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Afrikaanse Inligting
Species and Hybrids - Why so Confusing
Both Ld coniferum and Ld xanthoconus grow on the
Karbonkelberg. Whereas it is clearly Ld coniferum at the base and clearly Ld
xanthoconus at the top, the middle slopes have horrible intermediates.
Why do botanists insist on putting plants into boxes? Does taxonomy allow for a
non-distinct transition from one species to another? Why call it a hybrid? Why not a
different species? Is it not a cop-out to 'solve' the problem by calling it a hybrid?
Surely the answer lies in DNA analysis? Could the PAP encourage Botany Honours or MSc
students to tackle such a problem?
Nigel Forshaw, Oakridge
The problem with the brain is that it cannot know reality, it can just interpret what it perceives and try and make sense of it all. Animals and plants and evolution do not perform for our edification. In attempting to understand them, we will always be embarrassed by lack of data, poor understanding and the ingenuity and diversity of strategies of successful survival.
Populations are the units of evolution, not the individual, nor the species. Some populations are large and good mixing of genes, but most species consist of small populations that are isolated to some degree from other populations. The vast majority of species consist of these "meta-populations" and present no problems to taxonomists or "naming scientists." Two problems do exist, however. What does one do when two populations that look the same have never interbred for millions of years. Are these two the same species, or not? Typically for two populations to become genetically incompatible they must be sufficiently different for the cross to be less viable than the parents, and their distributions must overlap so that interbreeding is possible. Only under such circumstances can separate species evolve. (Even if offspring are impossible, a means of distinguishing "us" from "them" must evolve – this can happen by chance or by failed breeding). In some cases, only the populations in the contact zone develop the us/them sterility – populations outside of the contact zone may not develop the barrier, but still happily breed with the "us/them" populations of their own species.
If the hybrids are viable and they are not unfavourably adapted to the environment, then the two populations may simply merge into a single metapopulation (or intermediate species). Where hybrids are fitter in the environment than their parents, then hybrid swarms may erupt, swallowing the parent populations with a new metapopulation (or new combined species). If the hybrids evolve an us/them barrier, then we may have three species, with the new "hybrid" species outcompeting the original parent species.
Some populations can simply create themselves a new "us" identity, and thus become a separate species on some mate-recognition whim (fruit flies and Birds of Paradise may be examples). However, in some populations (Pr longifolia is a potential example) the "us" identity is sloppy so that it hybridizes with other species and absorbs the genes of distinct species. It is surmised that the interacting Protea species do not absorb the hybrid genes – they have an efficient "us/them" recognition system - and thus maintain their unique genetic and evolutionary integrity.
Over and above these complications is the fact that the isolated metapopulations (let’s just call them species) may be very old, or may have separated just "yesterday". Also some species may look identical to one another ("a" may differ from "b" in a myriad biochemical and physiological ways, none visible), whereas others of the same age and genetic isolatedness may look very different to the human eye.
So there you are. There is no such thing as a pure species or a perfect hybrid. Both are constructs to try and understand and communicate about the real world. Should we construct an infinity of terms to explain the possible interactions between metapopulations over time and their possible outcomes? But will we be any better able to understand or comprehend them? One thing is certain, new analytical techniques, such as "DNA analysis" will reveal new twists and unexpected complications. For instance, viruses might transfer genes between species. Or genes used as clocks to determine the age of species may run at different speeds in different lineages. So much for being a panacea for solving the problem!
If you have some ideas for revolutionizing the conceptialization of the species paradigm, then let’s hear them. It is about time that we had a worthy successor to Darwin.
Tony Rebelo
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