RT Journal Article SR Electronic T1 Natural selection maintains species despite widespread hybridization in the desert shrub Encelia JF bioRxiv FD Cold Spring Harbor Laboratory SP 2020.01.23.917021 DO 10.1101/2020.01.23.917021 A1 Christopher D DiVittorio A1 Sonal Singhal A1 Adam B Roddy A1 Felipe Zapata A1 David D Ackerly A1 Bruce G Baldwin A1 Craig R Brodersen A1 Alberto Burquez A1 Paul VA Fine A1 Mayra Padilla Flores A1 Elizabeth Solis A1 Jaime Morales-Villavicencio A1 David Morales-Arce A1 Don W Kyhos YR 2020 UL http://biorxiv.org/content/early/2020/01/24/2020.01.23.917021.abstract AB Natural selection is an important driver of genetic and phenotypic differentiation between species. A powerful way to test the role of natural selection in the formation and maintenance of species is to study species complexes in which potential gene flow is high but realized gene flow is low. For a recent radiation of New World desert shrubs (Encelia: Asteraceae), we use fine-scale geographic sampling and population genomics to determine patterns of gene flow across two hybrid zones formed between two independent pairs of species with parapatric distributions. After finding evidence for extremely strong selection at both hybrid zones, we use a combination of field experiments, high-resolution imaging, and physiological measurements to determine the ecological basis for selection at one of the hybrid zones. Our results identify multiple ecological mechanisms of selection (drought, salinity, herbivory, and burial) that together are sufficient to maintain species boundaries despite high rates of hybridization. Given that multiple pairs of species hybridize at ecologically divergent parapatric boundaries in the adaptive radiation of Encelia, such mechanisms may maintain species boundaries throughout this group.SIGNIFICANCE STATEMENT In Baja California, the deserts meet the coastal dunes in a narrow transition visible even from satellite images. We study two species pairs of desert shrubs (Encelia) that occur across this transition. Although these species can interbreed, they remain distinct. Using a combination of genetics, field experiments, 3D-imaging, and physiological measurements, we show that natural selection counteracts the homogenizing effects of gene exchange. The different habitats of these species create multiple mechanisms of selection - drought, salinity, herbivory, and burial, which together maintain these species in their native habitats and their hybrids in intermediate habitats. This study illustrates how environmental factors influence traits and fitness and how this in turn maintain species, highlighting the importance of natural selection in speciation.