Before revision:  We develop a simple theory that can account for this scaling relation using moment evolution equations and clarify the limit of applicability of the theory using time-backward observation}  .
After revision:  We develop a simple theory that can account for this scaling relation using moment evolution equations and clarify the limit of applicability of the theory using time-backward observation} This relation markedly differs from that predicted by a binary-trait model, where evolution becomes increasingly dependent on m as s decreases. We show that this difference is caused by the assumption implicit in binary-trait models that mutation causes convergent evolution even in the absence of selection. Our study illustrates how considering quantitative variation can be important for understanding the evolution of altruism during major evolutionary transitions .