question	potential approach(es)
origin, commonality, and importance of intrinsic postzygotic barriers	1. what are the major evolutionary drivers of intrinsic barriers and how frequent are they in nature?	a. genetically dissect reproductive barriers to determine if genes involved provide information of the evolutionary drivers (such as [50,85,91])
		b. compare the extent of reproductive isolation or diversification rates between groups that are known to differ in proxies for particular evolutionary drivers (e.g. mating system as a proxy for parental conflict and the evolution of hybrid seed inviability, circa [135]
	2. how common are less obvious intrinsic barriers (e.g. deficits in hybrid metabolism, neurology, or general ‘hybrid weakness’)? At what stage are these barriers important?	a. case studies quantifying hybrid defects, and when possible, the amount of selection against hybrids who carry them [e.g. 200]
		b. comparative studies that analyse the relative age of species pairs that produce hybrids with ‘hybrid weakness’ or other transgressive phenotypes
	3. is there interplay between intrinsic barriers and ecology? How important is this for speciation?	a. generate hybrids under multiple biologically realistic environmental conditions and measure viability or sterility [e.g. 41]
		b. assess hybrid sterility of viability in natural populations across an environmental gradient
intrinsic postzygotic barriers and introgression	4. how effective are intrinsic barriers at preventing introgression in nature?	a. map the genetic basis of incompatibility loci in allopatric populations, assess the fate of incompatibility alleles in contact zones (e.g. whether or not they are still present, or assess the steepness of clines across a hybrid zone for incompatibility loci versus neutral loci). Genetic mapping will of course be easiest in model systems in which laboratory crosses are possible. Although not as robust, if researchers are using non-model systems in which laboratory crosses are unattainable, using field collected hybrids for admixture mapping or RNAseq of barrier tissues (e.g. gametes) may be informative. We caution, however, that these natural hybrids may represent a non-random collection of incompatibility alleles
		b. experimentally evolve hybrid swarms between parental species with different types of reproductive barriers. Assess the prevalence of reproductive isolation through time and determine how quickly different types of barriers are lost from hybrid populations (or whether some persist)
		c. simulate genome-wide patterns of introgression for species pairs with differing types of reproductive barriers
	5. what is the relationship between the amount of introgression (or the composition of introgressed alleles) and divergence time?	a. assess rates and timing of introgression in natural or synthetic contact zones from species pairs of differing ages
		b. the same as above, but can use genomes of broadly sympatric taxa rather than explicit contact zones
		c. simulate genome-wide patterns of introgression between pairs of populations with differing divergence
	6. how do different evolutionary drivers that are responsible for the evolution of intrinsic barriers influence the stability of these barriers in the face of gene flow?	a. simulate secondary contact when incompatibilities are driven by different evolutionary mechanisms (e.g. neutrality, local adaptation, systems drift, conflict or other co-evolutionary dynamics), and assess the stability of simulated incompatibility alleles
intrinsic postzygotic barriers and species persistence and diversification	7. how important are intrinsic postzygotic barriers for species coexistence and diversification?	a. compare rates of accumulation of postzygotic intrinsic barriers in allopatry versus sympatry to determine if sympatric taxa are more likely to be strongly isolated (consistent with Templeton effect [136])
		b. simulate secondary contact between populations that are weakly or strongly reproductively isolated with pre- and/or postzygotic reproductive barriers and determine the probability of extinction
		c. determine whether levels of reproductive isolation are correlated with diversification rates [such as 244]