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KNOWING WHAT WE HAVE: THE EVER-CHANGING INVENTORY CLIVE STACE Introduction Now and again I am asked by some foreign botanist who is attempting a comparative floristic survey, or by a British non-taxonomist who is interested in genetic diversity, the number of species of vascular plant in the Britain. It is not easy to provide an accurate answer. Firstly, “the number of species” needs close definition. Are only natives to be counted; if so, what about the many doubtfully native species; if not, how well established should the aliens be?

What area are we talking about – the British Isles, the United Kingdom, Great Britain and Ireland, Great Britain, all different entities with different numbers of species? Are agamospecies to be included; if not, are only Rubus, Hieracium and Taraxacum to be excluded, or all agamospecies, and how do we compensate for genera like the ‘big three’, which can’t be omitted completely? Secondly, there are numerous cases where specific limits are disputed, like Polypodium, Euphrasia and Arctium. And thirdly, whatever our criteria, the numbers are constantly changing because research, ranging from field-work to biosystematic and molecular investigations, is leading to the discovery of new taxa and to changes in our taxonomic concepts, and because extinctions are taking place.

Over the past century the number of species and subspecies recognised in the British Isles has steadily risen, as shown in Table 1. As can be seen, this rise is true of native as well as of alien taxa and, although most of the rise in native taxa is due to the increasing number of agamospecies recognised, there is also an increase in the number of native sexual taxa. There have in fact been approaching 200 extra such taxa added to the list over the last 100 years.

These figures have been obtained by me by means of somewhat subjective assessments and decisions, and someone else would produce slightly different numbers, but they would not vary much, and the relative figures and the trends are indisputable. This paper seeks to analyse the reasons for these changes in our standard list. It is concerned with only Great Britain (England, Wales and Scotland), and mainly with native taxa.

The range of situations giving rise the changes outlined above are summarised in Table 2. Taxa can be deleted because of extinctions or earlier misidentifications; taxa added due to their arrival over here, their discovery here, or their new origin here; and changes to the names used for these plants are caused by either nomenclatural or taxonomic decisions.

Deletions Extinctions – The current British native flora must be seen very largely as a result of its immigration from the Continent of Europe over the past 10,000 years or so since the last glaciation. The first plants to arrive after glaciation, or which were already here in the Late-glacial, were highly cold-tolerant, and over the millennia they retreated northwards because of their inability to withstand both the warmer conditions in the south and the increased

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Table 1. Numbers of species and subspecies in the flora of Great Britain according to various sources.

Sexual spp.* Agamospp.* Total Native Naturalized Aliens Subtotal Casuals Total

Druce 1908

Druce 1928

Dandy 1958

1511 254 1765 233 1998 940 2938

1615 492 2107 399 2506 1706 4212

1630 611 2241 654 2895 —

Kent 1992 (+Clement†)

Kent & Stace 2000 (+Clement†)

1686 762 2448 1097 3545 c. 3038 6583

1698 806 2504 1274 3778 c. 3138 6916

*Numbers of agamospecies in the second row refer to those in the genera Hieracium, Rubus and Taraxacum only; those in other genera such as Alchemilla and Sorbus are included in the first row (“sexual spp.”). †

Reference here to ‘Clement’ implies Clement & Foster (1994) and Clement et al. (1996).

Table 2. Summary of processes causing alterations to the standard list of species in the British flora.

