Biologia 63/6: 881—887, 2008
Section Botany
DOI: 10.2478/s11756-008-0135-7
«Peut-être la desmidiologie future montrera-t-elle les liens qui unissent ces
types morphologiquement un peu différents et se rira-t-elle des vains efforts
faits de nos jours pour arriver à une conception plus rigoureuse de l’espèce.»
Ducellier 1918, p. 99
The species concept in desmids: the problem of variability,
infraspecific taxa and the monothetic species definition*
Frans A.C. Kouwets
RWS – Centre for Water Management, Hydrobiological Laboratory, P.O. Box 17, NL-8200 AA Lelystad, The Netherlands
& IBED, University of Amsterdam, P.O. Box 94062, 1090 GB Amsterdam, The Netherlands; e-mail: [email protected]
Abstract: Desmid systematics are much confused due to the existence of a very large number of infraspecific taxa, obscuring
a clear species concept. The basis of this confusion is the huge morphological variability of desmid cells. Moreover, these
ill-defined forms are frequently recombined back and forth with superficially related species, hence constantly changing
species characterizations. In the present contribution the author will argue in favour of the utmost reserve with respect to
the description of infraspecific taxa in desmids, the strict use of a monothetic species definition and a clear species concept.
Key words: desmids; infraspecific taxa; monothetic definition; species concept; variability
Introduction
Desmids (Zygnematophyceae, Streptophyta) are a wellcharacterized group of unicellular green algae. Most
desmids occur as single cells, but in a few forms the cells
are – sometimes very loosely – united into filamentous
or branched colonies. Desmid cells typically consist of
two semicells, connected by a so-called isthmus, somewhat like an hourglass. At the level of the isthmus, the
cells are more or less deeply constricted, and the constriction (sinus) may be open or closed. However, in a
number of genera such a constriction is entirely lacking.
In cross section, the two semicells may be circular (omniradiate), compressed (bi-radiate) or star-shaped (trito multiradiate), but with the exception of a very few
tropical forms, they generally are each other’s mirror
image. Accordingly, the cell shows a number of planes
of symmetry. Desmid cells may be morphologically simple and smooth-walled, but in most species the cell wall
is to a greater or lesser degree ornamented with a variety of smaller and larger pits, granules, warts, spines,
etc., either distributed evenly over the cell surface or
arranged in definite patterns. Moreover, the semicell
contours may show incisions and extensions (processes)
of all sorts and shapes. Most importantly, these morphological characters all show a certain degree of variability. Desmid cells roughly measure between 10 and
1000 µm.
The usual way of reproduction in desmids is by
vegetative cell division. After the division of the nucleus
and the formation of a cross-wall, the cell splits at the
level of the isthmus and each of the two parental semicells produces a new complementary daughter semicell.
Changes in growth conditions may result in smaller
or larger morphological differences between parental
and daughter semicells, disturbing longitudinal symmetry. These differences may concern cell wall ornamentation, but also general outline and radiation.
Sexual reproduction with the formation of zygospores
is only occasionally observed and in many species
it is even completely unknown. Well-known genera
are Cylindrocystis, Closterium, Actinotaenium, Pleurotaenium (omniradiate), Euastrum, Micrasterias, Cosmarium, Xanthidium (bi-radiate), Staurastrum, Staurodesmus (occasionally bi- but generally tri- multiradiate) and Hyalotheca (pseudo-omniradiate, cells united
into filamentous colonies). For more detailed information see e.g. Brook (1981).
Desmids occur exclusively in freshwater habitats,
with a clear preference for oligo- to mesotrophic,
slightly acidic environments. In general, the largest
species diversity can be found in shallow, mesotrophic
lakes with rich stands of submerged macrophytes such
as Myriophyllum, Potamogeton and Utricularia. Rich
communities may also be found amongst Menyanthes,
Carex or mosses such as Sphagnum, Scorpidium or
Drepanocladus. However, one should realize that preferences may vary between temperate and tropical areas, and also between lowland and mountainous regions.
