What is a species complex?

Dear Amanda,

I think to clarify complex species phylogenetic analysis is a powerful probe for investigating these aspects.In the case of fish the bichirs(Polypteriformes) are regarded the sister group of Teleosts, but future studies are needed for further confirmation.

As for my expertise, You may look at my recent book released by CRC Press in 2015 by title " Phylogeny, Anatomy and Physiology of Ancient Fishes" or the previous book By Academic Press on "Primitive Fishes " 2007(I did not edit).

Cheers,

Giacomo

I do not know if it is a help

A species complex may be many things as it is not restricted to a species concept, which are many. A species complex may be a collection of a species assumed once to be populations or subspecies of a single species, as in the case of Motacilla flava. Or it is a group of species identified morphologically with difficulty, but better via serological, anatomical etc. methods. It may be just synonymous with superspecies, a group of species often described under flagship of a representative species. Or a collection of distinct lineages fell within a monophyletic clade, which may represent extreme variations, hybridogenesis, intrrogression etc. Or a collection of sympatric monophyletic species or sometimes allopatric offshoots of a monophyletic clade.

Biologically speaking, there is no such thing as a ''species complex'', because what we identify as such is simply due to the lack of resolution of our methods to differentiate distinct species. For instance, in gall-inducing phylloxerids (Hemiptera: Phylloxeridae), taxonomists refer to species complexes when they are faced to morphologically identical specimens that induce very distinct galls (e.g. the Phylloxera caryaecaulis species complex). However, they may or may not be distinct species, there is just no way for us to tell... yet (I agree with Giacomo Zaccone's statement that phylogenetic analysis are useful to clarify species status). To sum up, species complexes are kind of like Shrodinger's cat: are the species distinct or not, we don't know until we ''open'' the species complex to find out!

Since the Motacilla flava (the yellow wagtail) has been brought up...  In that example and many other avian cases, the term species complex is used when one confronts a group of closely related populations/phenotypes that are not completely reproductively isolated yet can be distinguished due to some divergent characters (morphology, behavior, DNA, etc.)  Often, in such cases different taxonomic authorities or individual specialists working with these groups cannot agree on taxonomic treatments of these groups or change their treatments relatively frequently.  Often, it is also unclear how members of a species complex are related to each other.  Basically, due to continuous nature of evolution, in some cases of recent divergence it is hard to place "rigid, non-overlapping frames" we call species.  The idea of species is creationist in nature because Linnaeus, who proposed the concept of a species, believed in creation of discrete biological units that are distinct and cannot change.  His motivation was to discover the pattern of creation through describing the units of creation.  We know that process of evolution is continuous and there are no rigid units, so the original species concept is not an appropriate tool.  It works relatively well, however, on a small geographic scale because most of lineage divergence happens in allopatry at least in animals.  However, when geographic scale increases, or ecological divergence happens, we find cases when placing rigid frames on a continuum becomes difficult and taxonomic uncertainties arise.  To acknowledge these uncertainties specialists resort to using terms like "species complex" or "super species".  Therefore, in a very simplified way, members of a species complex display divergent characteristics that go beyond a population level (typical for a particular systematic group) but the pattern is difficult to interpret for creating a clear taxonomy.  

The Motacilla flava is a great example.  The number of species that are members of this complex varies from 1 to 11 according to different taxonomists.  The reason is a very complex variation in plumage, interbreeding patterns between different phenotypic forms and genetic lineages, discord in geographic patterns of phenotypic and genetic variation, etc.  At the same time, differences among phenotypes and genetic lineages of yellow wagtails far exceed these differences observed between populations in many Palearctic birds.

I agree with Umit and Eric with their basic statements.In the case of polypterid fishes(Polypteriformes), we may assume that they should belong to a monophyletic clade.The genus Polypterus contain 12 or 13 species(the bichirs) and the genus Herpetoichthys (the reedish) contains one species.Two distinct lineages that fall into monophyletic clade.I performed morphological and IHC analysis of the lungs that share in common the same structure and the presence of type 1 and type 2 pneumocytes, and immune cells, but a phylogenetic analysis is due.

Dear friend Amanda, I suppose a species complex is a set of species hypothesis that not necessarily reflect a monophily. Generally is grouped by any visual similarity. Too see more about it, search Fitzhugh.

