From: "Jay Norris" >How do species concepts deal with the inclusion/exclusion >of natural hybrids?
Dear Jay, Thanks for joining the thread. Natural hybrids are indeed a source of great interest. One must remember that the "natural" here refers to the fact that they are not man-made, and not to the fact that they are the normal occurence. The definitions of species that I use come from Albert Radford, they are not mine (I am not that bright). For the "biological species", in the "genetically related" part, it is implicit that such plants would be reasonably homozygous, and on further breeding would give rise to plants similar to those from which they arose. This would not be the case with a primary natural hybrid, except under very special circumstances. Natural hybrids - if their hybridity could be determined or infered - would of necessity need to be placed in a separate category when reviewing a genus, and not be included for placement with one of the various species that were being delimited. The "barriers" to be considered are of many types, and in the orchids, fertility barriers are the least important. Simply because one orchid will breed with another and make fertile hybrids does not make them part of the same species. The most important barriers to gene exchange here are relevant flowering times, geographic separation, altitudinal differences, pollinator differences and a host of other biological differences. None of these barriers are absolute and in the few instances when they break down, natural hybrids may be the result. It is interesting to consider the possible fates of natural hybrids out in the wild. Such a plant may have growth, flowering or fertility problems, leading to it's death before it could pass on its genetic material. A natural hybrid may exhibit sufficient features of one or other of its parents, and land up breeding back to this parent, adding its genetic material and hence the genetic material of another species into the gene pool of the dominant parent - this is known as introgression. A natural hybrid may exhibit novel characteristics; attracting for example a new pollinator, make seed with others of its type, and together with those progeny produced which exhibit suitable characteristics will slowly be incorporated into what is becoming a new gene pool which will ultimately cause the rise of a new species. This is probably the case with Phalaenopsis intermedia. Occasionally, when the genomes of the two parents of a natural hybrid are sufficiently dissimilar, there is total infertility; which is fully reversable should the rare event of chromosome doubling occur - forming an amphidiploid - a fertile plant capable of offering the full double genome to all its progeny. By this mechanism, in one generation one gets a plant that breeds just like a species. As an example of this, I think that Cymbidium borneense is an amphidiploid hybrid between Cym. atropurpureum and Cym. ensifolium. There is a man-made hybrid: Cym. Clare Natasha of the same parentage, and I wonder whether any of our members have a plant and a picture of this for me to look at? Regards, greig russell Kommetjie
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