DELETIONS

ADDITIONS

CHANGES

Extinctions Misidentifications

Arrivals Discoveries Novelties

Nomenclatural Taxonomic

competition there from more and more immigrants. Those first inhabitants are now confined to the coldest parts of Britain, on mountains or in the extreme north, and in some cases they have disappeared altogether. We know about this last group because of the painstaking analytical work on peat and sediments revealing the identity of the plants that left their pollen, seeds and other remains in Late- and Post-glacial deposits (Godwin, 1975). Their identification can be uncertain if the remains are fragmentary or when several different species are difficult to distinguish. There is, however, reasonably good evidence for the existence in what is now Britain of the following now extinct species at the end of the last (Weichsel) glacial period, i.e. in the Lateglacial period that ended about 10,000 years ago: Arenaria biflora, Arenaria ciliata, Ephedra distachya, Papaver alpinum/radicatum agg., Ranunculus aconitifolius, Ranunculus hyperboreus and Salix polaris. Although these are true natural extinctions, we should exclude most of them from our lists if we consider the modern era to date from the beginning of the Post-glacial. The only one of those species that is known to have remained until the Post-glacial

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is Ephedra distachya, which today still occurs on the French Atlantic dunes as far north as Finistère in Brittany. Ephedra is also exceptional among the above in that it is the only one that is not exclusively alpine or arctic-alpine today, and the possibility remains that its Post-glacial records, although quite numerous, are the result of wind-blown contamination. Perhaps, therefore, there are no species that we know to have been present in the Post-glacial that did not persist until historical times. Nevertheless, there are many examples of species that were widespread in Britain in the early Post-glacial and which are now well on the way to extinction here, such as Dryas octopetala and Betula nana, which were present in the London area after glaciation but which are now confined to the north of the country.

When it comes to historical extinctions we are on more certain ground (Table 3). Some of the species listed in Table 3 can be seen as extensions of the process of northwards migration in the face of increasing warmth referred to in the previous paragraph, e.g. Pinguicula alpina, Rubus arcticus, Trichophorum alpinum. Again, there are others on the same pathway: e.g.

Scheuchzeria palustris, formerly known in southern and central England but now confined to one area of Scotland; Lycopodium annotinum, gone from North Wales, Yorkshire and southern Scotland; and Meum athamanticum, Table 3. Native species known to have become extinct in Great Britain in historical times. Critical species in the genera Hieracium, Rubus and Taraxacum are omitted.

Species

Date

Vice-county

Trichophorum alpinum Carex davalliana Rubus arcticus Carex trinervis Cystopteris alpina Dryopteris remota Tephroseris palustris Pinguicula alpina Holosteum umbellatum Otanthus maritimus Euphorbia peplis Filago gallica Spiranthes aestivalis Bupleurum falcatum Arnoseris minima Bromus interruptus Schoenoplectus pungens Galeopsis segetum Saxifraga rosacea Crepis foetida Luzula pallidula

1804 1831 1841 1869 1881 1894 1899 1912 1930 1936 1949 1955 1959 1962 1971 1972 1972 1975 1978 1980 1992

Angus North Somerset East Perthshire East Norfolk Mid-west Yorkshire Dunbartonshire East Norfolk East Ross-shire Surrey West Cornwall (Scilly) East Cornwall North Essex South Hampshire South Essex Oxfordshire Cambridgeshire South Lancashire Caernarvonshire Caernarvonshire East Kent Huntingdonshire

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gone from the southern and central Pennines. All these are natural or mainly natural diminutions, and in my own opinion should not be conservation priorities. Their preservation amounts to gardening and I would prefer to let nature take its course; extinction is certainly a major and natural part of evolution, and is therefore instructive. If climate warming is a semi-permanent feature of this country, or even if it is a long-term upward trend in an oscillation, more losses will be sustained, particularly if our mountains lose their winter snow-cover or semi-permanent winter frost. Table 3 is as complete a list of historical extinctions as I have been able to compile. The earliest is Trichophorum alpinum in 1804, but it is surely almost inconceivable that there were not other extinctions in say the neolithic or medieval periods of which we have no record. It is likely that the natural process of extinction has been continual from the end of glaciation until the present time.

Others, the majority, in Table 3 are extinctions due to loss of habitat, mostly but not entirely caused by man, particularly the draining of wetlands, the felling of forests and the cultivation of heathland and grassland. Once again, one could easily compile a long supplementary list of such species showing great reductions in their ranges. Species preservation by habitat conservation is more logical for this category. Even so, and perhaps surprisingly, the total number of known native extinctions from Britain is only twenty-one.

