Blog

When is a ‘good’ species a good species?

June 26, 2025

True, long ago, animals such as the Brown Bear, Ursus arctos, were thought to comprise many races and possibly 90 subspecies, but since no one could be quite sure about the morphological assessments made at the time, more modern biologists have tended to err on the side of caution and lump such entities together as one ‘good’ species, rather than a species complex.

https://en.wikipedia.org/wiki/Brown_bear

Now this may not seem that important, but if we really don’t know what we are dealing with genetically when we study such and such a species, and wish to work on its biology and ecology, more especially in terms of its geographical range and what it eats with regard to its diet breadth, then we are surely not living in the real world.

But fortunately, help is now at hand with the implementation of high-resolution molecular (DNA) markers, including sequencing. Just as such approaches have revolutionised forensic science and solved many ‘cold cases’ going back decades, e.g. murders, the new technology has revealed that many apparent ‘good’ species in fact comprise a complex of morphologically-similar or even identical cryptic ones. For instance, in the case of the good old Brown bear, nowadays “15 extant, or recently extinct, subspecies [are] recognized by the general scientific community”, based heavily on DNA analysis, delimiting some 5-9 clades of extant members of the animal (See aforementioned Wikipedia article).

My interest and concerns with the presence of cryptic species goes all the way back to 2016 when I published a paper entitled ‘Known knowns and Unknowns in Biology’ (Biological Journal of the Linnean Society, 2016, 117, 386–398) with colleagues in Australia on the fact that many cryptic species await discovery and these will enhance estimates of biodiversity, whilst at the same time broadening our view of the natural world and bring home the fact that evolution is a ‘moving feast’ (so to speak) and never stands still. Thus many species populations ‘fray’ at the edges of their populations and enter new or vacant ecological niches and in effect, become cryptic species, initially at first, before developing more distinct morphological characteristic, if they ever do.

I recently wrote about this with a colleague in a Preface paper called ‘The Paradox of Generalism’ which presents a figure (Fig. 1) of many of the species, including killer whales and great white sharks, that until recently were thought to be ‘good’ species (i.e. predominantly homogenous genetically over their geographical range), but now have been shown to comprise true species or other, lower evolutionary entities following the application of high-resolution genetic markers). In the case of the killer whale, changes in behaviour have been found, related to geography, affecting behaviour and as such, the diet of these animals, i.e. whether they feed on fish, seals, or baleen whales.

https://orca.cardiff.ac.uk/id/eprint/178673/

It appears that such cryptic entities are being discovered ever more frequently, including, as I discovered yesterday, in terms of cryptic species of jellyfish of all things, more exactly the Portuguese man-o’-war, Physalia physalis, as a new paper informs us.

https://www.cell.com/current-biology/abstract/S0960-9822(25)00682-7

You would have thought that living in the marine currents and drifting around on these over vast distances to catch their food, the species would remain ‘good ‘due to their being adequate gene flow to prevent genetic divergence of populations. But no, it (the Portuguese man-o’-war) is not ‘good’, but rather comprises 4 apparently sympatric cryptic species. How about that for an unexpected finding!

Hugh Loxdale

Photo of Portuguese man-o’-war (Hydrozoa: Siphonophorae: Physaliidae)