Reproductive isolation and speciation apparently occurred before gametic incompatibility closed off the possibility of gene flow, and as in two sympatic species of asteroids in the genus Patiriella that do not have gamete incompatibility (Byrne and Anderson, 1994), other mechanisms need to be found to explain how species integrity is maintained in these closely related, interfertile
forgetiana are interfertile
species that grow sympatrically and parapatrically throughout southern Brazil, but they form no hybrids in the wild.
silvestris, two well-recognized species of the Hawaiian group, are interfertile
in the lab but are strongly isolated in mate-choice experiments (Kaneshiro 1976; Carson 1978).
Other models of hybrid zone maintenance include a greater role of ecological factors, with hybrids occurring in intermediate habitats along an environmental gradient (Endler 1977), or where a patchy distribution of habitats brings interfertile
species into close contact (mosaic hybrid zones: Rand and Harrison 1989).
Individuals from one habitat type are interfertile
with those from the other.
barbarae isolation via biotype depletion as it is completely interfertile
in artificial crosses with sister taxon (Pepper & Norwood, 2001) Ceanothus roderickii Selection against hybrids in parental habitats and possible genetic incompatibilities (Burge ct al., 2013) Cerastium alpinum Unknown; possibly, selection against hybrids (Ostevik et al., 2012) Collinsia sparsiflora FI hybrid sterility (Moyle ct al., 2012) Deschampsia cespitosa Unknown Geonoma macrostachys Flowering time and pollinators (Borchsenius ct al., 2016) Helianthemum spp.
However, cultivars within each ploidy group are interfertile
and hybridize freely (Nakagawa et al., 1987).
These two strains are completely interfertile
with one another and do not display any premating isolation.
Furthermore, the early ancestors of Cannabis and Humidas may have been interfertile
for a very long time, probably since before the oldest known macrofossils were found near the K-T boundary in North America (see Johnson, 2002; Plate 6:1 for aff.
RS-H is cytologically similar to and interfertile
with the cultivar NewHy (Asay et al., 1991; Jensen and Asay, 1996), which is an advanced-generation hybrid between quackgrass and bluebunch wheatgrass [Pseudoroegneria spicata (Pursh) A.
Furthermore, instances of deep genetic differentiation across zoogeographic boundaries need not reflect speciation: although the tidepool copepod Tigriopus californicus exhibits a minimum of 18% COI divergence across PC (Burton and Lee 1994), populations on either side of PC are interfertile
When one can distinguish three phases: (a) domesticated crop(s), (b) ancestral or at least closely related (at least somewhat interfertile
) wild plants which still have natural distribution ranges, and (c) weedy or ruderal relatives of the crop that interbreed with it, the assemblage is referred to as a "wild-weed-crop complex." When only (a) and (c) can be distinguished, it is simply a "crop-weed complex." Many crops like Cannabis exist in crop-weed complexes (Andersson & de Vicente, 2010), with domesticated forms in cultivation, and related ruderal (weedy) forms growing outside of cultivation.