In addition to all the above there are very numerous alien species that became established for a considerable time but eventually died out. Wellknown examples that were listed by Dandy (1958) are Ajuga genevensis, Armeria pseudoarmeria, Astragalus boeticus, Crocus sativus, Dorycnium pentaphyllum (D. gracile), Euphorbia pilosa, Inula britannica, Myriophyllum heterophyllum, Najas graminea, Orobanche ramosa, Rosa sempervirens, Scutellaria hastifolia and Vicia hybrida.

Misidentifications – A second group of taxa that can become deleted from the British list are those that have never been here at all, but were erroneously recorded because of misidentification. Asplenium cuneifolium, a diploid member of the A. adiantum-nigrum group found on serpentine soils in much of central and southern Europe, was recorded from Scotland (Roberts & Stirling 1974) and later from Cornwall, but the plants were soon shown to be unusual variants of the tetraploid A. adiantum-nigrum (Sleep, 1980). Koeleria glauca, a diploid species found in much of central and northern Europe, has often been recorded for Britain in recent decades (e.g. Humphries, 1980; Tutin, 1987; Sell & Murrell, 1996), but no confirmed records exist and all investigated plants in Britain have proved to be tetraploids. Festuca glauca, a commonly cultivated ‘blue’ fescue, figured in several British lists and Floras (e.g. Tutin, 1952; Hubbard, 1954; Dandy, 1958), but this species is now known to be confined in the wild to a small area of southern France; the British plants belong to F. longifolia. Several aliens belong to this category as well; species such as Cerastium biebersteinii, Leucanthemum maximum, Papaver lateritium, Phytolacca americana and Silene conoidea were claimed for the British list only on the basis of misidentifications.

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Additions Arrivals – As stated above, virtually all our present flora is the result of postglacial immigration, and the history of this influx is now well established. The accuracy and the length of any list of arrivals depends upon the date from which we begin our records. With regard to trees, for example, it is well known that Betula and Pinus preceded most of the forest genera, later came Quercus and Ulmus, later still Alnus, Tilia and Fraxinus, and last of all Fagus and Carpinus. Fagus did not become common until about 2,500 years ago.

The routes are fairly well understood too, showing that the old idea that everything came across on the Calais-Dover route, just because that is the narrowest stretch of sea now, is incorrect. Rose (1972) concluded that the Straits of Dover had not provided the final barrier to natural immigration, because there is no large assemblage of species queueing up at Calais waiting for transport. He suggested that one of the main routes could have been along the Somme Valley-South Downs axis; several southern species absent from East Kent are shared between these two regions (e.g. Bupleurum baldense, Phyteuma orbiculare, P. spicatum, Seseli libanotis and Teucrium chamaedrys), and there are more species ‘queueing’ in that part of France (e.g.

Anthericum ramosum, Digitalis lutea, Globularia punctata, Seseli montanum and Teucrium montanum). The similarity between the East Kent and Pas de Calais floras is exaggerated by the contribution of Orobanche and Orchidaceae, which have very light seeds that could have been blown across in relatively recent times after the English Channel was formed. Hence it is possible that some species in our flora, both trees and herbs, could have arrived here in only slightly pre-historical times. As a second example, recent work on oaks has shown that Quercus robur entered this country by at least two different routes, one eastern and one western, and each from a different southern European source or refugium (Ferris et al., 1995).

As with the extinctions, however, we can be more sure of the facts if we concentrate on historical times, although in this case there are no certain examples of species whose natural arrival here has been witnessed by man.

