28 GENE RECOMBINATION 



both space and time is implied. This concept is most useful for 

 the student of the species who has an essentially historical ap- 

 proach. He attempts to interpret the mosaic of present-day popu- 

 lations of a single species in terms of the action of evolutionary 

 directive factors on the totality of gene resources which have 

 been available in its ancestral populations. 



In yet another sense, the parameter of time may be omitted 

 and the contemporary genetic material of a species may be 

 viewed as constituting a pool of genes. Now in this case, the 

 emphasis is on the total genie material currently found through- 

 out the geographical range of the species. This gene pool is thus 

 the actual legacy willed by selection to the present-day series of 

 generations which a single human investigator may observe. This 

 type of pool is the province of the animal and plant breeder, the 

 eugenicist, and the evolutionist who may be tempted to predict 

 what may happen next. 



Neither of the above "pools," however, is the really crucial one 

 from the point of view of the population geneticist. His attention 

 is focused on a gene pool which can be defined much more rig- 

 orously and which may be studied with precise observational 

 and experimental methods. This is the gene pool found in a 

 single contemporary local breeding population, made up of ran- 

 domly, or almost randomly, interbreeding individuals. Wright has 

 encouraged the use of Gilmour and Gregor's ( 1939 ) term deme 

 to refer to this microgeographical unit, momentarily suspended in 

 time, which lies at the base of the hierarchal array of species 

 populations. 



Because breeding populations of species are entities distributed 

 in space, the individual genotypes formed in local populations 

 in any one generation are never drawn at random from the 

 entire gene pool of the species. Despite' dispersal and outcrossing 

 mechanisms, organisms tend to breed with those that are close 

 to them, in a physical sense, in their environment. This gives 

 reality to the deme and makes it the unit which most closely re- 

 sembles the fishbowl model. 



Of central importance is the fact that the local population is 

 the only part of the larger gene pool which is active from the 



