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Thread: Drosera dielsiana ?

  1. #9

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    Hi William,

    What do you mean with 'genetic purity' ? In my mind, a species should always be true from seeds. If not, it is a hybrid and can't be a species.

    Christian

  2. #10

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    Christian,

    If a plant species can hybridize with another plant species in the same geographical locality and form fertile hybrids, and there are no geographical barriers to isolate them, then there will always be intermediate forms between the two species.

    In the case of D. dielsiana, D. natalensis, D. aliciae, D. cuneifolia, D. nidiformis, D. burkeana, D. madagascariensis, D. capensis and other members of the South African Drosera this is exactly the case.

    The continental migration of Africa to the North has resulted in a concentration of these species in the Cape area in South Africa.

    In such a scenario, the term species is a verb, not a noun. At one time, members of the D. natalensis complex may have become stable through geographic isolation, allowing populations to become stable and uniform enough to warrant species seggregation as is the case with the genus Sarracenia. If the sarraceniae had all been forced into the same range millions of years ago, do you think we would be able to now find pure representatives? It is the geographic isolation that is essential.

    With the passing of time isolated populations can evolve (speciate) to the point where they are no longer capable of backcrossing with their original parents. This can result from changes in flower structure, specific pollinators, a difference in flowering times. In my opinion, these members should then be considered as distinct species. Such is not the case with some of the South African Drosera, at least not now in Southeast Africa.

    South Africa is a "melting pot" and has been so for millions of years. Even the species we regard as "true" species may in fact be hybrids which became more successful than their parents and won the niche once occupied by them in the distant past.

    The majority of the individuals in South Africa may be placed into the distinct taxa as stated above, but there are always going to be intermediate stations where the ranges overlap.

    D. dielsiana may actually be an intermediate that arose where the range of D. burkeana and D. natalensis overlapped. As such, the plant carries the genetic expressions of both of these populations, and the phenotypical expression of individual members of this species will express these traits.

    Where D. dielsiana currently overlaps the range of other of the South African Drosera, the process of forming hybridogenic intermediaries is continuing, producing new variations, many of which authors have tried to publish as species novae.

    Keeping all this in mind, how then can one begin to seek a purely representative form? Even the holotype is one member selected from a vast range of many differently appearing plants and cannot reflect the range of this variation. It is for this reason I have been cautioned not to place any ultimate reliance on taxanomic keys (e.g. Obermeyer)

    In the field, the most reliable determinator for D. dielsiana is seed morphometrics. Seed of D. dielsiana is oval, and much shorter than seed of D. natalensis or D. aliciae (which are identical). Shorter seed is more likely to be hybridogenic in origin, but be aware that under cultivation seed length varies wildly, and hence is not a reliable determinator!

    However, there is no single characteristic mentioned in the keys that is more completely significant than any other, and very few plants actually conform in all regards to the published descriptions, even when observed in habitat, and certainly not under cultivation!

    In cultivation, the identification of individuals seggregated from their populations will always be a matter of personal opinion. There is no expert that can answer those questions.

    For these reasons, I feel an open mind is the best plan, along with the free usage of "aff." (affiliated with) where there is some question of possible introgression, as in Drosera aff. dielsiana, as with the other South African taxa sharing identical karyotypes, contiguous stations, and possible hybridogenic orgins (even if regarded currently as "true" species).
    "Grow More, Share More"

  3. #11

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    Hi,

    Thanks for the answer William. It helped me a lot to understand the problem with these plants!

    It's absolutely clear, that there are always intermediates where two or more plants grow side by side. This will result in complex hybrids and lots of (most probably only slightly) different looking plants. This makes clear, why a word like species can't actually be used in this context. It also is clear to me, that the holotyp resembles only one plant of this population.

    What sense does it make to publish descriptions of new plants, when it is not clear if they are true from seeds? It don't see any sense in it. The genetic purity is, what makes out a species for me. I still have problems to accept, that a 'published' species might not be true from seeds. I think, before publishing new descriptions, at least this point should be cleared.

    Now i'm more confused than ever. What i will do is to use the aff. for every plant i have. I don't think anymore, that a positive determination of these plants is possible for me.

    best wishes
    Christian

  4. #12

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    Christian,

    I can sympathize with your confusion!

    Understand that I am not saying there are no examples of "true" species. There are valid grounds for assigning species rank to many of the South African taxa. The problems begin when individuals become seggregated from their populations. Once this happens, there is no way to be certain it has no hybridogenic involvement.

    Most of the forms I grow do come true from seed: that is, they produce identical plants in the F2 generation. Hybrids may become stable over time: this is all part of the process of speciation.

    The problems with introgression are what lead Dr. Schlauer to be very reluctant to grant species seggregation to species novae publications within the South African taxa. Paul Debbert has repeatedly tried to publish new species from South Africa, but these attempts have usually been discounted, and this is the reason why.

    Some have argued that gene studies will resolve the issue, but it must be remembered that phenotype is not dependent on the presence or absence of a single gene, but rather from the interplay of many genes. Studies involving L-ribosomal DNA markers are useful, but not definitive.

    Once I thought it would be a simple matter to photograph and record the some 130 Drosera species out there, but I soon came to realize the complexity of such an undertaking!

    There are no real answers, only opinion when it comes to these plants. I have grown CP on and off all my life, and I am very familiar with the genus Drosera under cultivation. Only after talking with Robert Gibson and Jan Schlauer have I become aware of the great difference between cultivated plants and material from the field. Herbarium and field studies go a long way toward understanding the dynamics of these populations. After leafing throunf many herbarium sheets, understanding becomes more defined, but IMO never absolute. Taxonomy is a very artificial system designed to make sense of a process that our senses can never apprehend. With the South African taxa, it will always be a best guess, no matter if it is Dr. Schlauer, Robert Gibson, you, or me. I think you have a good grasp of Drosera, and I think you can trust your own impressions regarding these plants :-)

    I will be happy to provide you with my personal opinions regarding your plants, but be aware that they are personal preferences based on a very limited experience of what is really out there.

    The good news is many Drosera species are better behaved than the South African taxa, and are distinct species with differing karyotypes, and incapable of producing fertile seed.
    (at least until Ivan Snyder came along, LOL>)
    "Grow More, Share More"

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