Some very recent discoveries have been claimed as natural immigrations, notably orchids in the genera Serapias and Ophrys (light seeds could have been blown here), and various other plants such as Pancratium maritimum, which is now established on shingle in South Devon (perhaps sea-borne seeds), but modern transport systems are so effective that their true mode of arrival might never be known. The above examples are, moreover, ‘desirable’ plants common in the Continental haunts of British field botanists! Molecular work might determine the precise sources in southern Europe of these new colonists, and hence throw light on the chances that they arrived here unaided by man.

In the case of alien plants, however, we can be sure that all the 4,400 or so taxa in the right-hand column of Table 1 fall into the historical ‘arrival’ category. The success of these varies from the transient appearance of species such as Phoenix dactylifera (date) and Arachis hypogaea (peanut) briefly thriving on the warm substratum of a council rubbish dump, or Poa flabellata, surviving on a few old walls in Fladdabister, Shetland, to major success stories such as those of Acer pseudoplatanus (forming a new habitat), Fallopia

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Table 4. Native species discovered in Great Britain in the 20th century.

Dates given are of discovery in a presumed native site, not necessarily of identification. Critical species in the genera Alchemilla, Euphrasia, Hieracium, Limonium, Rubus, Sorbus and Taraxacum are omitted.

Species

Date

Vice-County

Vulpia unilateralis Fumaria reuteri Luzula pallidula Schoenoplectus pungens Scorzonera humilis Hydrilla verticillata Centaurium scilloides Isoetes histrix Myosotis stolonifera Alisma gramineum Crassula aquatica Carex microglochin Galium constrictum Koenigia islandica Rumex aquaticus Calamagrostis purpurea Potamogeton epihydrus Teucrium chamaedrys Eleocharis austriaca Ophioglossum lusitanicum Artemisia norvegica Diapensia lapponica Trifolium occidentale Pilosella flagellaris subsp. bicapitata Gagea bohemica Dactylorhiza lapponica Epipactis youngiana Crepis praemorsa Utricularia stygia Callitriche palustris

1903 1904 1907 1909 1914 1914 1918 1919 1919 1920 1921 1923 1924 1934 1935 1940 1943 1945 1947 1950 1950 1951 1957 1962 1965 1967 1976 1988 1988 2000

South Lincolnshire West Cornwall Huntingdonshire South Lancashire Dorset Westmorland Pembrokeshire West Cornwall Westmorland Worcestershire Mid-west Yorkshire Mid Perthshire South Hampshire North Ebudes Dunbartonshire Angus Outer Hebrides East Sussex Mid-west Yorkshire West Cornwall (Scilly) West Ross-shire Westerness West Cornwall Shetland Radnorshire Kintyre South Northumberland Westmorland Westerness Dunbartonshire

japonica (becoming a major pest), Linaria purpurea (hybridizing with a native species and providing food for the rare Red Data moth Toadflax Brocade), and Senecio squalidus (hybridising with native species and forming a new amphidiploid species with one of them). There are many others likely to add to these examples in the near future, such as the South African Senecio inaequidens, which has now crossed the Channel from Calais and is beginning to colonise southern England, and the South American Conyza sumatrensis, which might have had a similar origin and is behaving similarly over here. The influx of alien species introduced by man, either deliberately for his many uses or accidentally as contaminants, is certain to continue for the foreseeable future.

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Discoveries – The discovery of new native species always arouses much interest, especially in a country like Britain which is supposed to be so well investigated. Despite this, native species that have been hitherto overlooked are still being found, as shown in Table 4, which is limited to discoveries of the last 100 years; probably there are others that I have overlooked. These are plants that were part of our flora for several or many millennia, but due to their rarity remained undiscovered until the dates shown. In addition to the latter there are several notable cases of rediscovery – species that were believed extinct but after a considerable time were refound, often in different localities, and so readmitted to the British list. For example Senecio paludosus was last seen in its classical sites in 1857, but found again in 1972; Atriplex pedunculata became extinct in its known localities after 1938, but was rediscovered in 1987; Gentianella ciliata was found once in 1875, but not refound (in the same place) until 1982; Crassula aquatica was last recorded from Yorkshire in 1938 but discovered in Scotland in 1969; and Hydrilla verticillata became extinct in the Lake District around 1945, but was found in Scotland in 1999. Only the last two of these are listed in Table 4, because their original discovery was not until the 20th Century. Omitted from Table 4 are agamospecies in the genera Alchemilla, Hieracium, Limonium, Rubus, Sorbus and Taraxacum, and microspecies in the genus Euphrasia, which would, if included, actually provide many more examples of new discoveries (as distinct from newly described species already known under some other name).

