However, Mallet found no evidence of this with H. himera and H. erato . Individuals living in the pure populations, outside the overlap zone, still show a strong mate preference for their own species. If reinforcement were a factor, one would expect these individuals to have less reservations about crossing with the other species because, not having encountered them before, they would not have been under pressure to evolve a mechanism to prevent crossing. Populations of another pair of butterfly species, H. melpomene and H. cydno have been studied in collaboration with scientists at the University de los Andes in Bogota, Colombia, and the Smithsonian Tropical Research Institute in Panama. These species overlap extensively, though not completely, across a large part of South and Central America.
Laboratory studies show that the hybrid females are sterile.
But hybrid males are fertile and can backcross, and this has been observed in the wild. The tools of gene technology are making it possible for the first time to measure the transfer of individual genes during cross species hybridisation and to relate this to fitness. This opens up the possibility of being able to make much better assessments of the potential ecological and evolutionary impact of gene flow, for example, from a genetically modified organism, or between pest species. Both pairs of Heliconius species, described above, differ in their colour patterns. These patterns fit them for survival in specific ecological niches, for example by protecting them against predatory birds.