464 BRADLEY 



expendable, but this is not the case (Heinle, 1970). Perhaps the 

 question of sex dimorphism cannot be answered until the mechanism 

 of sex determination in copepods is known. There have been some 

 suggestions of a multifactorial basis (e.g., Battagha, 1967; McLaren, 

 1976), but, aside from evidence of the lability of the sex ratio to 

 density and perhaps temperature and salinity (Heinle, 1970; Katona, 

 1970), Httle is known about the control of sex in copepods. 



The relationship between genetic and physiological variance in 

 the adaptation of organisms was discussed by Slobodkin (1968) and 

 Slobodkin and Rapoport (1974). Slobodkin's view is that environ- 

 mental stresses that can be dealt with physiologically cause a 

 minimum of genetic effects in later recurrences. Hence, genetic 

 variance in a particular characteristic would not even be maintained 

 in species with a high physiological flexibility in that characteristic 

 since genotypic differences would be obscured and the resistance of 

 the individual to the stress would be less related to its genotype. This 

 theory is supported by the results of Levins (1969) and Marshall and 

 Jain (1968). Levins found that Drosophila species with broad niches 

 had larger physiological tolerances to dissication and smaller genetic 

 differences than narrow-niched species, but he did not measure 

 genetic variance directly. Marshall and Jain found that one species of 

 wild oat (Auena barbata) was developmentally more flexible than 

 another {A. fatua) but was less variable genetically. 



The data presented do not give definitive answers to some 

 practical questions. On the basis of experiments on the range of 

 temperatures over which an individual can reproduce and of informal 

 observations of animals in cycling temperatures in the laboratory 

 (and the very low population densities in the Chesapeake Bay in 

 summer), Eurytemora affinis seems to be at the limits of adaptation 

 even in normal temperatures. A large potential for adaptation to 

 temperature exists, however, both genetically and physiologically. 

 This begs the question. What does this genetic and physiological 

 flexibility mean if it cannot be used to allow the population to adjust 

 to thermal stress? I must conclude that the measures of flexibility 

 used are not sufficient by themselves to predict the survival of 

 Eurytemora in an even wider range of temperatures. 



To answer the question of the apparently unusable flexibility, we 

 need to know (1) whether the most tolerant animals are also the 

 most flexible physiologically, (2) whether genetic variance can be 

 exhausted, and (3) whether reproductive capacity and temperature 

 tolerance are negatively related. 



If selection for temperature tolerance, which increases the 

 average tolerance in the population, results in lowered physiological 



