When good species have porous boundaries: weak reproductive isolation and extensive gene flow between Mimulus glaucescens and M. guttatus in northern California

Barriers to reproduction are often how progress in speciation is measured. Nonetheless, a key unresolved question is the extent to which reproductive barriers diminish gene flow in incipient species in nature. The Sierra Nevada foothill endemic Mimulus glaucescens and the widespread M. guttatus are considered to be distinct species based on contrasting vegetative traits, but barriers to reproduction are not readily apparent, although these species are not known to hybridize in nature. To explore boundaries between taxa, we examined 15 potential reproductive barriers between species in a Northern California area of broad sympatry. Most barriers, with the exception of ecogeographic isolation, were weak, and total isolation for each species was estimated to be incomplete. Population genomic analyses of range-wide and broadly sympatric accessions revealed that gene flow between these taxa is common across the range, and rampant within areas of sympatry. Thus, despite fairly strong ecological differentiation - which may be involved in maintenance of vegetative differences - ecological isolation is a weak barrier to gene flow in this system. This work underscores the value of combining classical measures of reproductive isolation with estimates of natural gene flow for studies of speciation in natural communities.