We now consider changes in the mean value of the threshold trait determining resistance.
We assume that both traits act as threshold traits (Falconer, 1989).
The existence of a threshold trait suggests that there are trade-offs associated with each morph (Roff 1984).
The analysis of the genetic basis of trade-offs associated with a threshold trait poses a more difficult problem than with the usual quantitative trait because there are two possible modes of determination.
The last question is important for our understanding of the evolution of tradeoffs, because in one case (dichotomy model) trade-offs are fixed, while in the other case (continuous model) trade-offs change as the threshold trait evolves.
By considering larval survivorship a threshold trait, I estimated the selection differential each generation and calculated realized heritabilities for each replicate.
The selection differential of larval survivorship was estimated from the proportion of the population that survived using standard methods for threshold traits (Crow and Kimura 1970; Falconer 1989).
As diseases has threshold nature, in order to estimate heritabilities, genetic and environmental correlations, TM program (Legarra, Varona, & Lopez de Maturana, 2011) was used that is capable of analyzing threshold traits
along with continues traits using Gibbs sampling approach, simultaneously.
Because group effects for threshold traits
are almost directly proportional to group size (Krebs and Loeschcke 1997), the estimate of heritability for adult thermotolerance is low ([[h.
Meristic and threshold traits, which can be classified in discrete categories, can also be considered to vary continuously on an underlying scale.
Included here is a comprehensive treatment of experimental designs for estimating heritability for continuous and threshold traits, with details of the statistical analyses and their genetic interpretation, guidance as to optimal experimental design, and standard errors of the [h.
Discrete plasticity, or polyphenism, has been modeled as a threshold response to continuous variation in the environment (Lees 1966; Vepsalainen 1974; Harrison 1979; Roff 1986; West-Eberhard 1992), and several recent models predict that threshold traits
evolve through genetic changes in the sensitivity of the threshold to the environment (Hazel et al.