breed : if it is not, hybridization will occur. The de- 

 tection of hybridization between two populations may 

 be difficult, for introgression commonly occurs ; that 

 is, the hybrids backcross with either or both parental 

 populations, and the backcrosses resemble the par- 

 ental populations very closely. The result of intro- 

 gression is the gradual intrusion or transfer of the 

 characters of each population into the other, so that 

 all distinction between them disappears (Anderson 

 1949). 



If two populations differing in habitat require- 

 ments interbreed, the first generation hybrids will 

 often show best adjustment to an intermediate envi- 

 ronment. A second generation, if it occurs, will then 

 consist of individuals of both ancestral and hybrid 

 types, each of which requires its own peculiar habitat 

 for optimum development. If there is a paucity or 

 absence of intermediate habitats, the intermediate 

 forms will be selected against, and die out. If the hy- 

 brids are sterile or have a lower reproductive capacity 

 than the parental populations, they also will be se- 

 lected against. Any ecological, ethological, or genetic 

 divergence between the two populations that reduces 

 the gamete wastage in hybrids will be selected for, 

 with consequent rcinjorcement of the isolating mecha- 

 nism. It happens often, therefore, that niche segre- 

 gation or differences in behavior between closely 

 related species will become decidedly more pro- 

 nounced in the overlap zone of their ranges than else- 

 where. The result, of course, is continued divergence 

 of the two populations until interbreeding between 

 them ceases, and they form distinct species (Brown 

 and Wilson 1956). 



On the other hand, there are circumstances in 

 which fertile hybrids are not selected against. If the 

 area in which the hybrids are formed offers a variety 

 of habitats, some of which are different than the 

 habitats occupied by the parent population, the hy- 

 brids may find themselves fully as well adapted to 

 them, and by so much be fully as well adapted to that 

 area as are their parents. Under these circumstances 

 the hybrids will survive, introgression will occur, and 

 the formerly isolated parental populations will fuse 

 into one. Introgression between populations has been 

 observed and recorded in areas where man by remov- 

 ing forests or producing other disturbances in the 

 environment has destroyed barriers that maintained 

 a sharp ecological isolation of populations ; the phe- 

 nomenon doubtless occurred repeatedly in the geo- 

 logical past with changes in physiography, climate, 

 and vegetation (Anderson 1948, Blair 1951, Siblev 

 1954, Hubbs and Strawn 1956). 



A third possibility exists. Where hybrids obtain 

 some advantage or show better adaptations than their 

 parents, they will be selected jor, and may eventually 

 replace both parental populations. This appears to be 



taking place with Colias butterflies in northern Can- 

 ada at the present time (Hovanitz 1949). 



Asexual and self -fertilizing forms 



Asexual and self-fertilizing organisms include 

 half or more of the Protozoa and many invertebrates 

 and plants. These organisms offer problems of spe- 

 cies recognition and evolution that are in many re- 

 spects different than those in bisexual forms. Each 

 individual is reproductively isolated from every other 

 individual, giving rise to offspring that are genetically 

 alike by fission, budding, sporulation, or self-fertiliza- 

 tion. Mutations, however, arise, and if favorable, may 

 transform a strain or clone as the result of natural 

 selection. Local clones may differ therefore in pheno- 

 typic and genotypic characters in the same manner as 

 bisexual populations, even though there is no oppor- 

 tunity for variation to occur through assortment and 

 recombination of the genes. Clones that are genet- 

 ically distinct are in the nature of sibling species. Such 

 clones, however, are for convenience considered as 

 belonging to one and the same taxonomic species, if 

 they show similar morphological characters that are 

 readily distinguishable (Meglitsch 1954, Boyden 

 1954, Sonneborn 1957). 



NATURAL SELECTION 



Natural selection is a continual force ex- 

 erted on each successive generation. Before natural 

 selection can take place, however, there must be 

 phenotypic variations between individuals, from 

 which selection can be made. In order for these se- 

 lected variations to have significance in speciation, 

 they must be genetically fixed and heritable. 



In most species, many more offspring are pro- 

 duced than can possibly survive. Because of this 

 overproduction, there is competition between the off- 

 spring for the necessities of life, which, together with 

 the strife between predator and prey and between 

 organisms and their physical environment, creates a 

 struggle jor existence. 



There is differential survival in this struggle for 

 existence because some individuals have structural, 

 functional, or behavioral variations that give them 

 advantages over individuals lacking those variations. 

 The superior genotypes will make a relatively larger 

 contribution to the gene pool of the next generation. 

 The result of differential survival and differential re- 

 production is popularly known as the survival of the 

 fittest. 



The accumulation of favorable variations in a 

 population brings the species generally to a better 



264 Ecological processes and dynamics 



