By Daniel Atha
Introduction
Everyone knows that trust in science, scientists and scientific institutions is at a low point. It’s not surprising the public is skeptical. The masses have been excluded from the language and practice of science. The public has little say in the goals of science. And many of the products of science have made our world more dangerous and inhospitable.
What I love about natural history is that it is comprehensible to everyone with a desire to look. The natural world is there for everyone to enjoy and study equally. There are no barriers to access and the possibilities for learning are infinite. You don’t need a Hubble telescope or a particle accelerator to study natural history.
As naturalists we know that we would all be better off if more people had a greater appreciation for and understanding of Nature and the earth system processes that make life possible. iNaturalist is democratizing science by empowering everyone to participate in the practice and process of natural history science. Every observation records what we find interesting and meaningful. And the totality of our observations reflect our values individually and collectively.
The literature on species concepts in biology is vast. Generally, most people would agree that a plant species is a lineage that has diverged from a common ancestor and has some genetic and morphological cohesion. The offspring inherit genetic, anatomical and behavioral traits from the parents. Put more simply, a species is a reproductively independent lineage (Rieseberg et al., 2006) with corresponding morphological traits. Disputes arise over reproductive capacity and how well genetic markers (genotype) are correlated with morphological character states (phenotype). See Mayer and the biological species concept (Mayr, 1992).
We might successfully classify a species using one or the other or both genotypic and phenotypic characters without knowing everything about the basic life history and reproductive mechanisms of an organism. In fact, most species are named before a great deal is known about their basic biology. But when either data set or the data in combination are ambiguous, we must seek additional data sets, such as reproductive strategy, chemical productions, chromosome number, etc… See Stewart-Wade et al., 2002 for a broad overview of Taraxacum biology.
It’s also important to remember that science is driven by curiosity and scepticism. When it is based on observable facts and informed by mature and reasoned analysis, science can reveal processes and patterns that may be hidden to the casual observer. The basics of science are the formulation of a hypothesis, testing the hypothesis (including the null hypothesis) and repeatability. Much of what the public thinks of as science deals with abstract concepts and astronomical values. But we must remember that a flora of a given area is really a whole series of hypotheses. Our concept of a species is really just a hypothesis that an organism is definable by a set of traits and that we can tell it apart from others reliably by use of a key. And because we are dealing with plants in the landscape, our floristic hypotheses can be tested by virtually everyone, regardless of their training or prior knowledge. That’s the beauty of a flora and the fun of writing keys.
As scientists (and citizen scientists) we must be careful not to make uninformed judgements or appear overly certain about our interpretations of natural phenomena. In our haste, carelessness or ignorance, we may base a hypothesis on insufficient evidence or poor interpretation of the evidence that is available. In such cases, when the hypothesis is overturned by evidence that should have been considered or analyzed properly, the public’s trust is eroded.
The subject of this post, the common Dandelion, is an example where careful studies carried out by scientists over decades can and should help us interpret and appreciate what we can all see in our lawns and gardens every day-- the common Dandelion. My goal here is to determine (by empirical evidence) whether the Red-Seeded Dandelion is a species as commonly understood by botanists and the public or whether it is a color morph that arises spontaneously from a large pool of genetic variants-- similar to an albino Rat.
Like the common Brown Rat, the Common Dandelion (Taraxacum officinale in the broad sense) is nearly ubiquitous across North America in areas disturbed by humans. The Common Dandelion thrives with ample sun, moisture and nutrients, especially in lawns, garden beds and exposed urban soils.
In North America, nearly all Common Dandelion plants are triploid and apomictic (Lyman and Ellstrand, 1984), meaning that they have three sets of chromosomes and reproduce by parthenogenesis (a process called apomixis whereby seeds are formed without fertilization). In other words, they are clones. Common Dandelions with a “normal” complement of chromosomes (diploid, 2n=16) capable of sexual reproduction with other diploid Dandelions and with triploids (2n=24) are known from Europe, where the species is native. Diploids are extremely rare in North America (Verduijn et al., 2004; Lyman and Ellstrand, 1984) and most plants studied in North America are triploid clones, reproducing asexually by apomixis.
Red-Seeded Dandelions (given the name Taraxacum erythrospermum in 1822 by Antoni Andrzejowski and Joseph Besser.) is treated as a species in most of our floras, including the Flora of North America.
