Many flowering plants have the potential to be world travelers via wind, ocean currents, and animals because their seeds are well adapted to surviving the vagar

Biological invasions occur when organisms are transported and become established in a new range in which they persist, proliferate, and spread (1). The negative consequences of invasions include loss of native biological diversity and community structure (and in extreme cases, the extinction of native species) (2); modification of ecosystem processes such as nutrient cycling and productivity patterns; alteration of disturbance regimes, especially the frequency of wildfires (3); reduced agricultural productivity; human health concerns; and enormous economic costs (1, 4). Consequently, invasive species are now considered to be one of the leading contributors to global change (5) and thus have been the focus of an extensive amount of ecological and ecosystem-level research. Much of this research has focused on answering a series of questions associated with predicting invasions: Which species will become invasive? Which life-history traits contribute to invasiveness? Which communities are susceptible to invasion? What will be the ecological and ecosystem-level consequences of invasion? Unfortunately, answers to these questions remain elusive (1). It is surprising to note, however, that little research has focused on the evolutionary aspects of biological invasions and addressed how evolutionary mechanisms may contribute to the success of an invasion (2, 6, 7). In contrast, the research of Lavergne and Molofsky (8) reported in this issue of PNAS examines the factors that contribute to range expansion and the invasiveness of Phalaris arundinacea L. (reed canarygrass) in North America and provides powerful insights into the role of evolution in the invasion process. The invasion process can be viewed as a series of steps that are initiated when propagules of a species (seeds, eggs, larvae, vegetative material, mature individuals, etc.) are sampled in their native range and transported to a new area (1, 4). These immigrants probably experience high mortality rates while in transit, and …

• Premise of the study: Hybridization is an important evolutionary force in the history of angiosperms; however, there are few examples of stabilized species derived through homoploid hybrid speciation. Homoploid hybrid species are generally detected via the presence of genetic additivity of parental markers, novel ecological and spatial distinctions, and novel morphological traits, all of which may aid in the successful establishment of hybrid species from parental types. Speciation and diversification within the genus Castilleja (Orobanchaceae) has been attributed to high levels of hybridization and polyploidy, though currently there are no examples of homoploid hybrid speciation within the genus. We employed multiple lines of evidence to examine a putative hybrid origin in C. christii, a rare endemic, known only from 80 hectares at the summit of Mt. Harrison (Cassia Co., Idaho).• Methods: We used granule‐bound starch synthase II (waxy) sequences and 26 morphological characters to address hybridization between C. christii and widespread congeners C. miniata and/or C. linariifolia in an area of sympatry. Chromosomes of C. christii were also counted for the first time.• Key results: All 230 direct‐sequenced C. christii individuals had the additive genomes of both C. miniata and C. linariifolia. Castilleja christii shares traits with both parents but also has floral characters that are unique and transgressive. Cytological counts indicated that all three taxa are diploid.• Conclusions: We conclude that C. christii is a stabilized homoploid hybrid derivative of C. linariifolia and C. miniata and is likely following an independent evolutionary trajectory from its progenitors.

Many studies have focused on how leaf litter depth affects seed germination and seedling growth because the seedling stage is the most vulnerable portion of a plant’s life cycle. Invasive plants with the most severe ecological consequences are those that modify ecosystems, and this can occur through the formation of thick litter layers which can suppress the emergence, survival, and recruitment of native plant seedlings; in addition, in some cases, these litter layers can suppress invasive plant seedling recruitment. Prosopis juliflora is a thorny shrub that is native to arid and semi-arid portions of North America, parts of South America, and the Caribbean. It has invaded millions of hectares around the world, including Saudi Arabia. The objective of this study is to evaluate whether P. juliflora leaf litter reduces the recruitment of its own seedlings under greenhouse and field conditions in Saudi Arabia. In both the greenhouse and the field, the number of days to first emergence increased and germination percentage decreased with increasing litter depth. With the 1, 2, and 4 cm litter depth treatments, the number of viable seeds generally decreased, with no emergence, germination, or viable seeds detected for the 8 cm litter depth treatment. Results of this study reveal that increasing the depth of P. juliflora leaf litter suppresses the survival and recruitment of its own seedlings. Future search should assess the actual mechanisms through which P. juliflora seeds are suppressed, the role of allelopathic compounds in this process, and whether viable seeds are dormant and will persist in the soil seed bank.