One could just as well ask: what is a species? Do we even "know" that?

Species are hypotheses and many of the examples brought up here so far still essentially adhere to a classical "biological species concept", i.e., one focusing much on sexual reproduction: fishes, birds - larger animals that exclusively reproduce sexually. For these organisms the concept of sexually isolated populations that do not reproduce with one another generally works fairly well. Still, the examples brought up in the discussion so far show that even for these larger animals the concept is limited. Ans it becomes a lot more complicated in plants, fungi etc. Hybrids, asexually reproducing species, haplobionts, parthenogenesis, dikaryotism, etc. etc.

It still surprises me that biologists frequently tend to focus on a minority of organism that are the classical, sexually reproducing organisms, ignoring the fact that the majority of species diversity occurs among groups that have far more complex modes of reproduction and thus probably follow more complex  evolutionary patterns.

Any species is a hypothesis. If we call it a "species complex" we are essentially simply saying that things are even more complicated ...

Dear Frank,

I think nowadays that morhological analysis, in addition to visual similarity as well as histological tools do not suffice to include or not a species into a monophyletic clade; this because there is a need of the molecular analysis.

Dear Giacomo,

Sorry, I do not understand your comment at all.

Just to clarify: I did not mention or compare any techniques such as using morphological studies or. molecular characteristics to form a species hypothesis, did I?

To reiterate my points:

(1) species are hypotheses (no matter what tools you use to define them)

(2) a "species complex" is just a group of species where our hypotheses are not yet well defined

(3) a universal species concept so far does not exist

(4) much of the discussion here has focused on the "biological species concept", i.e., species that are defined by their reproductive biology, but...

(5) the majority of species out there do not exclusively reproduce only by sexual means...

Dear Frank,

Thank you for explanation.I only tried to say the importance that species complex form a challenging system for applying the general lineages concept of species to diagnose level lineages(any point  of your discussion ie 2).For instance the analysis of mitochondrial DNA haplotypes is useful to reconstruct the evolutionary history of a given group(a number of species to say of P.jordani complex).

Hi Amanda,

It is a good idea not to confound operational definitions with scientific concepts. For example, the term 'species complex' may be used in taxonomy as a way of saying a) we think that this group of organisms may represent more than one species; b) we cannot discern the species boundaries with certainty (e.g., due to morphological similarity, or simply insufficient data); and c) we speculate (or assume) that these 'species' are related in some way (not necessarily through phylogenetic relationships which may know nothing about). This operational definition would stand as long as statements a, b), and c) conform with our knowledge. Neither of these points should be assumed to be correct or set in stone, but it may be a convenient way to point out a potentially problematic area.

Depending on context, the term 'species' may be similarly used as an operational definition (often happens in taxonomy), or as a 'hypothesis' (e.g., in systematics or ecology). Operational definitions may also vary, depending on the species concepts used. If you stay within the pragmatic taxonomic realm (i.e., defining taxa based, ultimately, on expert opinion), then a species complex would be just as real as 'real' as a species, subspecies, genus, tribe, etc.

You may wish to load your 'species complex' with hypothetical assumptions, e.g.,on the higher probability of introgression, recent phylogenetic divergence, or morphological convergence, but then you would require to get a more in-depth understanding of the situation to test/falsify them. For some of these better understood conditions, researchers have coined other specialty terms, such as 'semispecies' or 'superspecies', although these are being debated just as thoroughly.

As someone who has worked with small vespertilionid bats, I can attest that defining 'species complexes' is a convenient way to circumscribe operational groupings of hard-to-discern species as a first attempt at sorting out the taxonomy of poorly known morphologically obscure groups. As you delve into the intricacies of morphological and morphometric differences and add molecular data as a second layer of information, the picture may change profoundly. However, as noted by others on this thread, the complexity of life does not always conform to the definitions or assumptions we try to impose on it.

Dear Stephen:Interestingly, most studies show that the degree of genetic and phenotypic divergence within a complex of cryptic lineages making up a species or species complex is not particularly consistent.The DNA sequences are minimally different.Since I study the neuroanatomy of the lung and its oxygen receptors in the polypterids, I wonder if these may be a cryptic species complex.This group includes the bichirs to which are assigned  13 species of the genus Polypterus, and the reedfish Herpetoichthys calabaricus.

