 The phenotype, say "cross-veinless", is presumed to be caused by a combination of multiple genes.The phenotype appears when the sum of gene effects exceeds a threshold; if that threshold is lowered by the perturbation, the phenotype is more likely to be seen.Continued selection under perturbing conditions increases the frequency of the alleles of genes that promote the phenotype until the threshold is breached, and the phenotype appears without requiring the heat shock.