The most recent estimate of the total number of “good”
* Presented at the International Symposium Biology and Taxonomy of Green Algae V, Smolenice, June 26–29, 2007,
Slovakia.
c 2008 Institute of Botany, Slovak Academy of Sciences
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882
F.A.C. Kouwets
From the early days of microscopy researchers were attracted by desmids because of their remarkable cell
morphology (e.g. Corda 1834, 1839, 1840; Ehrenberg
1838). Many new forms were described, compelling the
development of a clear, preferably natural classification
(compare e.g. Agardh 1824; Kützing 1849; Nägeli 1849;
De Bary 1858). However, the species concept itself in
general attracted only little attention.
Pre-Darwinian pioneers of desmid taxonomy, as,
e.g., De Brébisson and Ralfs, who published in the
first half of the 19th century, obviously regarded species
as separate, immutable and not subject to variation.
Their species concept was typological or morphological. Ralfs (1848; the starting-point of desmid nomenclature) didn’t present any discussion of species concepts
in his treatise of “The British Desmidieæ”. Only relatively few varieties were described, sometimes called
“states” of the species: of the large genera Cosmarium
(at that time including Actinotaenium) and Staurastrum (including Staurodesmus), 3 out of 33 and 4 out
of 38 described species, respectively, had varieties. Such
varieties were commonly indicated with “α” for the typical form (the nominate variety) and “β”, “γ”, etc. for
the other forms. Apart from a few exceptional cases, no
formal names were attributed to these varieties. On the
other hand, Ralfs (1848) frequently doubted whether
varieties that differed too much from the typical form
shouldn’t be better described as new species, especially
when he had not had the chance to study these forms
by himself. Variability obviously was mainly thought
to be the occasional result of environmental influences
during growth or reproduction (see e.g. De Bary 1858).
An interesting exception is a short discussion by
Nägeli (1849, p. 129) who had found 3-, 4- and 5-radiate
cells of Phycastrum (now Staurastrum) crenulatum. He
remarked that they should not be classified in different
species or even genera, since he had also found so-called
janus-forms with one 3- and one 4-radiate semi cell (but
nevertheless gave them different names at some unspecified infraspecific level). However, in general he was of
the opinion that a species concept could not yet be established for algae, due to a lack of knowledge (Nägeli
1849, p. 41). Instead he focused on the definition of
genera as a basis for a natural classification.2
In the last few decades of the nineteenth century,
the attitude with respect to morphological variability of
desmids changed. A steadily growing number of forms
were described as new taxa, and microscopists became
increasingly aware that variability is an important factor in desmid taxonomy.
Already in one of his very first papers, Archer
(1860) presented a short but interesting discussion on
the species concept in desmids. He stated that: “ . . . in
regard to what is a species and what is not, it seems to
me that naturalists are prone to err in one of two directions: they either restrict the number of species in their
lists within too narrow limits, or inordinately increase
their number by giving a name and specific rank to
almost every variation which they encounter.” (Archer
l.c. p. 75). In his opinion both extremes might be wrong.
He rightly considered it much easier to describe a new
species, than to demonstrate that two or more familiar forms are but developmental stages of one and the
same species. The variability of two related but distinct
species might overlap, suggesting the transition of one
species into the other, and intermediate forms would
then be only temporary or local variations of one or
both of the species in question. Clearly, such variations
should not be given the status of a true species. Archer
made clear that he did not recognise “varietas” as a separate taxonomic level, and that the identity of a reputed
species could only be established by “. . . tracing the or-
1
This definition stands opposite to the generally applied
polythetic definition, where infraspecific taxa are defined by
characters that distinguish them from the nominate form;
consequently there is no common character set and the description of a species does not change with the inclusion of
a new variety. This monothetic vs. polythetic concept was
developed by Sokal & Sneath (1963) and Sneath & Sokal
(1973).
2
In this respect, many similarities exist with the diatoms
(Bacillariophyceae), another large group of unicellular algae
with a remarkable morphological variability and symmetry.
Not by coincidence, diatoms and desmids were long thought
to be closely related (and belonging to the animal kingdom;
see the interesting discussion on this subject in Ralfs 1848).
An extensive discussion of the species concept in diatoms is
given by Mann (1999).
species known so far amounts to approximately 3000
(Gerrath 1993).
At present the author is working on a volume
of the newest edition of the “Süsswasserflora von
Mitteleuropa” containing the genera Actinotaenium
and Cosmarium. Cosmarium is generally regarded as
the largest, albeit artificial desmid genus. Following
Růžička’s flora “Die Desmidiaceen Mitteleuropa’s” (see
Růžička 1977, p. 1), in the present flora the monothetic taxon definition will be applied: the concept of
a higher taxon constitutes the sum of the concepts of
the lower, equivalent taxa classified directly within it.