In microbiology, we usually define a species complex , in general, as a group of species that cannot be separated by the usual barcoding sequencing,e.g. 16S rRNA gene in bacteria. We talk sometimes of 100% identity in the sequences of the species in these complexes.

When dealing with microeukaryotes, the 'cryptic species' concept is more common. Classic studies, and not so old ones, used to focus to great extent on the morphology of 'protozoa', despite of now being well-known to be genetically different.

Regarding arthropods, I'm not sure if the species complex or cryptic species concepts are commonly used but I'm aware that many taxa require relatively advanced examination (e.g. SEM) of e.g. copulatory apparatus, to be able to differentiate macroscopically identical species.

In a simpler context 'species complex' encompasses all the 'characteristics and variability' of a species sharing with allied species. Phylogenitically all have a common ancestor (i.e. monophyletic). Currently molecular studies (e.g. DNA barcoding) are useful in separating species within the species complex. 

Notwithstanding the foregoing very able answers to your question, the fact is much simpler: A species complex is a convenient way to refer to taxa that seem to be related, yet are too different.  A group of related taxa like Streptanthus morrisonii and its subspecies are called the "morrisonii complex" principally because the subspecies, although validly published, are not accepted by the FNA.  The subspecies are quite different morphologically and electrophoretically, but without acceptance, they don't exist for botanists.  The "streptanthoid complex" is used to speak collectively about species from three genera that share several character states.  Willis Jepson, Edward Greene and others lumped Streptanthus and Caulanthus into one genus. Thelypodium species have streptanthoid flower morphology but are otherwise distinct. It is not necessary for the members of a species complex to be monophyletic; indeed, it is the very ambiguity of their status that produced the convenient collective noun.

The natural sciences are well provided with non-technical words and expressions with no consensus meaning. "Species complex" is one of them. Use the expression when it seems to be the best, perhaps the only, description...no need to argue with someone else's use of the expression...there is no appeal to authority, no matter what someone else says. 

Hi Amanda

I have one example with oaks, which are a typical syngameon example (Grant, I do not recall the year).

Iberian white oaks complex is formed by six species which maintain leaves morphological differences along their distribution areas, but which can hybridize in some cases. These hybridizations apparently occur among all species pairs if they come into contact. However, interestingly, there is also mixed forest (containing more than one species) where hybrids are apparently missing.

So you sees, the species in the complex seem to hybridize in some cases but not in others, while they maintain along most of their ranges taxonomical differences.

One interesting possibility is that post-zygotic isolation mechanisms are not strong enough to avoid the formation of  hybrids, but they are strong enough to force next-generation individuals to come back towards one of the species instead of forming a hybrid swamp. This coupled to assortative mating (for example due to phenology differences) could result in a short life-span of the hybrid generations.

Christian Lexer has some interesting results regarding this issue in his work with poplars.

Cheers

Pablo

The term 'species complex', to me, simply means a group of individuals that are closely identical enough to be similar species but are not entirely and totally identical in typification. In other words, there is identifiable discontinuities in their features, which are not significant enough to separate them into distinct species. Such variations or discontinuities may not be capable of causing reproductive discontinuities or barriers within the group. This situation implies that sexual reproduction with genetic exchanges  among them may further lead to more diversity in their forms or features due to genetic recombination. Such group of individuals may be collectively identified as a species complex.

A species complex is generally a monophyletic group - there are exceptions. A species complex instead is a broad descriptor of organisms where distinction into species is difficult due to overlapping characteristics. Cryptic species and species aggregates are merely specific types of species complexes. It should be noted, though, that all of these groupings are a part of informal taxonomy to for the sake of communication and are not used in actual taxonomy.

I agree completely with Jonathan (and could not have said it better:): in taxonomy there is not such thing as an "species complex" (see ICZN). You can use the term to indicate a "complex" situation, but please: state clearly what you mean and do not give it a name. 

Hi Amanda

As you mentioned, if we call a group "species complex", we are already assuming that such group is composed by more than one species. I believe such groups are just starting to diverge. Thus, we can consider species complexes as composed by species situated at early stages of diversification. 