In floristics and systematic botany science, we use taxonomic keys (among other methods) to test our hypotheses. The key is a series of choices that are meant to be mutually exclusive. An organism (taxon) is supposed to fit the characteristics presented in one choice, but not the other. Sometimes key are long and have multiple series of choices that narrow down the options until we are finally presented with just two choices-- one leading to our organisms. Most keys are dichotomous, meaning that each set of questions consists of just two choices, as in the key below.
Every flora treats the "species" in nearly the same way. The detailed descriptions of the two entities (when provided) differ slightly, but the characters in the key here are the ones almost always used to distinguish the species.
A. Leaves usually deeply cut throughout their length, the lobes sharp and narrow, usually pointed back; flower heads smaller; floral bracts (phyllaries) hooded and/or with horns near their tips; seeds (cypselae) reddish or purplish at maturity..... Taraxacum erythrospermum.
A. Leaves less deeply cut, particularly toward the base, the lobes less sharp, not pointed back; flower heads larger; the floral bracts (phyllaries) not hooded and without horns near their tips; seeds (cypselae) brown, olive or tan at maturity..... Taraxacum officinale.
The material presented here demonstrates that these are continuous characters (subject to bias and observer interpretation) and are not correlated with each other or other traits. All research specifically designed to test the hypothesis that Red-Seeded Dandelions are a valid species have found that they are not.
Materials and Methods
Research on the leaf morphology of the Common Dandelions shows that leaf length to width ratio and degree of lobeing varies as the plant ages (Sanchez, 1971) and is influenced by light intensity (Wassink 1965; Sánchez 1967). Rounded leaf blades with less lobeing develop in low light and deeply lobed leaf blades develop in high light (Sánchez 1967; Slabnik 1981). Increasing light intensity increases the leaf lobeing and decreases the leaf length:width ratio (Slabnik 1981). These studies show that the character used by every flora writer to distinguish the two Dandelions (leaf lobeing) is not a trait inherited from the parents, but is rather is a response to environmental variables.
Another study found that achenes actually sort into seven color morphs, but these occur independent of other character states (citation needed).
A study of 20 individuals from several populations across Washington, Oregon, Idaho, Montana, Wyoming, Utah and Nevada representing plants identified as Taraxacum officinale and Taraxacum erythrospermum (Taraxacum laevigatum) found that there was no correlation among twenty-five character states of achenes, involucres, receptacles, leaves and phenolic (chemical) profiles. In fact, these characters showed more variation within populations than between them (Taylor, 1987, citation and abstract below). Achene color-- the single most consistent character state used to distinguish Taraxacum erythrospermum-- was found to be independent of any other character, including shape and degree of leaf lobing. As Taylor points out, it is improbable that the many previous studies have overlooked hitherto untried character combinations to distinguish these entities (e.g. micro-characters). Additionally, as an overwhelmingly asexual, apomictic triploid in North America, it is improbable that hybridization and introgression (back crossing) between putative Taraxacum officinale and Taraxacum erythrospermum has blurred species distinctions creating a continuum of character states bridging one species to the other. Finally, the author’s work and several works cited therein demonstrate a correlation between environmental stress and phenotypic (anatomical) expression including leaf lobing and achene color and that this variation is best explained by a single, weedy species exhibiting a range of phenotypic expression in response to environmental conditions.
Morphological and chemical (phenolic) variables were used in principal components and cluster analyses to determine patterns of variation among and within 22 wide-ranging populations of Dandelions. Intrapopulational morphological variation was as great as or greater than interpopulational variation. Morphological variables were poorly correlated, and plants failed to cluster into the two described species, Taraxacum officinale and Taraxacum laevigatum [Taraxacum erythrospermum]. Phenolic distinctions existed among populations but not between species-types, and chemical variables did not correlate with morphological variables. The data, therefore, suggest that morphological variation is largely due to phenotypic plasticity. This conclusion was supported by the observation of a strong relationship between microhabitat and morphological phenotype, with characteristics of T. laevigatum being expressed under conditions of environmental stress. The pattern of phenolic variability reflects the existence of chemical biotypes. -- Taylor, 1987
Another study of 518 individuals from 22 populations across North America found that there were up to 13 enzyme clonal profiles discernible within a single population of Taraxacum officinale, demonstrating considerable genetic variability within a single population (Lyman and Ellstrand, 1984). In fact, Taraxacum officinale was shown to have a total of 47 enzyme and morphological clonal types among all populations sampled and the highest number of clones per individuals sampled (0.091) of any plant known. By comparison, 108 clonal types have been identified by enzyme analysis for Oenothera laciniata, but only 0.051 clones per individuals sampled. Based on these data and the work of others cited by Lyman and Ellstrand, the authors suggest that multiple introductions of Taraxacum officinale from Europe and Asia have contributed to the high genetic diversity found in North American plants.
