Velocity dispersion normalised to its local value, δσ _⋆ , as a function of redshift. The dott-dashed line is the observed evolution, δσ _⋆  = (1 + z)^0.4. The shaded area marks the 68% confidence interval. The solid line is the evolution due to major and minor mergers (μ ≥ 1/10) expected from our results. The dotted line is the expected evolution when very minor mergers (μ < 1/10) are taken into account. The dashed line is the expected evolution when very minor mergers and the progenitor bias (PB) are included (see text for details).