Comparison of Tables 3 and 4 shows that 14 native species became extinct and 30 native species were discovered in Great Britain during the 20th Century, a surprising fact in the light of the witnessed destruction of our wild flora. However, the comparable figures for the second half of the 20th Century are more depressingly realistic: 10 and 9 respectively. More tellingly still, all but one of the 10 extinctions were from lowland areas of England and Wales, whereas all but one of the 9 discoveries were from less well explored parts of Wales, Scotland or northern England. There is little doubt that further new natives will be found in the future, but equally that discoveries will become much less frequent in an age where transport is no longer a limiting factor for botanists.

There are also several discoveries of species that have been considered, and actually might be, natives, but which are at present thought more likely to be aliens. For example, Spergularia bocconei was discovered in W Cornwall in 1901, and Cerastium brachypetalum in Bedfordshire in 1947; both are more probably alien than native. Some botanists will claim that some of the species in Table 4 are in reality aliens rather than native plants. In addition to the above a large number of hybrids have been discovered in Britain in the last century; in fact about 100 were first found here between 1975 and 2000.

Novelties – Some additions to the British list are the result of new taxa having originated here. Foremost among such plants are interspecific hybrids, of which about 800 different species combinations have been recorded in this country. Many of the newly formed combinations result from hybridization between recently arrived alien species and natives, which had not previously come into contact. These hybrids vary from highly fertile, as in Calystegia and

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Linaria, to highly sterile, as in Senecio and Spartina. In the latter situation the chance doubling of the chromosome number can create in one generation a new highly fertile polyploid which behaves like a diploid, an amphidiploid.

We know that about half of all angiosperms are polyploids, and most or all might have originated in this way, a high proportion in relatively recent (perhaps even post-glacial) times. At least two such new amphidiploids have originated in Britain in the last 120 years: Spartina anglica (from the hybrid between the native S. maritima and the introduced S. alterniflora) around 1890, and Senecio cambrensis (from the hybrid between the native S. vulgaris and the introduced S. squalidus) first noticed in 1948. Novelties can be much less obvious than these two. In the case of Senecio vulgaris × S. squalidus, Centaurium erythraea × C. littorale, and Fallopia japonica × F. sachalinensis, to name but three examples, hybridization has led to a complex pattern of variation, of immense evolutionary interest and potential significance, but until these products of evolution are given names they do not affect our checklists.

Changes Names often come and go in checklists without any change in the plants known to be present here, i.e. the names used for our plants can be altered.

Such changes are of two major types: nomenclatural changes, caused by a more correct application of the rules of nomenclature or by changes in the rules themselves; and taxonomic changes, caused by new research or reexamination of data leading to new ideas on classification (Stace, 2001). These two types of change are not always discrete and are often intertwined, because the correct epithet for a taxon is often different according to its rank or to which higher taxon it is assigned; examples are given in Table 5. But on the whole nomenclatural changes are mandatory, because they are based on the ICBN, whereas taxonomic changes are optional, because they are based on taxonomic opinion.

Nomenclatural changes – It is not necessary here to labour the point that nomenclatural changes to our plants still occur all too frequently. The claim that strict application of the rules of nomenclature will in the end lead to greater stability seems to be based on logic but has yet to be realised. In recent editions of the International Code of Botanical Nomenclature (ICBN) new rules have been introduced whereby well-used names can be conserved, i.e. retained even though the Code would otherwise dictate that they should be altered, and this has certainly helped to avoid some undesirable changes, e.g.

