There are many species that are distinguished from the closely-allied taxa by one or few very small characters. Many of them maintain the species rank and have not been lowered to a subspecific rank.
For example, Lysimachia foemina is distinguished from L. loeflingii (previously included in L. arvensis) mostly by the number of cells of its glandular hairs on petal margin and by their density. Another similar example is Melampyrum italicum that differs from M. catalaunicum for having two-celled hairs on calyx instead of 5-8-celled hairs. The subspecies of Pinus nigra are mainly distinguished by the shape of the epidermal cells and the number of layers of ipodermal cells in needles. And so on…
NB: I included “medium-large” species because it’s more or less obvious that smaller organisms (e.g. mosses or protozoa or small insects) are distinguihed by very small characters.
Some ferns, where not readily distinguished thanks to geography or ecology, are confidently identifiable by ploidy, or spore morphology, or minute details of reproductive organs (annulus cells, glands…)
Most frustratingly the very common (spam levels sometimes) Halysidota tesselaris is not separable from H. harrisii in adult form without taking a specimen and brushing the genitalia. So many of an otherwise very easy to ID moth are thus relegated to genus-only level for eternity.
there are lots and will soon be many more thanks to current taxonomic policy where any genetic difference is described at the species level even if it can never be identified without expensive, difficult genetic analysis
There are many bird species pairs that are mainly told apart by range and song, with levels of genetic difference that would be considered trivial in most other taxa, and hybrids in the parts of their ranges that overlap. This gives the bird taxonomists plenty to do lumping then splitting them repetitively.
Its basically impossible to tell impatiens capensis apart from impatiens pallida if theyre not in flower - at least i cant. They love to grow next to each other too, which makes it even more annoying.
I really need to just DQA my non flowering observations lol
Species boundaries are often frustrating! Well worth complaining about. As we do that, though, we should (I think) remember that at least three different causes with different consequences make species difficult to distinguish.
The species look alike but are really distinct by anybody’s definition – they don’t interbreed even when they do live in the same place. Cope’s and Gray Tree Frog are an example (distinguished by chromosome number and song). So are insect taxa distinguished by details of the genitalia.
The taxa are closely related and interbreed sometimes but not always, so what do we do about that? Ignore the hybrids if they are few, OK, but if they’re common? Maybe swing eternally between calling them species and subspecies.
Asexual species. With these, we would be justified in separating them into groups based on what we can easily distinguish (though whose ID skills count for drawing the lines?).
Cases, and there are many, when some picky little detail is what we write in the keys for identifying the species because that picky little detail is consistent and clear if you can see it but actually the species do look different in more vague ways involving size and shape and behavior. Cases where the gestalt works, but can’t be communicated.
Groups like dandelions and members of the European Blackberry complex where the combination of sexual and asexual variation produces a pattern of variation that simply cannot be reduced to human needs for distinct species. In these cases, no species boundaries will satisfy everyone, and indeed no species boundaries will satisfy anyone who looks closely.
And then the groups that are split but shouldn’t be because the differences are so trivial. And who decides what shouldn’t be split? Ha, ha, ha, ha, ha.
And for examples . . . I guess the ones that annoy me most right now are bumblebees that I will accept are real different species (incompatibility of the genitalia, probably). Theoretically you could distinguish them by the color of certain abdominal hairs if you could see them and if they weren’t shaded by the abdomen. You wait and wait as the bees clamber around the flowers and finally you get the photo you need! Gleefully you post it. And somebody comes along and says, “Yes, the trait is there and yes, it usually distinguishes the species, but it may be inconsistent,” so Zap! your species ID is denied. Sigh.
Two seriously big hornet-mimicking hoverflies - Volucella zonaria (Common throughout the Palearctic), Volucella bella (South-Central Asia, kind of Tien-shan-ish). What colour are the bristles at the tip of the first segment of the antennae? Is there a black smudge on the top of the base of the hind femur? Is the second plate on the underside of the abdomen completely black, or is there a little bit of yellow in the hind corners?
Well, there is research going on to test if the leaves can be used to distinguish these species and there’s an iNat project that recruits citizen scientists to collect these leaf samples. https://www.inaturalist.org/projects/jewelweed-leaf-traits
Oooh thank you for sharing, i need to look in to that. I definitely know some spots where they grow
EDIT: and by some spots, I mean I know of a park less than a mile from my house that has both species growing adjacent to each other, they’re currently in bloom, and not a soul will care if I snag a few leaves.
So yeah. Will definitely be grabbing some thing week to help out. Thank you again for the share!