“…The size of offspring from matings between populations can depart from additive expectations for several reasons. In positive heterosis (hybrid vigor, or simply “heterosis”; Shull, ), offspring from both reciprocal crosses would be expected to be similarly larger (or healthier, or fitter) than offspring form the parental populations (e.g., Shikano, Nakadate, & Fujio, ), whereas outbreeding depression would cause smaller or less fit offspring in both interpopulation crosses. These effects are expected to be symmetrical, due to either the amelioration of mutational load (Keller & Waller, ) or the breakdown of co‐adapted genomes (Templeton et al., ) and are thus unlikely explanations for the phenotypic effects seen here (but we note that M‐Z hybrids were somewhat, but nonsignificantly, larger than M‐M offspring).…”