Figure 4: Evolution of buffering mechanisms.

A) Non-linear model of genotype-phenotype relationships, where genotype is represented as functional protein produced by a gene (x-axis). Protein level variance is represented by the vertical bars; horizontal bars represent variation in phenotype. Note that, based on the threshold model, the same variance in protein levels has a significantly different effects on phenotype depending on protein levels relative to the threshold, where dark grey is wild-type (WT) and light gret is mutant (Het^+/−). Either heterometry or increases in GRN complexity may shift the position of a threshold value in a non-linear genotype-phenotype curve (black to green). B) Alterations to inputs regulating a gene of interest (GOI) may affect its levels (heterometry). C) Alterations to the number of co-regulators of protein complexes regulating a developmental process can increase gene regulatory network (GRN) complexity.