Populations 79 



presence of both plant and rock does not guarantee that the butter- 

 fly also will be there. On Jasper Ridge, areas of serpentine with 

 abundant Phntago remain unoccupied although they are imme- 

 diately adjacent to the colony. Not only must all environmental 

 conditions be suitable for a habitat to be occupied, but chance must 

 supply access to the suitable area. Thus some suitable areas for E. 

 editha may not support colonies simply because no fertilized fe- 

 males have ever reached them. Man has provided many organisms 

 with access to previously uninhabited but suitable regions, as star- 

 lings, English sparrows, cabbage butterflies, honeybees, dandelions, 

 and mustard constantly remind us. 



STRUCTURE 



The structure of a population is considered here to be the totahty 

 of all the factors that govern the pattern in which gametes from 

 various individuals unite with each other. The structure can vary 

 from situations in which combinations might seem to be essentially 

 random (e.g., certain marine animals that release gametes into the 

 sea, some wind-pollinated plants) to those in which the probability 

 of certain combinations is much higher than others. The latter case 

 is certainly the rule, if for no other reason than that close neighbors 

 usually have higher probabilities of mating than more distant ones. 



Such factors as length of generation and size of individuals also 

 are important. If the variable to be measured is the number of new 

 gene combinations produced in a given area per unit of time, then 

 small organisms will differ from large ones. In any place there are 

 fewer large organisms than small ones and thus less recombination. 

 Organisms with a short life cycle produce more gene combinations 

 than those with long generation time, and their mutation rates also 

 differ. 



Especially in higher animals, there have evolved many behavioral 

 systems that profoundly affect the structure of a population. Many 

 animals are effectively sedentary in spite of great dispersal potential. 

 Birds often return from long migrations to exactly the same breeding 

 location as was occupied in previous years. Twitty has shown that 

 California newts have incredible perseverance and navigating ability, 

 returning precisely to a particular segment of a stream to breed. In- 

 deed, displaced individuals have returned to their home pool over 

 several miles of mountainous country. Specificity within a stream is 

 clearly shown in Fig. 5.3. Butterflies often use their powers of flight 

 merely to patrol a restricted area. In the Jasper Ridge colony of 

 Euphijdryos editha, 625 out of 647 recaptures of marked adults 