In yet another study of 318 Dandelion individuals, Lynn Mertens King found 145 chloroplast (cp) and ribosomal (r) DNA profiles among them. In her work, King states….
"North American dandelions with red achenes do not form a natural group based on either rDNA or cpDNA, so lack of differentiation between North American aggregate species Taraxacum officinale and Taraxacum laevigatum [Taraxacum erythrospermum] in rDNA and cpDNA is consistent with Taylor’s (1987) observations based on morphology and phenolic compounds and suggests they are not separate evolutionary lineages." - King, 1993
Citizen science data from iNaturalist demonstrate that the characters most often used by flora writers to distinguish Taraxacum erythrospermum do not stand up to the test. The characters are: 1. leaf lobeing correlated with achene color; 2. floral bracts (phyllaries) with horn-like appendages correlated with achene color.
The following observations show with empirical evidence that seed (cypselae) color is not consistently correlated with either of these commonly used morphological characters. Rather it appears to occur randomly within populations of "regular" Dandelions. (Field studies here in New York City will be designed to test whether distinctly red seed color occurs randomly in populations and is correlated with any other objective character.)
The following observations were culled from a review of approximately 300 of at least 17,000 global Dandelion observations. https://www.inaturalist.org/observations/identify?page=102&verifiable=true&place_id=any&taxon_id=47602. Observations were reviewed quickly and some which are ambiguous may remain.
Seeds (achenes or cypselae) red but floral bracts (phyllaries) without horns
https://www.inaturalist.org/photos/47571589 – https://www.inaturalist.org/photos/73078829 – https://www.inaturalist.org/observations/46074005 – https://www.inaturalist.org/observations/45680775 – https://www.inaturalist.org/observations/45305683 – https://www.inaturalist.org/observations/45241684 – https://www.inaturalist.org/observations/44993437 – https://www.inaturalist.org/observations/44992893 – https://www.inaturalist.org/observations/45680775 – https://www.inaturalist.org/observations/44870840 – https://www.inaturalist.org/observations/44545929 – https://www.inaturalist.org/observations/44515723 –https://www.inaturalist.org/observations/44416050 – https://www.inaturalist.org/observations/44218822 – https://www.inaturalist.org/observations/43544408 – https://www.inaturalist.org/observations/42918761 – https://www.inaturalist.org/observations/42200603 – https://www.inaturalist.org/observations/41687607 – https://www.inaturalist.org/observations/41633065 – https://www.inaturalist.org/observations/40355663 – https://www.inaturalist.org/observations/38709065 – https://www.inaturalist.org/observations/38195521 – https://www.inaturalist.org/observations/30666920 – https://www.inaturalist.org/observations/27168210 – https://www.inaturalist.org/observations/24093003 – https://www.inaturalist.org/observations/22916353 – https://www.inaturalist.org/observations/22439535 – https://www.inaturalist.org/observations/21959839 – https://www.inaturalist.org/observations/21910431 – https://www.inaturalist.org/observations/19276795 – https://www.inaturalist.org/observations/14042249 – https://www.inaturalist.org/observations/13356869 – https://www.inaturalist.org/observations/13013560 – https://www.inaturalist.org/observations/12069364 – https://www.inaturalist.org/observations/5708934 – https://www.inaturalist.org/observations/4935390
Seeds red but floral bracts without horns and variable leaves
https://www.inaturalist.org/observations/30449058
Red and brown seeded plants growing in mixed populations
https://www.inaturalist.org/observations/40355663 – https://www.inaturalist.org/observations/40355599 – https://www.inaturalist.org/observations/40333898 – https://www.inaturalist.org/observations/26018331 – https://www.inaturalist.org/observations/18144048
Seeds reddish brown
https://www.inaturalist.org/observations/22916353 – https://www.inaturalist.org/observations/48937056 – https://www.inaturalist.org/observations/46463545 – https://www.inaturalist.org/observations/46366025 – https://www.inaturalist.org/observations/43585532
