A better example would be lichen taxonomy. Lichens are symbiotic organisms which can contain members of up to three different kingdoms (a fungus, an algal protist, and a cyanobacterial moneran). However, since the fungal symbiont gives the organism its structure naming is based primarily on the the fungus.
Another problem for pigeonholing, if you are following a Five Kingdom system, is what to do with green algae. Unicellular green algae are very closely related to multicellular green seaweeds. So if you consider unicellular green algae as Protists you must either separate them from their very close relatives or else consider seaweeds to be Protists (which some classifications do, calling the Kingdom the Protoctista instead of Protista).
But beyond that, the green algae are very closely related to the multicellular Plantae, so no matter what you do you are going to have closely related organisms in different Kingdoms.
as well as to multicellular green plants. (The Plantae certainly are descended from them.)
More precisely, if life arose more than once, the others left no decendants. It’s quite possible that life arose multiple times, but got immediately eaten by the better-adapted life already present.
Colibri, what would be the problem with lumping the unicellular green algae and the multicellular seaweeds alike in with Plantae? You’d have to change the definition of the plants, but that’s no worse than changing the definition of the protists.
There’s no problem as far as cladists go. The problem is mainly with fitting them into a Five Kingdom system.
While three of the usual kingdoms are monophyletic, two, the Protista/Protoctista and Monera, represent “grades” of organization and are polyphyletic or at least paraphyletic. The unicellular Protista include groups that are more closely related to multicellular Plants, Animals, and Fungi than they are to each other. The Monera (procaryotes) includes two “Domains” that are very different from one another genetically, the Archaea being closer to the eucaryotes (the other four Kingdoms) than they are to the Bacteria.
With regard to Plantae and green algae, one course is to classify the multicellular plants as Plantae sensu stricta (also called Embryophyta), and recognize a taxon called Viridiplantae or Chlorobionta for the Plantae plus the green algae.
The problem, as it generally is for Linnean taxonomy, is what rank to give these categories. As I mentioned before, if you call either of them a “Kingdom” you end up with probably 20 or more kingdoms among the eucaryotes alone if you recognize each equivalent clade at the same level.
In a sense. The roots of the sea-vent food chains are various archaebacteria (or just archae, depending on which terminology you use), and the discovery of those ecosystems put the spotlight on the archae, and were possibly a contributing factor leading to them being recognized as one of the current three domains. But they’re not the only examples of archae, nor the first discovered: The organisms which give the distinctive color to Yellowstone’s hot springs, for instance, are also archae. From our anthropocentric viewpoint, the archae superficially resemble the bacteria (and were at one time classified with them in the kingdom Monera), but genetically and evolutionarily, they’re about as distantly related to the bacteria as we are.
This is going to be a problem with any sort of exhaustive cladistic classification, though. There’s some organism which is ancestral to everything, and the only clade you can fit that organism into is the clade Vitae. That ancestral organism is almost certainly extinct, but there are still some modern organisms which are more like it than are others, and it’d be nice to have some category for such organisms. But if the Ancestor is going to be part of that category, and if there’s anything at all which isn’t going to be, then that category can’t be a clade.
