Table 1. Summary of the deleterious substitution rates in the different adaptive regimes.

Adaptive regime	Deleterious substitution rate	Typical passenger cost	Related work
No drivers	r(x) ≈ 2x/(e2x − 1)	$d{s}_{\text{d}}^{\ast }/dN<0$	Fisher (1930); Wright (1931)
Rare drivers	r(x) ≈ 1/(1 + x)	$d{s}_{\text{d}}^{\ast }/dN>0$	Johnson and Barton (2002)
Multiple drivers, $x_{\text{c}}\approx s_{\text{b}}^{\ast }$	r(x) ≈ (1 − e−2x)/(2x)	$d{s}_{\text{d}}^{\ast }/dN>0$	Schiffels et al. (2011)
Multiple drivers, $x_{\text{c}}\gg s_{\text{b}}^{\ast }$	r(x) ≈ e−x	$d{s}_{\text{d}}^{\ast }/dN\approx 0$	Desai and Fisher (2007)
Infinitesimal limit	r(x) ≈ e−x	$d{s}_{\text{d}}^{\ast }/dN<0$	Hallatschek (2011)

To compare functional forms, we show the scaled substitution rate, r(x) = c₁R(−c₂x), where the scaling parameters c₁ and c₂ are chosen so that r(0) = −r′(0) = 1.