Betula pubescens has been conserved against Betula alba. Examples of the numerous changes to the names of British plants that have occurred in the past ten years are given by Stace (2001).

Taxonomic changes – These can be of three main kinds: amalgamating or splitting taxa; reduction or elevation in rank of a taxon within one genus; or swapping a species from one genus to another, or a genus from one family to another (the latter resulting in a change of systematic sequence rather than a change of name).

Examples of names that have been ‘lost’ from our list, except as synonyms, because of the amalgamation of taxa are Centaurium latifolium and C. capitatum (both now considered variants of C. erythraea; in addition the

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Table 5. Examples of the need to use different epithets for a taxon according to its rank or to which higher taxon it is assigned.

Reynoutria japonica = Polygonum cuspidatum = Fallopia japonica Polypodium cambricum = Polypodium vulgare subsp. serrulatum Polypodium interjectum = Polypodium vulgare subsp. prionodes Festuca richardsonii = Festuca rubra subsp. arctica = Festuca rubra var. alaica former has become extinct), Tamarix anglica (now considered not distinct from T. gallica), Epilobium lamyi (now treated as part of the variation of E. tetragonum), Poa balfourii (now considered a shade growth-form of P. glauca), and Nasturtium (now amalgamated with Rorippa). Conversely, some of the new names that have become part of our list of accepted taxa due to splitting are Brachypodium rupestre (now considered separate from B. pinnatum, and in fact to be the common plant known as Tor-grass), Rorippa palustris (now considered separate from R. islandica, and in fact to be the common ruderal species), Aphanes australis (A. inexspectata, A. microcarpa auct.) (separated from A. arvensis and just one example of the many agamospecies added to our standard list in recent decades), Festuca rubra subsp. scotica, an additional subspecies of F. rubra described in 1991, and Seriphidium (now segregated from Artemisia). As mentioned later, the Nasturtium and Seriphidium examples above are in line for reversion to their former states, just as Chamerion has been split from, re-amalgamated with, and recently re-split from Epilobium.

Examples of taxa that have been elevated or relegated in rank are Arctium nemorosum versus A. minus subsp. nemorosum, Dactylorhiza lapponica versus D. traunsteineri subsp. lapponica, Schoenoplectus tabernaemontani versus S. lacustris subsp. tabernaemontani, Polypodium interjectum versus P. vulgare subsp. prionodes (see Table 5), and Festuca nigrescens versus F. rubra subsp. commutata.

Finally, species that have changed their name because they have been swapped from one genus to another are exemplified by Polygonum cuspidatum/Reynoutria japonica/Fallopia japonica (see Table 5), Artemisia maritima/Seriphidium maritimum, Nasturtium officinale/Rorippa nasturtiumaquaticum, Nardurus maritimus/Vulpia unilateralis, and Senecio integrifolius/ Tephroseris integrifolia.

Name changes are counter-productive in that an unfamiliar name does not convey to the user the sense intended. It takes some time for the biological community to realise that Polygonum cuspidatum and Fallopia japonica are the same thing, or that the common Tor-grass is not Brachypodium pinnatum.

The taxonomist has a duty to keep changes to the necessary minimum but, where the evidence is very strong or unequivocal, changes do have to be made, because the names of plants are not simply handles that could equally well be served by a numbering system but they additionally reflect the position of the

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taxon in the taxonomic hierarchy and hence the relationship of taxa. For any one group of organisms there are frequently competing systems of classification constructed by different researchers. Sometimes some of these are wrong, or less well supported by the evidence, but very often the differences are due to different aims of the classifier being best served by different classifications. In my opinion the over-riding purpose of a taxonomic classification of organisms should be to produce a predictive system, i.e. one that groups organisms together according to the greatest number of characters that they possess in common, so providing the greatest chance that closely classified taxa are also more likely to agree in other, new or so far unstudied characters. This is the most commonly held, although not the universal, view.

