In Richard Fortey’s book Life: A Natural History of the First Four Billion Years of Life on Earth, he keeps referring to amoebas as animals. But if you look at the kingdoms of life, don’t amoebas belong to the protists? Or is it a case that the animal kingdom’s proper name – metazoans – means it only includes multicellular beings?
What is the difference between a cladogram and an evolutionary tree (or maybe the former has supplanted the latter)? Related question: where does one draw the line between phylogenic systematics and taxonomy?
Current thinking is that there are five kinds of organisms on earth: bacteria, protozoans, plants, fungi, and animals. Under that scheme amoebas would not be animals. But the difference between a single celled protozoan, a group of protozoans, a colony of protozoans, and a simple animal is not clear.
A cladogram is a phylogenetic tree made by using the rules of cladistics. But other trees can be made that don’t follow the exact cladistic rules, even though they make intuitive sense. For instance, whales and dolphins are usually put in the order Cetacea, while cows, sheep, deer, pigs, hippos and such are put in the order Artiodactyla. This makes sense to us, since whales are so different. But genetic evidence now suggests that whales are more closely related to hippos than they are to any other members of the Artiodactyla. That means that hippos are more closely related to whales than they are to–say–deer. Therefore, hippos could be put in the order Cetacea and removed from Artiodactyla. But that would mean that Artiodactyla is an invalid group according to cladistic rules, because one branch is missing, and a parent group has to include all child groups. According to cladistic rules whales would Artiodactyls, even though they share very little morphologically with the traditional Artiodactyls.
A cladogram is a hypothesis about the relationships of various groups, based on shared characters (each taxon itself is placed at the end-points of the diagram). A phylogenetic (or evolutionary) tree adds the element of time to such a hypothesis, postulating not only relationships, but times of divergence. Further, the divergence point, or node, is representative of the ancestral taxon in a tree.
Taxonomy, essentially, is the science of classifying things. Emphasis is on which taxon goes in which box (it also deals with the naming of those taxa and their boxes). Phylogenetics places emphasis on who is related to whom. Together, phylogenetics and taxonomy make up the field of Systematics.
Note that, to muddy the waters further, a cladogram is not necessarily cladistic. Cladists are hard-line phylogenic systematists, with DNA-derived data taking precedence over gross characters. As far as a cladist is concerned, paraphyletic groups must be eliminated, at all costs. Thus, a cladist would degrade Aves into a sub-group of Reptilia.
In molecular biology, most people think that cladistic phylogeny is the only way to work. This has led to some unnecessary brick walls of people spend aeons analyzing comparative primary DNA or protein sequences when taking a quick look-see at secondary or tertiary structures would reveal remarkable similarity of function and higher-order structure that is not trivially deduceable from similarity of primary DNA or amino acid sequence.
I understand this classification, which is why asked the question. In other words, if amoebas are protists, then why does Fortey (and I assume hs is not alone) keep calling them animals? Adding to the confusion is that Fortey prefers the word “protist” over “protozoan,” as the latter implies animal-ness.
I haven’t read the book and I have nowhere near the biology cred of previous posters to this thread, but my understanding is that the division of life into five kingdoms is a moderately recent development, and while, today, the word “animal” properly refers only to one of the kingdoms, it can be loosely used to refer to anything that moves – including van Leeuwenhoek’s “wee beasties.”
Yep. Cladistic taxonomies are based on nested groups (akin to set theory). Birds are a subset of dinosaurs (since they evolved from dinosaurian ancestors), and dinosaurs are a subset of reptiles. Thus, birds are dinosaurs, and dinosaurs are reptiles (so, of course, birds are reptiles, as well). One major sticking point, as Dogface alludes, is that only monophyletic groups (those groups composed of a common ancestor and all descendants) are said to represent real groups. Reptilia sans Aves (which is the Linnaean version of Class Reptilia) is paraphyletic: it only includes some of the descendants of a common ancestor, and is therefore not representative of a natural taxon.
