Da. Roff et al., The evolution of threshold traits: effects of selection on fecundity and correlated response in wing dimorphism in the sand cricket, J EVOL BIOL, 12(3), 1999, pp. 535-546
The quantitative genetic basis of traits can be determined using a pedigree
analysis or a selection experiment. Each approach is valuable and the comb
ined data can contribute more than either method alone. Analysis using both
sib analysis and selection is particularly essential when there are likely
to be nonlinearities in the functional relationships among traits. A class
of traits for which this occurs is that of threshold traits, which are cha
racterized by a dichotomous phenotype that is determined by a threshold of
sensitivity and a continuously distributed underlying trait called the liab
ility. In this case, traits that are correlated with the liability may show
a nonlinear relationship due to the dichotomy of expression at the phenoty
pic level. for example, in wing dimorphic insects fecundity of the macropte
rous (long-winged) females appears in part to be determined by the allocati
on of resources to the night muscles, which are almost invariably small or
absent in the micropterous (short-winged, nightless) females. Pedigree anal
ysis of the cricket Gryllus firmus has shown that wing morph, fecundity and
the trade-off between the two have additive genetic (co)variance. It has a
lso been shown that selection on proportion macroptery produced an asymmetr
ic correlated response of fecundity. The present paper details the results
of direct selection on fecundity and the correlated response in proportion
macroptery. Selection for increased fecundity resulted in increased fecundi
ty within both wing morphs and a correlated decrease in proportion macropte
ry. Similarly, selection for decreased fecundity resulted in a decrease wit
hin morphs and a correlated increase in the proportion of macropterous fema
les. This provides additional evidence that the trade-off between fecundity
and wing morphology has a genetic basis and will thus modulate the evoluti
on of the two traits.