To put this into a different perspective:
Most animal species have a genetic barrier to inter-species breeding. Some that are very close (horse x donkey = mule) can produce off-spring, but the hybrids are usually sterile. There are some cat breeding programs, using various wild species (and also lion x tiger). While their off-spring are not sterile, they are 'low fertility'.
Orchids do not breed the way animals do. They rely on a pollinator, whether it be an insect, bird, reptile or whatever. Since the flowers have evolved together with their species specific pollinator, there has been less of a need to evolve genetic barriers to cross pollination with other species. As a result, breeding is possible within broad groups of orchids such as:
Abt 1000 Vandaceous species
Abt 1000 Cattleya family species
From time to time we run into hybrids, which are infertile, but more often than not it is because the plant is triploid (3N) and not 2N (std) or 4N (rarer, but normally fertile).
In some cases the infertility can be traced to incompatible number of chromosomes. For example, the 2N number in Paphiopedilum varies considerably from species to species.
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