I do not think we need to run for fast asnwers, when studying species complexes. Indeed, species complexes may deserve an entire ife of studies, because it can be difficult to find limits in lineages at early stages of diversification. Dont feel guilt if you dont find these limits, these groups only need to be investigated in deep.

Congratulations for bringing this subject into discussion

Fabio you are right.I think also that in a monophyletic group, there may be more species have been separated without evolving morphological differences, ando so DNA barcoding is necessary.

Giacomo

All of the replies on the meaning of "species complex" are reasonable and sufficient for operational purposes. Most differ from one another in at least one particular. The range of the differences demonstrates the informality of the expression. Some replies contain what can be understood as requirements; an informal expression has no requirements. For example, monophyly is given as a requirement but it is not, nor is it only about multiple species. A species complex might consist of several species, but can also consist of varieties of one species.  It is not a convenient way to circumscribe because circumscription is a term of art, but to the extent that "a way to circumscribe" means only to put in the spotlight, that is consistent with informality.  Sometimes a species complex is a group of taxa that are beginning to diverge but it can refer to taxa at other stages too. One might include here ecotypes or endemic taxa. A few examples: the results of recent molecular work on the genus Hesperolinon (Schneider et al. AJB 103(2)) indicate very early stage divergence, so much so, in fact, that only a few of the species are distinct.  The varieties of Streptanthus morrisonii, validly published, are not accepted - thus creating a species complex which in this case refers to a species with several fairly similar but clearly different infraspecific taxa.  Several other streptanthus species are in the same boat. The Intermountain Flora does not define species complex but uses the idea in two entries randomly discovered inVol 4.: with reference to Frasera/Swertia - "Until a comprehensive study is made of all the species in the complex..." P. 18. With reference to penstemon: "About half a dozen species from beyond our area belong to the same complex..." p.96. Finally, although they do not include the expression in the index to Plant Systematics (1999), Judd, Campbell, Kellogg and Stevens use it:  "Members of the Andropogon virginicus species complex...." p. 130.

When all is said and done species complex is a reasonably useful informal way to refer to a group of taxa at or near species that appear related but the assumed relationships are completely or partly unknown, ambiguous, unclear or unrecognized. The expression has all the precision of a sentence like "I think those white things over there are gulls".

I don't really see why there should be a missunderstanding of the term 'species complex'. In nature, organisms respond genetically to the environment. ariations of the environment of organisms may drive their response in different directions depending on the operational genetic force in the individual. Sometimes there could be mutatinal changes that may translate to various levels, extents or forms of modofication of the genotype and/or the phenotype.

In addition to this, recombination, whether during cell division or during normal vegetativ cell life can occur, and depending on whether it is transmitted to sebsequent generations of cells from the originally affected cells can lead to formation of variations. Such variations may find their way to new generation of individuals through any of several genetic ways.

Such natural genetic events that can originate variations can affect any or some of several genes. In this situation, differet individuals could originate different directions, dimmensions and limits of variability. Such differences could be noticed in different individuals of the same taxonomic species. Similarly, recombination itself through sexual reproduction is known to originate variation. All such variations in various individuals of the same species do not necessarily look alike. Therefore one could notice a pool of slight changes, which of course could be significan, but may not necessarily bring about reproductive bariers or discontinuities. Sometimes cytotypes of a species could arise. One can conveniently use 'species complex' to describe this situation.

All 'species complexes' may not always arise through monophylly. There is a possibility that it could arise through parallel events of interspecific hybridization. In other words, if two pairs of individuals of closely related species successfully interbreed, their respective offsprings may not be exactly similar enough to be conveniently represented by one type specimen that comes from one of them. Once they have sufficient structural, genetic and biochemical similarity in addition to environmental range and reproductive compatibility, they may be seen either as a species or a 'species complex'. In other words, the term 'species complex' could be used to indicate existence of some reasonable or clearly observable level of heterogeneity among members of a species.

