ECOLOGY AND ISOLATION 



623 



uals that are haploid, bisexual and game- 

 tophytic." Diplont-haplont types are found 

 among the flowering plants in wluch there 

 may be an inhibiting effect of self-pollina- 

 tion and cross pollination occurring particu- 

 larly during the "reactions between the 

 diploid tissues of the pistils of flowers of 

 the sporophytic generation and the haploid 

 pollen tubes of the alternating gametophy- 

 tic generation." These physiological reac- 

 tions may be determined by an allelic pair 

 of genes, an allelic series, or they may be 

 polygenic. 



Where the genetic mechanism affecting 

 crossability is simple, one might expect oc- 

 casional initial isolation between popula- 

 tions through physiological incompatibility 

 (Crombie, 1947). Where the genetic mech- 

 anism is more complicated, one might as- 

 sume long separation of the populations 

 through various types of reproductive isola- 

 tion. 



Because of the difficulty of detection, 

 physiological isolation may be more com- 

 mon than the evidence indicates at present. 

 As in sexual isolation in general, physiolog- 

 ical isolation would be expected to follow 

 the effects of the various extrinsic factors 

 of isolation. Initial separation of popula- 

 tions on the basis of physiological incom- 

 patibility is probably rare. 



GENETIC ISOLATION 



Genetic change characterizes all evolu- 

 tionary progression or regression. Popula- 

 tions as well as individuals exhibit genetic 

 differences. Genetic distinctions, however, 

 may or may not prevent interbreeding be- 

 tween populations; in order to affect isola- 

 tion directly, they must affect interfertility 

 (p. 676). Physiologic incompatibility be- 

 tween the sexes of the organisms carrying 

 the gametes is discussed under Physiologi- 

 cal Isolation (p. 622). Lack of fertility 

 because of chromosomal or gene balance 

 is termed genetic isolation and lies largely 

 in the field of genetics rather than in 

 ecology (White, 1945; Castle, 1946; Pat- 

 terson and Wheeler, 1947; Hughes-Schra- 

 der, 1948). 



Initial infertility between individuals 

 may rarely give rise to species divergence 

 among animals. Hubbs and Hubbs (1946) 

 report a species of fish (Mollienisia formo- 

 sa) composed wholly of females that is 

 physiologically dependent upon copulation 



with other species in order to stimulate 

 oviposition and development. The offspring 

 never inherit any characters from the males 

 of these other species. Species divergence 

 in this case must have occurred without 

 any other type of isolation than infertility 

 and parthenogenesis. 



Polyploid plants and animals with many 

 parthenogenetic generations may develop 

 intersterility suddenly (Jackson, 1931). Re- 

 cent knowledge of such isolating mecha- 

 nisms is summarized by Dobzhansky 

 (1941), Cain (1944), Crombie (1947), 

 and Hughes-Schrader (1948). 



Genetic isolation is the clinching intrinsic 

 type of isolation that ultimately separates 

 diverging species regardless of fluctuations 

 in extrinsic factors. A high proportion of all 

 taxonomic species of both plants and ani- 

 mals have developed genetic isolation. To- 

 gether with the other intrinsic mechanisms 

 (sexual bars to gene flow, hybrid invia- 

 bility, and hybrid sterility; see Table 52, 

 Isolation Factors, on p. 606), the develop- 

 ment of a permanent prevention of gene 

 flow is a criterion used by some authors 

 in their definition of a species. We use the 

 attainment of reproductive isolation be- 

 tween genetically different natural popula- 

 tions whether by intrinsic or extrinsic mech- 

 anisms, as the criterion of the species cate- 

 gory (p. 625). Genetic isolation commonly 

 results from regressive evolution of fertility 

 (p. 676). 



Genetic isolation, hybrid inviability, and 

 hybrid sterility are analyzed in genetic and 

 embryological literature and are consid- 

 ered to be internal physiological phenom- 

 ena outside the field of ecology. However, 

 inasmuch as these intrinsic factors influence 

 the extrinsic relations of one organism or 

 population to another, ecologists must take 

 them into account. 



SELECTIVE HYBRID ELIMINATION 



Hybrids may fail to perpetuate them- 

 selves because of inviability, sterility (Dob- 

 zhansky, 1941), or through selective elim- 

 ination. As ecologists, we are here con- 

 cerned with the last type of incapacity. If 

 two populations have become differentiated 

 by adaptation to two different habitats 

 through selection, the hybrids would prob- 

 ably not be so well adapted to either 

 habitat as would the parent forms and 

 might thus be selectively eliminated. The 



