“…Quantitative variation of SI among individuals has often been observed within species (Kokubun et al, 2006;Levin, 1996;Lloyd, 1968;Reinartz and Les, 1994;Stebbins, 1957;Stephenson et al, 2003), and the geographic distribution of individuals categorized by SI strength was also confirmed (Goodwillie and Ness, 2005;Inoue, 1988;Kokubun et al, 2006;Lipow et al, 1999;Lloyd, 1968;Lyons and Antonovics, 1991;Ortiz et al, 2006;Stone et al, 2006;Tsukamoto et al, 1999). These previous studies, however, covered only a few populations (but see Kokubun et al, 2006;Lloyd, 1968) and estimated SI/SC as a qualitative trait, but not a quantitative trait.…”