32 GENE RECOMBINATION 



geographical nature of most species means that only a small por- 

 tion of the total pool of genes in the species is drawn into com- 

 bination at any one time. Thus it is that the geneticist finds 

 himself forced, like the members of all other disciplines which 

 deal with species, to consider both geographical and historical 

 matters. 



The Species in Space and Time 



As we have seen previously, the local population occupies a 

 really pivotal position for the genetic nature of the species. Its 

 composition depends not only on the past history of the local 

 population itself but also on the history of the populations near 

 it, and from which it may have inherited genes. Thus the pa- 

 rameters of time and space in the study of local populations are 

 so interrelated that it is often difficult to separate their effects. 

 The words "gene flow" are sometimes used to describe the process 

 whereby the local population obtains influx of hereditary ma- 

 terial from the outside. This is, in most cases, an extremely slow, 

 gradual process, usually extending over many generations. 



In a larger sense, then, the gene pool of the species is dis- 

 tributed in time as well as in space. The local population where 

 the actual recombination trials are taking place may owe its 

 contemporary composition to very complex interactions with 

 the populations in its immediate ancestry. In certain cases, cyto- 

 genetic studies enable us to prove that this past history involved 

 polyploidy or hybridization or both. In other cases, it is possible 

 to interpret the present genetic peculiarities of a species popula- 

 tion as being traceable to recent strong isolation of a local popu- 

 lation. In any event, the net result is an alteration of the content 

 of the gene pool and thus both qualitatively and quantitatively 

 of the raw materials with which recombination operates. Thus, 

 for instance, a newly formed hybrid swarm, such as one that 

 has resulted from the breakdown of ecological barriers by hu- 

 man disturbance of the habitat, may be looked upon primarily as 

 a process resulting in the sudden release of recombinations con- 

 sequent upon the original wide outcross. Hybridization between 

 species with high chromosome numbers which maintain high 



