  Plant domestication involved a process of selection through human agency of a
series of traits collectively termed the domestication syndrome. Current debate
concerns the pace at which domesticated plants emerged from cultivated wild
populations and how many genes were involved. Here we present simulations that
test how many genes could have been involved by considering the cost of
selection. We demonstrate the selection load that can be endured by populations
increases with decreasing selection coefficients and greater numbers of loci
down to values of about s = 0.005, causing a driving force that increases the
number of loci under selection. As the number of loci under selection
increases, an effect of co-selection increases resulting in individual unlinked
loci being fixed more rapidly in out-crossing populations, representing a
second driving force to increase the number of loci under selection. In
inbreeding systems co-selection results in interference and reduced rates of
fixation but does not reduce the size of the selection load that can be
endured. These driving forces result in an optimum pace of genome evolution in
which 50-100 loci are the most that could be under selection in a cultivation
regime. Furthermore, the simulations do not preclude the existence of selective
sweeps but demonstrate that they come at a cost of the selection load that can
be endured and consequently a reduction of the capacity of plants to adapt to
new environments, which may contribute to the explanation of why selective
sweeps have been so rarely detected in genome studies.
