518 Cytogenetics and Evolution 



in some respect. Hybrids according to this usage might be 

 mendelian or chromosomal, depending upon whether the gametes 

 differed merely in gene loci or in some unit larger than a gene 

 locus. Chromosomal hybrids would be of several different kinds. 

 The classification of plants that are not strictly true-breeding 

 into the categories of "hybrids" which we have just mentioned 

 ignores completely the taxonomic position of the parents. It also 

 has the drawback sometimes of classifying one plant as a hybrid 

 and another that is identical as not a hybrid. For example, if 

 one gene is completely dominant over its allele, the heterozygote 

 or hybrid will be indistinguishable phenotypically from the homo- 

 zygous dominant which is not a hybrid. Also some structural 

 hybrids might exhibit evidences that they are heterozygous for 

 inversions or translocations but might be indistinguishable pheno- 

 typically from the types that were homozygous for the inverted 

 segment and homozygous for the noninverted chromosome. If 

 these two cytological types did not differ as to any alleles, and 

 if the inversion was not accompanied by a position effect, the 

 fact that the two gametes that formed the "hybrid" zygote were 

 different could not be detected except by cytological examina- 

 tion. It is perhaps unfortunate that the term hybrid has been 

 used in several different senses, but the fact is that it has. 



Isolating Mechanisms 



If two species can cross if given the opportunity and if Fi off- 

 spring can be produced which are fertile, these offspring should 

 be able to produce further offspring either by self-fertilization, by 

 crossing with one another, or by backcrossing to the parents. 

 Furthermore, these Fo and backcross offspring would also prob- 

 ably be fertile, in which case they could also produce a new 

 generation by the same methods. If the two original species 

 were separated by geographical barriers which had become 

 broken down so that the two species could come close enough to 

 cross, such an array of Fi, backcross, and subsequent generations 

 would be produced in a small area after several breeding genera- 

 tions. If the organism was one whose life span exceeded several 

 reproductive periods, individuals (or clones) of all these genera- 

 tions would be present at once. Such a heterogeneous mixture of 

 types would be a hybrid swarm. 