For my point of view, species complex is a monophyletic group of specimens (or populations) with wide variability among them relatively to the others definite species from same systematic group, but with no clear molecular or morphologycal clusters within, which could be described as separate species. Another situation could be when the clustering could be detected, but the differences is too narrow for species level (ex setae number, colors, etc)

From the taxonomic point of view (officially) "species complex" does not exist as an entity (Check the International Code of Zoological Nomenclature). However, it is a very common way to give a name of a group of species that may or may not be in fact closely related (e.g. monophyletic group). The most common example is a group of cryptic species that could be historically be named under one species name. With new technology and data that species can be in fact a number of different species and can be, for the sake of operational ease, be called as a "species" complex. Especially, when you can't still solve the taxonomic issue properly and you just want to point out the problem...

If you think species complex is complicated to define, just ask what an actual species is...

Cheers!

Basically ndividuals that seem to belong ti the same species but they Have little differences not easy to recognize

A collection of populations that are differentiated enough to qualify as more than one species but that have fuzzy boundaries and lack full reproductive isolation (Alagona, 2016).

In practice, species complexes often result from widespread lineages that were historically viewed as a single species, but with the addition of novel data (molecular, increased geographically comprehensive sampling, etc.) has been split into multiple diagnosable species. Similarly, species that may have not been viewed as closely related historically (morphologically disparate), are now shown to be part of an evolutionarily cohesive unit of closely related species - a species complex - based on novel data.

Dear Amanda, Francisco, Francesco, Ahmad, Luke.

As myself a zoo owner of giraffes, big cats and elephants, etc, there seems to be an evolution to species level, at which it stops. For example my lions and tigers look related at some time in the past, even to dogs, bears, as most characteristics are in common. Likewise giraffes and okapis look to have had a common ancestor, the giraffes having now become become 9 subspecies, with very minor (cosmetic) differences, occasional dwarfs. These all cross with each other and dominant features prevail. Likewise some species, like lions, tigers will breed into, for example a liger, in a collection in England. Such experimentation is frowned on in zoo circles.

These may point to the limits of evolutionary ancestors. Mammals appear suddenly in an explosion of placental forms in the Cretaceous., Likewise insects with trachea, in the carboniferous, and many life forms in the Cambrian. These do not adhere to any Darwinian tree of life and do not have proven ancestors. So speciation may be pre-written on the DNA by our Creator. (And the fossil record is the story of recovery after a wipe-out, see www.earthhistory.org.uk) If you are allowed to consider that? Or is Epicureanism the only philosophy allowed?!

It seems, Anthony, that you are attempting to goad someone into a discussion about evolution versus creationism. In my opinion, this isn't the place or thread for such a conversation.

I think it is just a way for people to say "this is considered a single species today but we know/asume/suspect there is actually more than one

I would like to recommend and read my recent paper in this connection:

Problems of species and the features of geographical distribution

in colonial volvocine algae (Chlorophyta). Novosti sistematiki nizshikh rastenii 54(2): 299–311. 2020. https://doi.org/10.31111/nsnr/2020.54.2.299

Available on ResearchGate.

Historically the term was used to identify more than one cases:

For species once believed to form a morphospecies (cryptic species),

OR

a species having populations with unresolved taxonomies,

OR

a group of species among which reproduction isolation is not complete (ring species, semi-species, hybdridogenetic taxa),

OR

a group of closely allied species within genera (species groups, superspecies).

Thus, collectively all above.

species group is not clear identification, sometimes can't find evidence to separate all of them.

The 'species complex' is an informal taxonomic term, comes under 'open nomenclature qualifiers' that showing the uncertainty of a taxonomic identification (Sigovini et al. 2016). For practical purposes, a species complex may define as, a group of very closely related species that require a critical revision in order to clarify the actual taxonomic position of the taxa involved and diagnostic characters. However, this term is generally understood in the taxonomic literature as, a group of morphologically similar species to the point that the boundaries between them are often unclear (Sigovini et al. 2016); and usually refers to either a group of cryptic (two or more distinct species hidden/classified under one species name) or sibling species (morphologically indistinguishable species/populations that are closest relative to each other and reproductively isolated or showing incomplete reproductive isolation (Mayer 1963; Bickford et al. 2007; Sigovini et al. 2016; Kwon-Chung et al. 2017).

As a neurobiologist of fish respiration,this may apply to the fish species Monopterus albus and and Monopterus cuhia inhabiting the poorly aerated ponds in India.They do not carry or share in common morphological characters,but differ in that one of the two ie M.cuhia has an air breathing pharyngeal organ originating from the transformation of gill respiratory system,and the other one lacks it,but has a richly vascularized mouth mucosa.Perhaps they are two genetic variants ,so a trascriptomic analysis may help.