Consequently, the description of a species comprises the
descriptions of all infraspecific taxa (if any, of course),
including the nominate variety (or form). All these
taxa share a monothetic character set that differentiates them from other species. The nominate variety
includes the type of the species and is only nomenclaturally distinguished from other varieties; it is automatically established when the first “true” variety of
a species is described. Moreover, the description of a
species is not static but will change each time a new
variety is included.1 Since species form the basic taxonomic level to which a key should lead, a well-defined
species concept is a prerequisite for a workable flora.
The most important factor in the correct establishment
of a species is detailed knowledge about its variability.
Most curiously, the species concept in desmids is only
weakly developed.
Pre-Darwinian species concepts
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Species concept in desmids
ganism through its whole course of life . . . “ (Archer
l.c. p. 76). But he also emphasized that it is essential
to distinguish a distinct, undescribed form by a name,
and to record its diagnostic characteristics.
In later papers (Archer 1862, 1865) he further elaborated this view. He rejected the practise of grouping
together a number of allied but well-distinguished forms
under a common name as presumed varieties of a single
species, stating that he did not believe it to be more in
accordance with nature, nor that it was more convenient in determination books. He remarked that: “The
very fact of recognizing them as varieties presupposes
and acknowledges their distinctions.” (Archer 1865, p.
258). He furthermore strongly argued against the use
of var. α, β, γ, etc., in favour of giving these forms
their own names, considering these “ultimate forms” as
separate species. Most interestingly, he also pointed at
another aspect of the species concept, i.e., the status of
the “standard or typical form”, what we now call the
nominate variety, and other varieties of a species. He
stated that this typical form is “. . . not in itself, perhaps, more decidedly or distinctly separated from any
of the so-called varieties than they are from each other,
nor less so . . .”. In this respect too, Archer clearly was
ahead of his time.
Post-Darwinian species concepts
Around the time Archer wrote his first papers, Charles
Darwin was developing his views on evolution and variability. In total six editions of his famous book “The
Origin of Species” were published between 1859 and
1872. The 6th edition (Darwin 1872) was the most elaborate one, for the first time including the word “evolution”. As a result of sexual reproduction, species are
variable, and new species arise from a gradual process
of inheritance of characters, natural selection and survival of the fittest. Hence, the morphological complexity
of desmids is brought about by evolutionary processes.
Moreover, in Darwin’s opinion a variety should be considered as an incipient species. It should be emphasised
here that this genetically fixed variability has nothing
to do with the minor variations in cell morphology due
to environmental pressures affecting the growth of the
cell.
However, Darwin’s ideas about variability, inheritance and natural selection obviously were only scantily penetrating the microscopist’s world. The previous
rather narrow species concept in desmids was dramatically broadened by some workers in the next decades,
giving rise to much taxonomic and nomenclatorial confusion.
At least partly due to the apparent ignorance of
the work of Archer discussed above, Reinsch (1867a, b)
presented a very aberrant view on variability of desmid
species and the effect on systematics and nomenclature. Most importantly he considered cell-dimensions
as a minor factor in distinguishing different forms at
the species level. He described many new species, frequently subdivided into two or more so-called “Formen”
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– generally only indicated by diagnostic characters –
which in turn were sometimes further subdivided into
unnamed categories. (see, e.g., Cosmarium botrytis and
Staurastrum meriani in Reinsch 1867b).
Some years later, Klebs (1879) pointed at the poor
general knowledge of variability in desmids. Although –
judging from his list of references – he obviously knew
the papers of Archer, he presented a system with species
that are further subdivided in up to three different levels, called “Variationen”, indicated with a, α, 1, etc.
Only some of these forms were provided with a name.
The ultimate example of this “classification” was Cosmarium botrytis (which he considered the most variable
desmid of all: Klebs, l.c., p. 38; compare also Nordstedt 1893), admitting that every classification based on
such an abundance of different forms is fully artificial.
