  Both gas accretion (infall) and winds (outflow) change a galaxy's metallicity
and gas fraction, lowering the effective yield. Low effective yields in
galaxies with rotation speeds < 120 km/s have been widely interpreted as the
onset of SN-driven winds below a characteristic galaxy mass, but gas accretion
is also a viable explanation. However, calculations presented here prove: (1)
that metal-enriched outflows are the only mechanism that can significantly
reduce the effective yield, but only for gas-rich systems; (2) that it is
nearly impossible to reduce the effective yield of a gas-poor system, no matter
how much gas is lost or accreted; and (3) that any subsequent star formation
drives the effective yield back to the closed-box value. Thus, only gas-rich
systems with low star formation rates (such as dwarf irregulars) can produce
and maintain low effective yields, while massive gas-poor galaxies can never
show low effective yields, even after experiencing substantial infall and/or
outflow. The drop in effective yield seen in low mass galaxies is therefore
less likely to be due to the onset of SN-driven winds than to the galaxies'
surface densities falling entirely below the Kennicutt SF threshold. Additional
calculations confirm that the fraction of baryonic mass lost through winds
varies only weakly with galaxy mass, shows no sharp upturn at any mass scale,
and does not require that >15% of baryons have been lost by galaxies of any
mass. SN feedback is therefore unlikely to be effective for removing large
amounts of gas from low mass disk galaxies. In addition, the dependence between
metal-loss and galaxy mass is sufficiently weak that massive galaxies dominate
metal enrichment of the IGM. The calculations in this paper provide limiting
cases for any arbitrary chemical evolution history, as proven in an Appendix.
