TABLE 5

Comparison of the ratchet with the hitchhiking model

Ratchet: N = 125,000, U = 0.01Hitchhiking: N = 125,000
nSDPow 1 (%)SDPow 1 (%)
25-fold reduction
s = 0.0002, k/k0 = 0.04, θ = 5Thh = 0.04N gen, θ = 5
n S D Pow 1 (%) S D Pow 1 (%)
120.79–0.5621.12–1.2212
401.66–0.84102.93–1.5148
1003.00–1.11325.39–1.7184
θ = 10θ = 10
nSDPow 1 (%)SDPow 1 (%)
121.63–0.6372.16–1.3431
403.38–1.06376.01–1.8591
1005.92–1.365711.1–2.1099
θ = 20θ = 20
nSDPow 1 (%)SDPow 1 (%)
123.21–0.73264.26–1.5559
406.78–1.294511.93–2.1999
10011.98–1.677322.08–2.38100
33-fold reduction
s = 0.0005, k/k0 = 0.03, θ= 5Thh = 0.03N gen, θ = 5
nSDPow 1 (%)SDPow 1 (%)
120.58–0.6720.82–1.186
401.29–0.91112.27–1.4337
1002.35–1.12294.66–1.6980
50-fold reduction
s = 0.001, k/k0 = 0.02, θ = 5Thh = 0.02N gen, θ = 5
nSDPow 1 (%)SDPow 1 (%)
20.46–0.6920.53–1.194
401.09–1.0491.63–1.3625
1002.01–1.15243.53–1.5665

The reduction in genetic diversity was chosen to be of the order of that observed in the D. miranda neo-Y chromosome data (Bachtrog and Charlesworth 2002). The S and D values are means over 1000 coalescent trees for each model. For the ratchet, results are based on the structured coalescent; for hitchhiking, results are based on coalescent simulations (Hudson 1990) from a population that expanded from size 1 to N instantaneously at time Thh in the past. Thh was chosen to produce the same mean level of variability as with the ratchet.