454 BRADLEY 



TS were found to be closely related to survival time at high 

 temperatures (Bradley, 1976). This means that the assays are not 

 only convenient (short term) and practical (unambiguous) but also 

 realistic in measuring the resistance of the animals to thermal stress. 

 Population and individual flexibilities were defined in terms of 

 components of phenotypic variance. Observed phenotypic variance 

 in temperature tolerance can be partitioned into genetic and 

 nongenetic components: 



ol = ol +a^A +^Eg +^3 



where o\ = additive genetic variance (caused by genes themselves), a 

 measure of population flexibility 

 a^^ = nonadditive genetic variance (caused by difference be- 

 tween genes at the same or different loci) 

 ag = general environmental variance (nongenetic variance be- 

 tween individuals) 

 Qg = general environmental variance (within individuals), a 

 measure of individual flexibility 



The additive genetic variance, o\, indicates the degree to which a 

 population can adapt or change genetically in temperature tolerance. 

 The rate of change is directly proportional to the heritability of the 

 characteristic, a^/ap. This ratio varies from 0, when none of the 

 variations between animals is caused by additive effects of the genes 

 themselves, to 1, when all the variation is additive and genetic. The 

 additive portion rather than the total genetic variance is the critical 

 measure because combinations of genes at a locus are not passed on, 

 nor are most combinations of genes at different loci. 



Therefore, the heritability measure indicates the potential of the 

 population to change genetically the characteristic concerned. In the 

 present case the heritability of temperature tolerance indicates the 

 potential of a local population of copepods to increase in average 

 temperature tolerance, assuming other factors (e.g., fertility) do not 

 change. I should emphasize that heritability itself will change as 

 selection for increased tolerance proceeds since additive genetic 

 variability generally decreases because of the restricted range of 

 genotypes present after selection. 



Tolerance to cold temperature was measured in a fashion similar 

 to that described, with some differences. Obviously the shock 

 temperature was different (1.5°C), and animals were removed after 

 10 min (at which time most had succumbed) and allowed to recover 

 at room temperature. Again TS and TR were observed and included 

 in the index (30 + TS — TR), but the maximum tolerance for animals 