I wouldn’t call this the most up-to-date thinking. About thirty or forty years ago, the so-called “Five Kingdoms” classification gained popularity. This included Monera (bacteria and blue-green “algae,” which don’t have a cell nucleus), Protista (one-celled organisms with a nucleus), and three multicellular groups: Fungi, Plants, and Animals. Although better than the traditional two-kingdom organization of Plants and Animals, according to cladistic thinking this still leaves a lot to be desired.
Cladistics demands that co-ordinate categories (e.g. Kingdom, Phylum, Class, etc.) must branch at the same level. The problem is that some kinds of bacteria are as different from one another as either is from multicellular forms. Likewise, the different kinds of protists are as different from one another as each is from the multicellular forms.
What this boils down to is that, if you classify such relatively closely-related forms as multicellular plants and animals in different Kingdoms, one must do the same for all the various kinds of protists. That is, Amoebas would form their own Kingdom, as would Ciliates (Paramecia), as would Flagellates, etc. In short, there would be about 15-20 Kingdoms just among the non-Monera (Eucaryotes). The non-Monera would form one “Realm” (a higher level than a Kingdom), while the Monera themselves might be split into two Realms, the “true” bacteria and the archaic bacteria.
To get back to the OP:
If one defines an “animal” as a non-photosynthetic mobile organism (a non-technical definition), then Amoebas are animals. This would be the traditional classification.
According to the Five Kingdoms system, Amoebas belong to Protista, not Animalia.
According to the most modern cladistic viewpoints, Amoebas would belong to one of about 20 Kingdoms among the non-Monera. (I might also note that there are amoeba-like organisms in several of the various protist Kingdoms.)
Sorry, in the previous post I used the term “Realm” for the level above Kingdom; the proper term is “Domain.”
This site gives some information on the Three Domain System, as well as cladistics. (It also mentions Six-Kingdom and Eight-Kingdom systems.)
In the diagram showing the three Domains of Bacteria, Archaea (archaic bacteria), and Eucaryota (protists, fungi, plants, and animals) note the many branches within the Eucaryotes at a deeper level than those between plants, animals, and fungi. These branches would represent separate Kingdoms among the unicellular eucaryotes.
In general, however, most cladists frown upon trying to assign a Linnaean rank to a given clade (or, they simply don’t bother because of the vagueries of said system). The current “best guess” for relationships of the various “kingdoms” are that there are three basic Domains / clades / whatever you want to call the taxonomic level itself: Bacteria, Archaea, and Eukaryota. Among the Eukaryotes one finds animals, plants, fungi and protists [which, upon preview, I see Colibri has already mentioned].
The archetypal Amoeba sp. belong to the taxon Ramicristates within an ill-defined – and probably polyphyletic – group (referred to as “the other protists” at The Tree of Life website), itself placed within the Eukaryota.
Quite so. I should have clarified this. However, people do persist in trying to shoehorn clades into the Linnaean categories. One problem is that every branch point technically demands its own category, and there are just enough categories to go around. So there ends up being a horrible proliferation of Infraorders, Superfamiles, Tribes, and, I dunno, probably by now Mega-Uber-Classes and Fraternal Associations.
This sort of thing I think has caused a certain resistance to Sibley’s cladistic classification of birds based on DNA hybridization, because he ends up having to put penguins in the same order as eagles because he runs out of enough categories to accomodate all his branch nodes.
To say that birds are dinosaurs is about as correct as saying that humans are the same species as the ancestor of all Hominidae. We can point to extinct species with which there are similarities but we don’t have that ancestor around to make a direct comparison.
Birds are birds. At one time, birds evolved from dinosaurs, but we do not have enough extant dinosaurs around to compare them directly.
If one wishes to get cladistic, one will have to abandon the Linnean categories. Instead, there should be an un-rooted diagram that only denotes similarity, not bothering to name branch nodes.