Hermaphroditic individuals can produce both selfed and outcrossed progeny, termed mixed mating. General theory predicts that mixed-mating populations should evolve quickly toward high rates of selfing, driven by rapid purging of genetic load and loss of inbreeding depression (ID), but the substantial number of mixed-mating species observed in nature calls this prediction into question. Greater average ID reported for selfing than for outcrossing populations is consistent with purging and suggests that mixed-mating taxa in evolutionary transition will have intermediate ID. We compared the magnitude of ID from published estimates for highly selfing (r > 0.8), mixed-mating (0.2 < r > 0.8), and highly outcrossing (r < 0.2) plant populations across 58 species. We found that mixed-mating and outcrossing taxa have equally high average lifetime ID (d = 0.58 and 0.54, respectively) and similar ID at each of four life-cycle stages. These results are not consistent with evolution toward selfing in most mixed-mating taxa. We suggest that prevention of purging by selective interference could explain stable mixed mating in many natural populations. We identify critical gaps in the empirical data on inbreeding depression and outline key approaches to filling them.

weakmoderatestrongInbreedingDepressionInPlantsData were collected as part of the NESCent working group, “The Paradox of mixed mating in flowering plants”. The group searched for inbreeding depression data for populations of species for which outcrossing rates were available (see Goodwillie et al 2005, Ann. Rev. Ecol. Evol. Syst 36: 47-79). The spreadsheet includes some data that were not used for the analyses reported in the paper. For example, if not enough life stages were measured by the original authors for us to calculate 3- or 4-stage inbreeding depression, we could not use the data for our analyses. Our decision rules and a table that includes all data used in our analyses are included in our publication (Table 2) For further information, please see the ReadMe file.