In agreement with De Bary (1858) and Archer (1860),
he further considered it impossible to define desmid
species after mere morphological characters without detailed knowledge of the complete life-cycle. Both Reinsch (1867b) and Klebs (1879) applied a broad, morphological species concept, and included very different
forms that had previously been described as separate
taxa within their own species.
However, the most drastic “lumper” of desmid
species undoubtedly was Playfair (1907, 1910) who
stated that “. . . something like ninety percent of the
“species” are polymorphic forms of the other ten; and it
is only by tracing out their life-histories through the observation of transition-forms, that the specific connection of their innumerable variations can be established”
(Playfair 1910, p. 463). As an example he cited Cosmarium rectangulare, described by Grunow in Rabenhorst (1868), and besides the “type” he included 25
varieties and formae (so-called “growing forms”), in a
number of cases supplemented with “formae mixtae”.
These growing forms were considered immature, and
were presumed to be able to grow out to fully developed “typical” forms. However, many of these forms
were well established species that at most only have a
superficially similar outline, and Playfair’s view rightly
met with no approval (compare also G.S. West 1909).
On the other hand, Playfair was completely right when
he stated that “. . . in warm weather and in shallow,
stagnant waters, cell-division does (italics from Playfair, FK) take place a second time before the nascent
semicells have become fully developed, . . .”, but clearly
these immature cells cannot be considered as “missing
links” between two different species.
Most curiously, none of the three phycologists
Reinsch, Klebs and Playfair mentioned above have presented any evidence in their papers that they were familiar with the theories of Darwin. The same holds true
for one of the most distinguished desmid workers of the
last decades of the 19th century, Nordstedt. In his early
papers (Nordstedt 1870, 1872), he described new varieties as separate taxa (sometimes indicated with “α”,
“β”, “γ”, etc.) provided with an infraspecific epithet,
and formae as mere growth forms, indicated as “forma
minor”, “forma major”, etc., without the intention to
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describe new taxa. One confusing exception was made
in Cosmarium crenatum, where he described 4 forms,
two of which were described only briefly, and two were
described as a var. bicrenatum, and as a subsp. costatum, respectively. In later papers Nordstedt also gave
formae with epithet and brief latin diagnosis (e.g. Nordstedt 1873), but it remained unclear whether he really
intended to describe new taxa.
In one of his very first papers, G.S. West (1899)
gave an overview of “. . . Variation in the Desmidieæ
and their Bearings on their Classification”. Some of
the ideas of Darwin can be found in his text (albeit
somewhat hidden), such as development and evolution
(G.S. West l.c., p. 370). West further stated that: “In
these restricted areas (small ponds and quiet margins
of lakes, etc. FK) the unceasing effect of the struggle
for existence will result in a gradually increasing diversity of form, and this is to be correlated with the immense numbers of individuals that are sometimes found
in these situations” (G.S. West l.c., p. 367). Moreover,
he suggested that the most numerous variations are to
be found amongst the commoner and more widely distributed species. In this context West even stated that “
. . . by a gradual process of natural selection or survival
of the fittest, the present morphological complexity was
in all probability for the most part brought about.”,
without actually referring to Darwin’s work!
According to West, this process showed a tendency
in the direction of an increase in the complexity of morphological characters, such as warts and spines etc., as a
defensive ornamentation against many forms of predating aquatic animals, but partly also serving as anchors
in time of floods. In addition he stated that “. . . the
occurrence in large quantity of a particular species is
most conducive to the production of deviations from
the normal form.” (G.S. West l.c., p. 368). However,
he emphasised that variability of a character does not
reduce its usefulness to identify species.
The latter point made by West brings us to the
possible consequences of morphological variability for
the taxonomy of desmids. G.S. West (l.c.) was of the
opinion that “Many of these differentiations are combinations of characters which are repeated in hundreds,
and I may say thousands, of individuals, and can therefore be rightly considered as constituting distinct varieties.” Most curiously, this would mean that taxonomic
status (variety) depends on the frequency of occurrence
of a morphological variation. West considered such variations “permanent”. On the other hand, permanent
variations wouldn’t occur readily, since the character
of species would then be totally changed in a very few
generations (G.S. West l.c., p. 371). Many species apparently retain their original characters in an extraordinarily constant manner, and West concluded “ . . . that
the natural production of a permanent variation in a
Desmid-species is a more difficult matter than at first
imagined.”
