Overlooked shepherd's purse diversity

This is a follow up thread to this one: Curious About Shepherd’s Purse. In that thread, @blue_celery agreed with me that other species besides Capsella bursa-pastoris are likely to have spread outside their native range. Now, in the current iNaturalist dataset, we find, as expected, C. bursa-pastoris observations worldwide:

When we check C. rubella, we see that it is almost entirely confined to Europe, with a couple of interesting exceptions:

When I was first doing this investigation, the outlier in Iceland – research grade – was the furthest from the native range. The one in Kansas was added just two weeks ago, and the one in California, four days ago. Neither is yet at research grade.

One of the links that @blue_celery provided was documentation of C. rubella occurring cryptically in Chile – a datapoint that does not exist on iNaturalist, although there are many observations of C. bursa-pastoris in Chile. My hypothesis is that C. rubella is more widespread in North America than the two observations suggest; some may even be misidentified on iNat as C. bursa-pastoris due to “field guide bias,” i.e. the field guides only say that bursa-pastoris is present, so that is what users assume every Capsella they see is.

The other link from @blue_celery has this handy illustration of the seed capsules:

Now, I have some shepherd’s purse that I have been growing out from seed, and now that it has capsules, I am certain that it is rubella. You can see its capsules here:

Note that the lobes have concave sides, and flare outward, in keeping with the above illustration for rubella. Now, I do not feel that I can post this as an observation, since it is captive/cultivated, and the location and date are inaccurate. But suppose I return to the original collection locality at the appropriate time of year, and post an observation of the parent plants? My concern is, if I ID it as Capsella rubella, will that get a fair consideration by the community, or will it automatically be reassigned to C. bursa-pastoris “because the literature says so”? I am aware that my being certain has no scientific standing; so how would I demonstrate it?

I also wonder: if I went through the North American observations of “Capsella bursa-pastoris,” carefully examining them to see if any are really rubella, how much would the community support me?


I think a more fundamental problem than the field guide syndrome is that C. rubella is only controversially recognized as distinct from C. bursa-pastoris in the first place; several prominent taxonomists seem to disagree. For example, Al-Shehbaz seems to think there are too many intergrades to warrant its segregation.

But hey it’s in POWO [along with a lot of other crap], so by all means, if you think you can identify C. rubella go ahead. Please use more than one character-state to distinguish it from C. bursa-pastoris, though, since the central criticism of C. rubella seems to be that its alleged distinguishing character-states do not consistently covary. iNaturalist might be a good way to test whether they in-fact covary.

C. rubella, it it’s real, has already been reported from North America E.g. in Weakley’s latest flora of the SE USA (2020) - see discussion under C. bursa-pastoris.

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there are enough evidences that they are distinct. One can hypothesize that morphology cannot sometimes match with genetics but I would be careful to raise doubts on the independence of a taxon that seem to be neither the closets relative of C. bursa-pastoris.

I agree, it is sometimes difficult to get a strong identification because there seem to be intermediate individuals.

Given the bias against non-natives in general I would not be shocked that other species are hiding in plain sight. A book I am reading currently has several examples of invasive species not being critically looked at, multiple species are then discovered. To others points, I recently posted a photo of a non-native dandelion species* and all my comments back were “it’s not in the book!” By not critically IDing or even questioning certainly leaves out biodiversity. For example a local birder in CO found a population of African Collared-Doves amongst the throngs of Eurasians. I sure wasn’t looking.

If you can help with a little write up of what marks to look for in this complex I’d love to help look though images, feel free to send me a message. I think this one of the strongest parts of iNaturalist. Even if we are currently mis-IDing a species now we have the photos to reassess.

*Turns out I was wrong about the dandelion but the point remains!

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“there are enough evidences that they are distinct.”

This is by no means a consensus view of taxonomists. But for our purposes, if it’s in POWO, that’s usually good enough.

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For “our” purposes? But of which purposes are you writing about? Speak for yourself

blue_celery may correct me if I´m wrong, but the last I read about Capsella rubella and bursa-pastoris was that rubella is an allogamous (outbreeding) species with chromosome numbers 2n= 16 whereas bursa-pastoris is largely autogamous (self-compatible) rarely outbreeding species with 2n= 32. Why those two should not be separate species is somewhat of a miracle to me. Of course the biology of bursa-pastoris leads to a large number of faintly distinct microspecies (Jordanons) that perhaps sometimes approach rubella in morphology especially as the morphological differences are not as pronounced as the biological ones. And hybrids between the two species occurr spontaneously where they meet (not sure about their fertility or infertility, probably tending towards the latter)

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Maybe I did not get the point or I have written something not clear, but I think I am not the one you should address to for the inclusion of rubella into bursa-pastoris.

O.K., you didn´t get the point. But I think I made it clear that I´m all for the separation of rubella and bursa-pastoris as distinct species. “Why those two should not be separate species is somewhat of a miracle to me.” Sounds clear enough to me.

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Me too. I experienced the existence of possibly intermediate individuals but I agree that they are two distinct species.

That is one of the reasons I am growing these indoors: to control for self- vs. cross-pollination.

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Ah, good thinking. Do you know how the protection against self-pollination works in C. rubella? Protandry/protogyny (hope I haven´t missspelt those, but you know what I mean…) or some other mechanism?

I thought I saw it, but I can’t find the article now.

Although this article suggests that C. rubella does self-pollinate, and that this is what separates it from C. grandiflora. Which just goes to show how confused and confusing this genus is.

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