A multi-disciplinary approach, including phylogenetic analysis, population biology, and quantitative genetics, has helped to elucidate the selective factors that have promoted speciation and shifts in breeding systems in Schiedea (Caryophyllaceae). Schiedea is the fifth largest lineage in the native Hawaiian flora and the most diverse lineage with respect to breeding systems. The genus is monophyletic and shares a common ancestor with a clade consisting of two arctic or boreal-north temperate species. Most inter-island colonizations were from older to younger islands, and most movement between islands led to sufficient isolation to result in formation of new species that are single-island endemics rather than species with multi-island distributions. Closely related species pairs occurring on older islands tend to differ in habitat and are isolated ecologically on the same island, while species pairs on younger islands tend to be in similar habitat on different islands. Speciation within this lineage has been associated with shifts in habitat, pollination system, and breeding system, including evolution of selfing (obligate autogamy, facultative autogamy), mixed mating systems, and dimorphism (gynodioecy, subdioecy, and dioecy). Dimorphic breeding systems appear to have been derived independently twice in Schiedea, and facultative autogamy and obligate autogamy have both evolved three times. The colonization of windy, dry habitats appears to occur before changes in sex allocation patterns, and the evolution of dimorphism in this lineage has been promoted by the combination of high inbreeding depression and high selfing rates. Many morphological traits associated with allocation to male and female function are highly heritable, and genetic correlations in general do not appear to constrain the evolution of dimorphism in Schiedea.