But there is less agreement on the most efficient way that such a classification should be achieved. It is probably true that the most frequently used method today is to attempt to produce a phylogenetic classification, i.e. one that reflects the phylogeny or ancestry. Surely, the perfect phylogenetic system is bound to be the most predictive, although of course certain characteristics can vary enormously in a closely related group of taxa (e.g. growth-forms varying from annual weeds to rhizomatous perennials, cactus-like succulents or shrubs and trees in the genus Senecio). (It should perhaps be mentioned here that some taxonomists would aim to produce a phylogenetic classification even if it were shown unequivocally that such a system were not the most predictive).

The popularity of the phylogenetic approach has soared tremendously in the last decade due to the development of cheap and quick ways of sequencing DNA, which is probably the most certain way of deducing ancestries. Because of this I will conclude with some comments on some of the results so far obtained from this technique.

Taxonomic changes suggested by DNA sequencing – Considerable prominence has been given to the results of some systematic DNA sequencing that has suggested classifications differing considerably from the current consensus. In such cases, assuming that the DNA work has been done efficiently, and that the phylogenies produced do truly reflect the ancestry, it is probably the case that the previous classification was not highly predictive, but was unduly influenced by certain conspicuous characters the use of which actually produced a somewhat artificial classification. I do not intend to discuss here the methodology of systematic DNA sequencing, nor how one can ensure that the results of it do represent the true phylogeny, but shall use examples where rigorous research has produced a convincing phylogeny that conflicts with the current classification. (Unfortunately not all DNA research survives such a close scrutiny!).

In order to construct a classification that reflects ancestry from a phylogenetic analysis (whether based on DNA or other evidence), only monophyletic groups (i.e. groups all the members of which are derived from one common ancestor) are recognised as taxa. It must also be borne in mind that these phylogenies do not fix the absolute taxonomic ranks of the taxa, but only their relative ranks. Hence, if two taxa are revealed as being contained within one larger taxon, the former could be considered as two species in the same genus, or as two subspecies or varieties in the same species. Figure 1 is a

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DNA sequence-based phylogeny of certain orchids, reproduced from the work of Pridgeon et al. (1997), in which a series of ancestral relationships is revealed. (Such phylogenies are usually drawn ‘sideways’, like this one, rather than ‘upright’ like most family trees, merely to enable one to write in the names horizontally). All the taxa shown form one monophyletic group (‘family tree’), within which many other monophyletic groups can be found.

The six largest of the latter are delimited by horizontal lines (drawn in by me) and lettered A to F. How this tree or cladogram is interpreted taxonomically is to a considerable, but not unlimited, degree a matter of opinion. Any monophyletic group could be recognised as a distinct taxon. For example, to take two extreme opposing and probably ridiculous views, all the species listed in Figure 1 could be placed in one genus, or every species could be placed in a genus of its own. Almost certainly something between these extremes is desirable; usually the closest to the current system is ideal. Often, in fact, the present classification is found to be fully supported by new DNA phylogenies, so that no changes are implicated. This is, however, not the case with the orchid phylogeny.

