It is therefore clear that it is only the outward appearance of the two species that inhibits hybridization. The authors attribute this to assortative mating, the advantage of which is not clear, and it would lead to the rapid appearance of streams of new lineages, and possibly even species, through mutual attraction between mutants.
The outward appearance of the two species prevents hybridization, as stated by the authors. This is attributed to assortative mating, whose advantages are not clear, and it would result in a swift emergence of new lineages and possibly even species due to mutual attraction between mutants.