Date of Award


Document Type

Distinction Paper

Degree Name



Biology & Earth Science


Dr. Jeffrey Lehman

First Committee Member

Dr. Halard Lescinsky

Second Committee Member

Dr. David Robertson


ecological factors, Los Cusingos, Dioecy, Imperfect Flower, Perfect Flower, Outcrossing

Subject Categories



There is a considerable disagreement about the selective forces that have led to the evolution of dioecy (i.e., the condition of having separate male and female flowers on separate plants). Historically, selection to enforce outcrossing has been the primary explanation despite there being little empirical evidence. Hence, many scientist argue that the selection of dioecy is driven by ecological traits that influence male and female fitness and seed dispersal. The objective of this study was to examine the association between plant sexual system (particularly, dioecy) with various ecological traits for the flora of a tropical premontane wet forest in Costa Rica. Our study included 313 angiosperm species in 216 genera and 83 families that were characterized for plant sexual system, growth form, flower size, fruit type, and seed number. Fisher’s exact tests were conducted to determine the relationship between each of the ecological characteristics and plant sexual systems at the species- and generic-level. The null hypothesis for all comparisons is that the proportionate representation of hermaphroditic, monoecious, or dioecious species in the various categories is not significantly different from the distribution of species for the entire flora. Of the total 313 species used in this study, 229 (73.2%) were hermaphroditic, 41 (13.1%) were monoecious, and 43 (13.7%) were dioecious. In analyses of species, dioecy was associated with woody growth (X2 = 8.53; P = 0.03); tiny flower size (X2 = 7.37; P = 0.04), few seeds (X2 = 13.82; P < 0.001), and fleshy fruit (X2 = 9.18; P = 0.003). Analyses of genera, yielded similar results except there was no association with woody growth. Our results agree with those of other published works on regional floras and suggest that dioecy is heavily driven by 1) woody growth (i.e., the long-lived, perennial growth form), 2) flower size (i.e., tiny, inconspicuous, flowers possibly pollinated by generalized, non-specific pollinators), 3) fruit and seed characteristics (i.e., fleshy, single-seeded fruits that are often dispersed by specialized frugivores, resulting in “high quality” dispersal).