The Botanical Review

, Volume 63, Issue 1, pp 65–92

Sexual specialization and inbreeding avoidance in the evolution of dioecy

  • D. Carl Freeman
  • Jon Lovett Doust
  • Ali El-Keblawy
  • Kathleen J. Miglia
  • E. Durant McArthur


Dioecy has evolved independently, many times, among unrelated taxa. It also appears to have evolved along two contrasting pathways: (1) from hermaphroditism via monoecy to dioecy and (2) from hermaphroditism via gynodioecy to dioecy. Most dioecious plants have close cosexual relatives with some means of promoting outcrossing (e.g., herkogamy, dichogamy, self-incompatibility, or monoecy). To the extent that these devices prevent inbreeding, the evolution of dioecy in these species cannot logically be attributed to selection for outcrossing. In these cases, the evolution of dioecy is, we believe, due to selection for sexual specialization. However, in other species, that lack outbreeding close relatives, dioecy may have evolved from gynodioecy (males and hermaphrodites) as an outbreeding device. Subsequent disruptive selection and selection for sexual specialization may have also shaped the evolution of dioecy from gynodioecy in these species, resulting in two genetically determined, constant sex morphs.

Both pathways for the evolution of dioecy require the operation of disruptive selection, though the gynodioecy route involves more restrictive disruptive selection and a genetic designation of gender. In contrast, the monoecy route is not dependent on the genetic designation of two sex morphs, but, rather, allows the possibility of sexual intermediates and sexual lability. Both pathways produce one morph in which maleness is suppressed and another in which the female function is negligible or nonexistent—the reproductive mode recognized as dioecy.

Evidence is presented here to support the thesis that instances of sexual lability, the presence of an array of sexual intermediates, sex-switching, and sexual niche segregation can be explained in terms of the pathway that was taken in the evolution of a particular dioecious species. In addition, the degree of sexual dimorphism seen in dioecious species is correlated with mode of pollination (insector wind-pollinated) and other ecological factors.


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Copyright information

© The New York Botanical Garden 1997

Authors and Affiliations

  • D. Carl Freeman
    • 1
  • Jon Lovett Doust
    • 2
  • Ali El-Keblawy
    • 3
  • Kathleen J. Miglia
    • 1
  • E. Durant McArthur
    • 4
  1. 1.Department of Biological SciencesWayne State UniversityDetroitUSA
  2. 2.Department of Biological SciencesUniversity of WindsorWindsorCanada
  3. 3.Department of BiologyUniversity of Suez CanalEl-ArishEgypt
  4. 4.Shrub Sciences LaboratoryU.S. Department of Agriculture Forest ServiceProvoUSA

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