Wednesday, 31 August 2011

Bright eyes

Euphrasia is a genus of hemiparasites in the family Orobanchaceae.  In England they’re called “eyebrights” and, in most of the European species, plants are annual, like this E. nemorosa, introduced in New Zealand.
Euphrasia nemorosa, Waituna, Southland, New Zealand.
Euphrasia are called hemiparasites because they steal only water, but not food, from their hosts.  Their leaves are green, unlike the related broomrapes (Orobanche) which are pale brown holoparasites.  
Our native Euphrasias are mostly alpine.  Many are small perennial shrubs, like this E. cuneata on the Rimutaka Range.
Euphrasia cuneata, Rimutaka Range, New Zealand
Others are annuals that live in alpine boggy sites, like this E. dyeri.
Euprasia dyeri, Lammermoor Range, New Zealand
I have to show you where it was growing, among Oreobolus sedges, which it was probably parasitizing, in a string bog under a typical Otago summer sky.  The mountain range in the distance is the Rock & Pillar Range, and on the other side of it is the Maniototo plain, where Graham Sydney paints landscapes with lenticular clouds just like this one.
String bog in alpine tussock grassland, Lammermoor Range, Otago, New Zealand
Among the trends in the evolution of Euphrasia in New Zealand is a reduction in flower size, possibly associated with self-pollination, but some flowers are larger.  Two odd extremes are reached in E. wettsteiniana which grows at low altitude in Westland.  In flowers of E. wettsteiniana, there are only two seeds, one in each half of the capsule, and the corolla tube of the flower is hugely elongated.  Does this lift the flower above the water level of the swamps, raise it out of the wet grass, or is it a way to control the pollinators' behavior?
Euphrasia wettsteiniana, Kangaroo Lake, Westland, New Zealand.
In the South Island, several species, like E. dyeri and E. repens, vary from one population to another.  E. cuneata is also variable—compare the flowers of this plant on Mt Ruapehu with the Rimutaka plant above. 
Euphrasia cuneata, Mt Ruapehu, New Zealand.
Botanists' views vary about how to handle this sort of variation (and in what follows I’m talking generally and not referring to Euphrasia).  Does it mean several species are involved, or are these just local forms of single but variable species?  This is a question that’s too often not settled by applying the methods of science; rather, botanists sometimes fall back on assertion and authority.  Different appearance isn't sufficient, because there are many explanations for it besides speciation.
In my opinion, these questions need to be tested explicitly.  I think the most powerful approach is to start with the hypothesis that such populations are conspecific (i.e., they belong to the same species).  That’s a hypothesis that can be tested.  Any data that clearly falsify the hypothesis will compel us to reject it, and treat such populations as different species.  But if the evidence isn’t compelling, I advocate waiting for more data before rushing ahead with naming a new species.
What sort of data would compel us to reject such a hypothesis of conspecificity?  The data would have to be evidence that the two populations aren’t able to exchange genes, or that they haven’t exchanged genes for so long that they have diverged genetically.  If they couldn’t be crossed, or if their hybrids were sterile, that would be convincing.  Population genetic analyses might show a lack of gene flow between them.  A number of inherited independent differences between them would also be evidence they don’t cross.  Chemical, morphological, genetic, even behavioural differences can be used.  
Ecological data, such as the soil type or other aspects of the plants' habitats, don't help much with the question of how many species are involved.  Such data may tell us something about the process of speciation, but they're not powerful evidence about whether speciation has actually happened.  Many species can occupy a range of habitats and look very different as well—our own species is one of the best examples.
When different looking populations can’t be shown to be separate species, botanists can still recognise their differences using ranks that are lower than species, like subspecies, variety, and form.

1 comment:

  1. Gorgeous photos and great article, thanks. Am wondering if there is a better description of E wettsteinianna than Allan 1961 p.860?