A group of soo closely allided complex organisms with unclear boundaries of separation.

Giacomo Zaccone the two seem to be distinct enough to be classified under different genera. Species complex term is applied when two or more taxa are apparently closely related.

Dear Umit

The taxonomy of asian swamp eels is not well known.But Monopterus albus and Monopterus javanensis form a species complex because of genetic research that recognized more cryptic forms(Collins et al.,2002).The genus Monopterus include 10 species including Monopterus cuhia

Umit Kebapci

Enclosed is a relevant paper

A species complex is a group of closely related individuals believed to be confined naturally within the taxonomic boundaries of a distinct species; but which have clear phenotypic discontinuities that are not sufficient enough to discriminate them as different species. Members of a species complex may, to a relatively good extent, may be phenotypically distinguishable. They are not likely to have reproductive boundaries. Therefore cross-fertilization between various members of the group create further phenotypic diversity. Naturally, this diversification continues until an interplay of factors such as genome-by-environment interactions, which may lead to phenotypic plasticity, genetic polymorphism, significant structural, geographic and finally reproductive barrier redefine them as different species.

@Julian Osuji, its seems, that if these species are reproductivelly not isolated, phenotipically, they likely would be continues one in raltion to the other, due to genetic flow among them. However, in this case, frequences of the different forms will not be uniforms, and rarity of certains phenotypes could indicate to theirs intermediate or hybrid position among the species.

@Andrey Rudoy, if the members of the species complex have reproductive affinity, there would be continous genetic flow among them leading to continuous phenotypic variation. It's good to note that some species are in breeders while some others could be outbreeders. If inbreeding species forms a species complex, there may not be much phenotypic variation compare with a case where the complex is interspecific (safer seen as interspecifically complex). Frequencies of the different forms of variation within the species complex may not be uniform in outbreeders quite unlike inbreeding species complex. Inbreeding species evolve convergently thus making them more uniform.

@Julian Osuji convergence in so short distances could be, however, if species are isolated completly, factors making them to be different should be, which cause disruptive selection, thus convergence by the selected traits, unlikely

Species complex contains a number of "Cryptic Species"; which are genetically distinct species erroneously classified as a single species due to at least superficial morphological similarity.

A species complex or a complex species?

About species concept, no one definition has satisfied all taxonomists. In fact, more than 30 alternative species concepts have been proposed with the biological, ecological, phylogenetic and typological species concepts being the most widely used.

As a matter of fact, one of the grand challenges in zoological taxonomy today is still what describes a species. From a pragmatic standpoint, morphological characters remain as the main criteria for species delimitation in zoological taxonomy according to the International Code for Zoological Nomenclature. However, when only a typological-based approach is used for delineating closely related species, bona fide species may be considered as a single nominal species due to few or no morphological differences.

A group of closely related organisms that are very similar in appearance which the boundaries between them are often unclear, can be defined as a species complex informally. Similarly, species group, species aggregate, species series, macrospecies, and superspecies are also used as informal taxonomic ranks.

As an informal assemblage of taxa, members of a species complex (also referred to as “species group”, “species aggregate” or “species series”) are related phylogenetically and share morphological similarities (resemble each other). In some cases, members of a species complex may display few or no phenotypic differences. Eventually, these traits might be considered insufficient to create new formal taxonomic categories (e.g., genus or subgenus) for grouping them. This concept is closely related to the concepts of sibling species and cryptic species, but not necessarily all members of the complex must be cryptic or sibling species. Species that are morphologically indistinguishable, but belong to distinct evolutionary lineages are referred to as cryptic species or sibling species.

So, I think the question should be “A species complex or a complex species?”.

In conclusion, it should be noted that species complex etc. are also informal, just like categories that were used in the past as infraspecific categories (e.g. variation, morpha, aberration etc.) that are currently considered informal and unavailable according to ICZN. The existence of both infraspecific and supraspecific changes and formations is an undeniable reality. This is probably why there has not been a single consensus on the species concept among taxonomists.

Thanks.

Species complexity mainly refers to the structure of species and is closely related to biodiversity