West obviously distinguished between occasional
variability of species, and permanent variability with
taxonomic consequences. Referring to papers by Schmi-
F.A.C. Kouwets
dle (1893, 1894) and Borge (1896) he suggested that
occasional variability was brought about by prolific
growth and rapid division, and mainly affects the outward characters of the cell. Permanent varieties should
be the result of evolutionary adaptations to environmental conditions, provided that changed environmental conditions continue to exist for a prolonged period
of time.
In a subsequent chapter, G.S. West (1899) described the variability of a large number of desmid
species. However, his attitude towards the classification of morphological variations seems somewhat ambivalent: on the one hand he stated that many described species and varieties are forms of other species,
but on the other hand he seemed to recognize many
such varieties that should – at least partly – rather be
merged with the nominate variety, or classified as separate species.
Recent developments: polyphasic taxonomy
After these first decades of rapidly developing desmid
research, the need for taxonomic and nomenclatorial
rules became ever more urgent. One year after the International Botanical Congress held in Vienna in 1905,
the so-called “Vienna Rules” were published, marking
the beginning of a series of the “International Code of
Botanical Nomenclature” (ICBN) of which the “Vienna
Code”, published in 2006, is the most recent offspring
(McNeill et al. 2006). In addition, Nordstedt (1906) argued about the designation of Ralfs’ “British Desmidieæ” published in 1848 as starting point of desmid
nomenclature, in a final attempt to bring about some
stability, and Nordstedt’s proposal was adopted at the
3rd International Congress held in Brussels in 1910 (but
see counterarguments given by Silva in 1958, 1960, and
references given herein)
In the next period many of the above mentioned
growth forms were given formal status as separate infraspecific taxa, provided with epithets, author(s) and
year of publication. In many cases no original figure or
authentic material existed, bringing about much confusion concerning species concepts and environmentally
induced morphological variability (“ecomorphae”). In
this respect two contrasting practices were employed.
In stead of merging such doubtful “forms” with the
species they belong to, they were frequently classified
as separate infraspecific taxa of this species, possibly
in respect for the original author of the form, in this
way creating a large series of taxonomic synonyms. On
the other hand, morphologically clearly different forms
were frequently described as infraspecific taxa of apparently unrelated, only superficially similar species,
possibly out of laziness (it’s easier to describe a new
variety than to describe a new species) or maybe even
for fear of too drastic taxonomic revisions. In contradiction with ICBN rules, the “typical”, nominate form
(i.e., the first described form of a species, marked by
the autonym when other forms are described) was still
frequently listed as “var. typicum” or “var. genuinum”,
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Species concept in desmids
etc., suggesting that other varieties are but aberrations
of this “type”, hence generating a polythetic species
concept.
In this way, a large number of so-called collective species was created. A quick scan of the Fritsch
Collection of Algae (FBA, Ambleside) revealed that at
present, e.g., Cosmarium bioculatum comprises – give
or take one or two – 24 varieties, C. contractum 36
and C. botrytis 40. From the accompanying descriptions and figures it is clear that most of these varieties
have no (genetic) relation with the species (or rather:
the nominate variety) at all; they are only confusing the
(polythetic) species concept and should be described as
separate species. The remaining varieties most probably are rather ecomorphae, and should be included in
the species, without any separate taxonomic status.
From the above it is clear that a different approach is needed to come to a workable species concept in desmids. As in other groups of algae, a multidisciplinary approach may be the answer, frequently
called the “holistic” or “total evidence” approach, or
“polyphasic taxonomy”.3 All available information on
morphology, the life-cycle, genetics, etc. should be considered to define a species.
As discussed above, traditionally the species concept in desmids (and most other groups of micro-algae)
is purely morphological. Desmid taxa are generally distinguished on the basis of a large variety of more or less
clearly defined morphological characters, mainly relating to cell wall outline and sculpture (see introduction).
However, the taxonomic value of these characters, especially when they show a different degree of development, obviously was rather a matter of personal preference. This in turn led to a very inconsistent use of
infraspecific taxa (subspecies, varietas, forma).
A different approach is the biological species concept, as it was principally developed by Mayr (1942,
1982).4 In this approach, a species is defined as a group
of organisms whose members have the potential to interbreed with other members of the group, or, conversely, are generally unable to interbreed with organisms of other such groups. It is unfortunate, however,
that desmids only occasionally show sexual reproduction (by means of the formation of zygospores; see e.g.
