ECOLOGY AND ISOLATION 



617 



extremes of a seasonal range, with inter- 

 breeding only between those individuals 

 with similar seasonal adjustments, might 

 spHt the species. As in habitat isolation, 

 there might be a selective eUmination of 

 the intergrades in some instances. The gene- 

 tic complexity and probable gradual di- 

 vergence are illustrated by the intersterility 

 of many species separated by seasonal iso- 

 lation. 



In many other cases, interfertility is not 

 impaired between species inhabiting the 

 same geographic region that are reproduc- 

 tively separated by seasonal breeding 

 periods. Epling (Dobzhansky, 1941) has 

 studied flowering seasons in the genus 

 Salvia. Salvia niunzii and S. clevelandii are 

 wholly isolated because the first is past its 

 flowering season when the second begins. 

 Two other species of the same genus, S. 

 mellifera and S. apiana, have overlapping 

 flowering seasons and produce hybrids 

 when found together in the same locality. 



Two closely related species of the bee 

 genus, Andrena (A. peckhami and A. par- 

 nassiae) , visit different flowers in the same 

 region in Wisconsin and also fly in differ- 

 ent seasons, coincident with the flowering 

 seasons of their hosts (Cockerell, 1931). 



The moth, Eupethecia innotata, feeds on 

 Artemisia, and a closely related species, E. 

 unedonata, emerges earlier and feeds on 

 Arbutus (Hogben, 1940). Pupae of the 

 species feeding on Arbutus were cooled, 

 thus delaying their emergence, and, when 

 mated with the species feeding on Artemi- 

 sia, fertile hybrids were produced. Of 

 course, the seasonal isolation is associated 

 here with habitat differences in food plants 

 as well. It should also be pointed out that 

 the production of fertile hybrids in the 

 laboratory does not necessarily mean that 

 these hybrids could survive through many 

 generations under natural conditions. Al- 

 though it is difficult to separate such a 

 factor as seasonal isolation from all other 

 evolutionary factors, it would appear that 

 in some cases it may be a real hindrance 

 to interbreeding. 



Two species of grasshoppers, Arphia 

 sulphurea and A. xanthoptera, have similar 

 ranges from Nebraska to Texas, Florida 

 and New England. They occupy similar 

 habitats, except that A. sulphurea remains 

 in the nymph stage in the winter, matures 

 in the spring, and largely disappears be- 



fore A. xanthoptera, hatching from eggs in 

 the spring, becomes mature. In Illinois, 

 adults of A. sulphurea have been taken 

 from April to July 29, and adults of A. 

 xanthoptera have been taken from July 14 

 to October 18 (Blatchley, 1920; Hebard, 

 1934). 



Miller and Miller (1943) report differ- 

 ences in the season of the colonizing flights 

 of two closely related species of termites, 

 Reticulitermes hageni and R. virginicus 

 These two species have similar geographic 

 distribution and inhabit the same ecologic 

 areas. The flights of R. virginicus occur in 

 south Florida from March through May, 

 while the flights of R. hageni occur in the 

 fall and winter months. In the vicinity of 

 Washington, D. C, Reticulitermes flavipes 

 flies from outdoor colonies late in April or 

 early in May; R. virginicus flies in early 

 June; and R. hageni flies in July and 

 August (Snyder, 1935). 



The "land-locked" sockeye salmon or 

 kokanee of Cultus Lake, British Columbia, 

 spawns in August and September, while 

 the residual sockeye, from which the koka- 

 nee is probably derived, spawns in the 

 same lake from October to December. This 

 seasonally isolated form shows some color- 

 ation differences in the mature male and 

 other slight differences, including a rela- 

 tive immunity to the parasitic copepod, 

 Salmincola. It may be presumed that these 

 seasonal differences have established either 

 partial or total reproductive isolation and 

 thus constitute a major factor in the diver- 

 gence of these forms, which have not yet 

 been named. The kokanee has been dis- 

 tinguished as a subspecies, but, as defined, 

 it inhabits many lakes and may well be a 

 polyphyletic group (Ricker, 1938, 1940). 



After consideration of the evidence, it 

 would seem possible for seasonal isolation, 

 together with selection, to separate popu- 

 lations gradually, even within the same 

 geographic and ecologic area, thus leading 

 to speciation (Crombie, 1947). Seasonal 

 isolation would thus be a sort of temporal 

 habitat isolation. Doubtless this form of re- 

 productive isolation is a rather minor mech- 

 anism as compared, for example, to topo- 

 graphic or habitat isolation. 



If the life cvcles of the individuals ex- 

 tend two years or more, such annual isola- 

 tion within the same geographic and habi- 



