Simultaneous selection for two correlated traits inTribolium

Abstract

Simultaneous selection by independent culling levels of two correlated traits in all four combinations of directions was investigated with Tribolium in a replicated experiment extending over nine generations. In addition to the two primary traits, 13-day larval weight and pupal weight, four secondary traits (pupation time, adult emergence time, adult weight and larval number) were observed.The observed responses for both selected and unselected traits agreed with theoretical expectations after the latter were adjusted for changes which occurred in genetic and phenotypic parameters. Phenotypio variances for the selected traits were correlated positively with population means, yet genetic variances and heritabilities declined in all selected populations. No change was detected in the genetic correlation between selected traits even though the divergent two-trait selection was designed especially to ' break' the positive correlation of + 0-55 + 012 present in the base population.Striking changes in growth and developmental patterns resulting from the divergent selection were discussed in terms of metamorphic limits and 'stabilizing' genetic correlations.

Rapid Independent Trait Evolution despite a Strong Pleiotropic Genetic Correlation

“…While in none of these cases was the mechanism of correlation known, and in only one case was there a rigorous published estimate showing an additive genetic correlation that was high in magnitude (Frankino et al 2007), it seems likely that the genetic correlations were at least moderate in magnitude and caused at least in part by pleiotropy, because the traits were mostly size related. In addition, two studies (Bell and Burris 1973;Frankino et al 2007) reported an unchanged genetic correlation after selection, two others showed a relatively unchanged bivariate phenotypic distribution before and after selection (Emlen 1996;Frankino et al 2005), and another two show a correlated phenotypic distribution after selection (Weber 1990;Wilkinson 1993); these results also suggest pleiotropy. Clearly, the similar results in both plants and animals for a variety of different kinds of traits strongly suggest that genetic correlations by themselves are insufficient to quantify the nature and severity of evolutionary constraint.…”

“…6). Second, artificial selection on several species of insects and a wild population of the domestic radish has been able to independently change the means of a wide variety of correlated traits (Bell and Burris 1973;Weber 1990;Wilkinson 1993;Stanton and Young 1994;Emlen 1996;Beldade et al 2002;Frankino et al 2005Frankino et al , 2007; but see Allen et al 2008); in other words, these studies were able to change the position of the bivariate cloud of points in spite of a correlation (cf. fig.…”

“…Selection in this direction should result in the slowest possible response and thus the maximal evolutionary constraint. Several studies using this basic idea have been performed (Bell and Burris 1973;Weber 1990;Dorn and Mitchell-Olds 1991;Wilkinson 1993;Stanton and Young 1994;Emlen 1996;Beldade et al 2002;Frankino et al 2005Frankino et al , 2007Allen et al 2008), and all but two (Dorn and Mitchell-Olds 1991;Allen et al 2008) have shown that short-term evolution of correlated traits can occur perpendicular to the major axis of the correlation.…”

Impacts of genetic correlation on the independent evolution of body mass and skeletal size in mammals

“…In the first approach, the constraining effect of the correlation between traits is assessed using artificial selection, by imposing selection regimes that would "push" the phenotype in multivariate space at right or near-right angles to g max , i.e., along the hypothetical line of greatest evolutionary resistance (reviewed in [21]). Owing in large part to the labor-intensive nature of selection experiments, such studies have been done almost exclusively on organisms that are short-lived, breed easily, and produce relatively large numbers of offspring, such as plants (e.g., Arabidopsis, [29], wild radishes, [21,30]) and insects (e.g., butterflies, [31][32][33][34], beetles [35], Drosophila, [36]). Although most studies show that evolution at right angles from g max is possible, despite strong genetic correlations, a few have shown the existence of seemingly unbreakable, developmentally based constraints on the short-term evolution of traits perpendicular to g max (e.g., color variation between eyespots in the wings of the butterfly Bicyclus anynana [34,37,38].…”

Experimental comparison of methods for simultaneous selection of two correlated traits in Tribolium. 2. Index selection and independent culling levels : a replicated single generation test

“…In the N line, the males and females were separately ranked for k, and X2 , then selection was simultaneously practised in the 2 rankings till the proportion selected was achieved (BELL & B URRIS , 1973 The average value throughout the 10 replicates of the intensities of selection obtained in the index line (tabl. 4) was slightly smaller than the theoretical value (1.39).…”