In Figure 1 each of the six monophyletic groups (clades) could be recognised as a taxon (e.g. a genus); this would confirm the current concepts of Serapias (group B) and Ophrys (group C). But within each of those two S. lingua and O. insectifera could be split off from the remainder of their respective groups, because each is distinct from the rest and would leave a one-species-smaller group which is still monophyletic. The same is true of Barlia/Himantoglossum, Neotinea/Orchis and Traunsteinera/Aceras/Orchis in groups D, E and F respectively. But in group A it is not possible to split Anacamptis from Orchis because they are not separate from each other (Anacamptis is said to be ‘nested within’ Orchis), so removal of Anacamptis would not leave a monophyletic group. Moreover, it is clear that all the species hitherto placed in Orchis do not form a monophyletic group, but are disposed in three separate groups interspersed with other non-Orchis taxa. Orchis is thus polyphyletic, and cannot be maintained as a single genus on its own unless we are happy to accept obviously artificial taxa of low predictivity. The only logical move is to split the erstwhile Orchis into three groups: one to include Anacamptis; one which is close to Neotinea and could include it; and one which is close to Aceras and Traunsteinera and could include them or just Aceras (but not just Traunsteinera). If we cite only the British species (plus Orchis laxiflora from the Channel Islands and Neotinea maculata from the Isle of Man and Ireland), Pridgeon et al. (1997) chose to include Neotinea maculata with Orchis ustulata (Neotinea the correct generic name), and Aceras anthropophorum with Orchis mascula, O. militaris, O. purpurea and O. simia (Orchis the correct generic name); the inclusion of Anacamptis pyramidalis with Orchis morio and O. laxiflora (Anacamptis is the correct generic name) is uncontentious (Table 6). Their conclusions were based on a consideration of other evidence along with the DNA sequence data, such as pigments, cytology and hybridization. This last point is instructive, because the advent of systematic DNA sequencing does not, as sometimes suggested, herald the end of other systematic research, which will continue to be just as important as it always has been.

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Figure 1: Cladogram of European non-palmate-tubered orchid species based on DNA ITS sequences, taken from Pridgeon et al. (1997), on which the six most distinct subclades (labelled A–F) have been drawn.

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Nevertheless, this radical reclassification of Orchis and relatives is based primarily on DNA sequence evidence, and there is a natural ‘fear’ that such events will become commonplace. The newly defined orchid genera are not easy to recognise on the basis of morphological (such as floral) characters, and if this becomes a frequent situation taxonomy will soon lose its appeal for many biologists and amateurs. There are certainly more cases coming to light where similar studies are leading to taxonomic changes. For example it seems likely that Seriphidium should not have been separated from Artemisia, because the former seems nested within the latter. There is now quite good evidence that Nasturtium should be re-separated from Rorippa, because in their cladogram other genera come between them. Several well-known families, such as Scrophulariaceae, are seen to be in need of a new circumscription. However, the great majority of these changes are not radical, but were implicated before by other more conventional evidence which was perhaps too contradictory or equivocal to warrant taxonomic changes. In many cases the DNA sequence evidence merely confirms one out of two or more conflicting classifications. If one were to predict a family in which the most radical changes would be suggested by molecular evidence it would surely have been the Orchidaceae. This is the largest family in the world (over 20,000 species), and has undergone an enormous and rapid surge of evolution in flower structure connected with pollination syndromes. This primarily affects the floral structure, which is traditionally the set of characters most used in classification. Thus it is not surprising that closely related taxa have evolved very different structures, or that similar structures have been retained or been convergently evolved in less closely related taxa. This is just the situation where new, more objective evidence is likely to provide the most surprising and novel results.

Certainly many more taxonomic changes will alter our standard plant list in the years to come, many of them indicated by the results of DNA sequence analysis, but the signs are that this will most often amount to an improvement, fine-tuning and clarification of our system of classification, not its replacement. One cannot stand in the way of progress just because it requires us to learn a few new names!

Table 6. The new disposition of British species formerly in Orchis in three distinct genera.

GENERA

SPECIES

ANACAMPTIS

Orchis laxiflora

Orchis morio Anacamptis pyramidalis NEOTINEA

Orchis ustulata Neotinea maculata

ORCHIS

Orchis mascula Orchis purpurea Orchis militaris Orchis simia Aceras anthropophorum

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Acknowledgements I am grateful to David Pearman and several B.S.B.I. recorders for help with some of the dates of discoveries and extinctions, and to Ann Conolly for assistance with the literature on Late- and Post-glacial floras.

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Prof. Clive A. Stace Department of Biology, University of Leicester, Leicester LE1 7RH

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