We examined reproductive isolating barriers at four postmating stages among 11 species from the morphologically diverse genus Nolana (Solanaceae). At least one stage was positively correlated with both genetic and geographic distance between species.Postzygotic isolation was generally stronger and faster evolving than postmating prezygotic isolation. In addition, there was no evidence for mechanical isolation, or for reproductive character displacement in floral traits that can influence pollinator isolation. For sexually reproducing plants and animals, the origin of new species involves the evolution of reproductive isolating barriers between diverging lineages. Studying these isolating barriers therefore provides insight into the process of speciation (Coyne and Orr 2004). Several approaches have been used to examine the evolution of reproductive isolation within the same closely related group of species, including examining the relative strength of isolating barriers operating at different reproductive or developmental stages among different species pairs, across different degrees of evolutionary divergence, and/or among reciprocal crosses (Coyne and Orr 2004, and see below). These approaches aim to evaluate how rapidly barriers accumulate and which isolating barriers contribute most during initial divergence between lineages, among other questions. In combination, they can provide insight into the evolutionary forces and the genetic mechanisms responsible for the evolution of new, reproductively independent, lineages.First, examining the reproductive compatibility of a single species pair at multiple developmental stages (e.g., premating, postmating prezygotic, and postzygotic) can be used to infer which stages are most effective at reducing current gene flow between these species, and therefore which stages might have been more or less influential during their initial divergence (e.g., Ramsey et al. 2003;Kay 2006;Martin and Willis 2007;Mendelson et al. 2007;Maroja et al. 2009;Dopman et al. 2010). For example, based on the estimated contributions of multiple preand postzygotic reproductive barriers between two monkeyflower sister species, Mimulus lewisii and M. cardinalis (Ramsey et al. 2003), factors acting prior to hybridization (specifically ecogeographic isolation and pollinator isolation) were inferred to be the primary isolating barriers in this system. Data such as these can also suggest the evolutionary forces that are most likely responsible for reducing gene flow between species. For example, estimates of pre-and postzygotic barriers between M. guttatus and M. nasutus (Martin and Willis 2007) revealed that prezygotic barriers contributed most to total isolation, likely as a result of adaptive divergence in mating systems (i.e., shift to self-pollination) and edaphic ecology (i.e., drought avoidance via phenological acceleration).