MAR APR MAY JUN JUL AUG SEP OCT 



FIG. 19-3 Seasonal changes In relative frequency of the third 

 chromosome with the standard (black), arrowhead (stippled), 

 and Chiricahua (cross-hatched) gene arrangements in a natural 

 population of Drosophila pseudoobicura (Dobzhansky 1951). 



species, these genes may be arranged in all sorts of 

 combinations to form an almost infinite mixture of 



character modifications (Caspari 1951). Heterozy- 

 gotes are, therefore, much more fle.xible in adaptively 

 responding to the environment than are homozygotes. 

 The more characters for which an individual is het- 

 erozygous, the more adaptable its offspring are likely 

 to be. Adaptive polymorphic populations are more 

 efficient in exploiting the environment than are ge- 

 netically uniform ones. Conversely, species that are 

 widespread geographically through many habitats are 

 genetically more diversified than are those restricted 

 to few or specialized habitats. There are limits, how- 

 ever, beyond which a character cannot change. The 

 continual tendency for characters to fluctuate around 

 a mean or intermediate condition gives a population 

 r/enctic homeostasis (Lerner 1954). 



Genetic drift 



Although certain characters may result from 

 the action of a single gene or pair of genes, many, 

 perhaps most, characters within a species are poly- 

 genic ; i.e., they are aifected by a multiplicity of genes. 

 The exact form in which a character is expressed de- 

 pends on the particular combination of genes which 

 the individual or population possesses (Waddington 

 1957). 



If a fertilized female, a single pair of animals, or 

 at most a few hundred individuals become separated 

 from the rest of the species, there will be represented 

 in them a considerable decrease in the number of 

 genes available to the main body of the species, since 



no individual or small group of individuals can pos- 

 sess all the genes that occur within the species' pool. 

 Inbreeding within small isolated populations may thus 

 bring into prominence traits which are expressed 

 only irregularly and inconspicuously within the spe- 

 cies as a whole. Establishment thus of restricted 

 genotypes in small population by loss of genes or acci- 

 dental changes in frequencies at which certain genes 

 occur is called genetic drift, or the Scwall Wright 

 effect (Wright 1931). Character variations formed 

 in this manner often appear to be non-adaptive, and 

 there is controversy as to whether such characters are 

 important in species formation. 



The possibility that genetic drift is a significant 

 factor in speciation under some conditions is shown, 

 however, by ground finches of the Galapagos Islands 

 (Lack 1945), species of which differ chiefly in the 

 size and shape of the bill. Species belonging to the 

 same genus consume the same kind of food. Differ- 

 ences in bill characteristics apparently developed as 

 small populations became isolated on different islands 

 within the Galapagos group, even though the islands 

 were similar in climate, vegetation, and habitats gen- 

 erally. The particular bill characteristics that the 

 various species possess apparently resulted from a 

 chance combination of genes that became segregated 

 on each island. With inbreeding, these bill character- 

 istics became genetically fixed. It is of interest that 

 the peculiar bill characteristics came later to have a 

 secondary significance in furnishing sign stimuli in 

 courtship and in territorial defense. This has given 

 ethological isolation to some species and prevented 

 interbreeding where species, thitherto dispersed and 

 isolated, have come again into contact. 



Similar fixation of special characteristics may 

 occur in species subject to catastrophic or cyclic re- 

 ductions in abundance. The genotypes of the few sur- 

 vivors will determine the genetic makeup of the entire 

 new population that develops in the area (Elton 1930, 

 Timofeeff-Ressovsky 1940). For example, the arctic 

 fox is a cyclic northern species possessing a white 

 and a so-called blue color phase. Over most of the 

 fox's range, the blue phase is much less common than 

 the white, but on certain islands only the blue phase 

 occurs. It is possible that at some time in the past, 

 at the bottom of a population cycle, only homozygous 

 blue foxes survived ; reproduction of these animals 

 and their offspring rendered the entire new popula- 

 tion blue. This does not mean, however, that blue 

 color is necessarily non-adaptive or that it may not 

 he genetically linked with a character tliat is adaptive. 



Mutations may be the result of chemical 

 changes in the individual gene or to chromosomal 



262 Ecological processes and dynamics 