There are many observations with olive colored seeds and with highly divided leaves. Of the observations where you can see both the seeds and the leaves, there are just about as many with olive seeds and highly divided leaves. This is an excellent series of photos with olive achenes, highly divided leaves and phyllaries with small projections. https://www.inaturalist.org/observations/80414881
https://www.inaturalist.org/observations/6104316 – https://www.inaturalist.org/observations/55330874 – https://www.inaturalist.org/observations/55135826 – https://www.inaturalist.org/observations/51994588 – https://www.inaturalist.org/observations/50983759 – https://www.inaturalist.org/observations/46723114 – https://www.inaturalist.org/observations/46700686 – https://www.inaturalist.org/observations/46623799 – https://www.inaturalist.org/observations/44853618 – https://www.inaturalist.org/observations/43719170 – https://www.inaturalist.org/observations/43659943 – https://www.inaturalist.org/observations/43585532 – https://www.inaturalist.org/observations/43308757
Discussion
I have found no scientific studies that support the recognition of Taraxacum erythrospermum as a valid species-- only descriptive accounts using continuous, qualitative characters that are subject to bias. It appears that every study actually testing the validity of the hypothesis with objective and quantitative criteria found that there is no basis for recognition of Taraxacum erythrospermum as a valid species. Don't get me wrong. I love all plants, including the ones we call weeds and invasives. An artificial ranking as species does determine the inherent worth of Red-Seeded Dandelions as living beings worthy of our love and respect.
Conclusion
Based on the evidence presented here, it is clear that the common Dandelion, nearly ubiquitous across North America, is an apomictic, triploid species that has very high rates of morphological, chemical and genetic variability within and among populations, but especially within a population. The author has seen no evidence that the red achene character state is anything but a mutant color morph that may appear randomly wherever clonal Dandelions occur. Until there is convincing evidence for the validity of the species, the available evidence argues for treating the common, weedy Dandelion as a single species (Taraxacum officinale).
This is how science is supposed to work. Someone has a hypothesis. That gets tested independently and objectively and either the evidence supports the hypothesis or doesn't. To ignore the evidence and persist with the unsubstantiated hypothesis is not scientific at all.
Literature Cited
King, L.M. 1993. Origins of genotypic variation in North American dandelions inferred from ribosomal DNA and chloroplast DNA restriction enzyme analysis. Evolution 47: 36–151.
Lyman, J.C. and N.C. Ellstrand. 1984. Clonal diversity in Taraxacum officinale Compositae), an apomict. Heredity 53: 1–10.
Mayr, E. 1992. A local flora and the biological species concept. American Journal of botany 79: 1537–2197 https://bsapubs.onlinelibrary.wiley.com/doi/abs/10.1002j.1537-2197.1992.tb13641.x
Riesberg, L., T.E. Wood and E.J. Baack. 2006. The nature of plant species. Nature 440: 24–527. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2443815/
Solbrig, O.T. 1971. The population biology of dandelions. Am. Sci. 59: 686–694.
Solbrig. 0.T. and B.B. Simpson. 1974. Components of regulation of a population of dandelions in Michigan. J. Ecol 62: 473–486.
Solbrig. 0.T. and B.B. Simpson. 1977. A garden experiment on competition between biotypes of the common dandelion (Taraxacum officinale). J. Ecol. 65: 427–430.
Stewart-Wade, S., S. Neumann, L. Collins and G. Boland. The biology of Canadian weeds. 117. Taraxacum officinale G.H. Weber ex Wiggers. Canadian Journal of Plant Science 82: 825–853.
Taylor, R.J. 1987. Populational variation and biosystematic interpretations in weedy dandelions. Bulletin of the Torrey Botanical Club 114: 109–120.
Verduijn, M., P. Van Dijk & J. Van Damme. 2004. The role of tetraploids in the sexual–sexual cycle in dandelions (Taraxacum). Heredity 93: 390–398. https://doi.org/10.1038/sj.hdy.6800515
Wassink, E.C. 1965. Some Introductory Notes on Taraxacum officinale L. as an experimental plant for morphogenetic and production research. Mededelingen Van De Landbouwhogeschool, 65–16. Wageningen: Veenman.
