this population migrate to an island, these founders of the island popu- 

 lation may, by chance, not exhibit that 1:2:1 ratio. Perhaps five wifl be 

 MM, four will be Mm, and one will be mm — or any other chance combi- 

 nation you can imagine. In the extreme case all ten might be MM (or, 

 alternatively, mm), in which case the island population descended from 

 the ten individuals would lack completely one of the genes under consider- 

 ation. In this manner the gene pool of the island population might be very 

 different from the gene pool of the population from which the founders 

 came. This founder principle, as it is called, may help to explain how small, 

 isolated populations have come to possess the unusual characteristics 