Brook 1981). Many taxa are hitherto only known in
their vegetative state, sometimes forming large clonal
populations. On the other hand, such populations of genetically identical individuals may give a clue to phenotypic (morphological) plasticity, not influenced by the
environment (Kouwets 1984).
3
Mann (1999) used the term “Waltonian species concept” after Izaak Walton’s “The Compleat Angler”, 1st ed.
published in 1653.
4
A detailed and very interesting discussion of species
concepts is given by John S. Wilkins, in an unpublished thesis submitted to the University of Melbourne, Australia (see:
https://webspace.utexas.edu/deverj/personal/test/species.
pdf). It is currently in press as “The origin of species concepts: The history of an idea”; a summary was published by
Wilkins (2006)
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In an extensive treatment of the subject, Grönblad
& Růžička (1959) referring to Mayr (1942) once more
tried to stop the unbridled increase of infraspecific
desmid taxa. They rightly advocated a strict separation of taxonomic and non-taxonomic variations, and
emphasised that only stable, genetically fixed and inheritable variations should be given taxonomic status.
At the same time they admitted that the unambiguous establishment of such variations is a very difficult
task. Because of the complex morphology of desmid
cells, they stated that the use of infraspecific taxonomic levels cannot be avoided in desmid taxonomy.
They recommended the use of the terms “varietas”
and “forma”, and suggested terms like “(eco)morpha”
for non-taxonomic variations. On the other hand, they
question the necessity to give every small but stable
variation a separate taxonomic status, and somewhat
vaguely leave the responsibility to the authors.
In addition to these morphological studies, mating experiments with members of the genus Closterium
have shown that, as in diatoms for example, cryptic
species do exist in the desmids (e.g., Coesel 1988; 1989,
Ichimura 1997; Sekimoto et al. 1995). However, it is
presently unknown to what extent this phenomenon occurs in the group as a whole.
Finally, during the last decade, a number of papers
has been published concerning the molecular phylogeny
of desmids (Gontcharov et al. 2003, 2004; Gontcharov
& Melkonian 2004, 2005, 2007). These studies show
the relationship between morphology and genetics, and
the first results show that traditional genera such as
Staurastrum and Cosmarium should be re-evaluated,
and split into a number of smaller groupings. An interesting conclusion is that the taxonomic significance
of some morphological characters apparently has been
greatly overestimated (Gontcharov & Melkonian 2005).
However, up till now, generally only one single clonal
culture per taxon was used to construct phylogenetic
trees. It would be very interesting to test different clones
of different infraspecific taxa of, e.g., Cosmarium botrytis (compare the citation at the head of this paper!).
Conclusions
Whilst preparing a volume of the “Süsswasserflora von
Mitteleuropa” containing the desmid genera Actinotaenium and Cosmarium, the author faced great difficulties in establishing clear boundaries between different – variable – species. From the discussion presented above it appears that during the past decades
morphological variability in desmids has been misinterpreted to a large extent, which has led to a
flood of ill-defined infraspecific taxa. These infraspecific taxa (mainly varieties and formae) obscure a clear
species concept, and hence hampered the development
of a sound taxonomy for desmids, which is the indispensable basis of every flora. Besides, it should be
kept in mind that any taxonomical or nomenclatural change of a species will affect all its infraspecific taxa too. But most importantly: infraspecific
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taxa are no absolute need for a clear species definition.
Therefore, in agreement with the ideas previously
expressed by Archer (see above), species are considered
here as the terminal taxa in the tree of life. Infraspecific
taxa are commonly defined as to differ genetically, but
yet (if sexual reproduction occurs) are able to form hybrids. They will only develop into separate species after
reproductive isolation. In effect, varieties are species “in
statu nascendi”. This idea is compatible with the generally employed monothetic species definition, where
all infraspecific taxa share a common character set. In
desmids, to achieve a feasible taxonomy, “stable” varieties should generally be given the status of a separate species. The taxonomic status of “forma” should
be abandoned altogether. Morphological variations induced by the environment should merely be characterized as “ecomorpha”, without any taxonomic status.
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Received September 1, 2007
Accepted March 17, 2008
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