Classification 101

Classification is as important to biologists as student’s names are to teachers. How can people talk about a plant, animal or bacterium if they don’t even know what it’s called or they have different names for it? (And how can an organism be classified if it doesn’t have a name?)

Is the giant panda more closely related to the red (lesser) panda or bears? (See the answer)

The need for a classification system is apparent when you consider the sheer magnitude of the animal kingdom - over 50,000 vertebrate species alone, many without common names. Add invertebrates to the mix, and we have over a million described animal species...and there may be several million more waiting to be discovered. Some authorities estimate there may be 100 million species, including plants, fungi, bacteria and other life forms.

Fortunately, biological classification is very simple.

Just joking! It can actually be amazingly confusing. It can even be hard to figure out what the science of biological classification is called...Taxonomy? Phylogenetics? Systematics?

From top to bottom: Can you name these misnamed North American hoofed mammals? (See the answers)

Does naming animals go hand in hand with classification, or are they two different disciplines? And what about the often fascinating stories behind the discovery of new species? Do these adventures in discovery have any scientific merit, or are they just anecdotal?

I’ve considered myself an amateur taxonomist most of my life, so I thought I was a minor expert on biological classification - until I began researching this article. I quickly discovered that some of the definitions burned into my brain might be inaccurate.

I even thought about coining my own name for the study of the Discovery, Naming and Classification of living things - DisNaCology, more properly spelled disnacology. Unfortunately, that would probably only add to the confusion.

At any rate, GeoZoo is organized in a way that makes it relatively easy to find animals and plants you’re looking for even if you don’t know a lot about classification. So, if you’re one of the many people who find taxonomy boring or hopelessly confusing, feel free to skip this article and continue to the next page in this series, Homes (distribution, habitat, etc.).

On the other hand, if you’re a serious student who really wants to understand living things, then I highly recommend you continue reading. I can almost guarantee it will help you with your studies.

Imagine if species never became extinct. Every species of trilobite, dinosaur, pterosaur and prehistoric cockroach and human that ever lived is still alive today.

That would be incredibly exciting - but also incredibly confusing for taxonomists - people who classify living things.

You don’t have to be a zoologist to know the difference between a mammal, bird and reptile. But mammals, birds and reptiles didn’t just appear out of thin air; they evolved over countless generations.

In other words, there were prehistoric creatures that might be described as half bird, half reptile, for example. They are sometimes referred to as intermediate forms. Since evolution often occurs in relatively rapid bursts, these intermediate forms are typically not well represented in the fossil record. When a fossil of such a creature is discovered, it’s often hailed as a missing link.

When biologists first began classifying animals, they knew little about evolution and prehistoric life. They simply observed that mammals, birds and fish are different from each other and placed them in separate groups. In other words, animal classification was based largely on morphology (anatomy and physical appearance).

Today, the focus has shifted to evolutionary relationships. Thus, North American mice and Australian marsupial mice could not be classified in the same family, even if they looked almost exactly alike. Nevertheless, morphology remains a useful aid, because species that are closely related do tend to resemble each other.

Four Classification Schemes?

That’s right - there are a number of classification schemes to choose from, though most are related. Technically, some can more properly be described as naming, rather than classification, schemes. You may find it useful to use all four in various situations.

And if four isn’t enough, don’t worry - there are still more for you to research.

1. Taxonomy

Taxonomy is the science and practice of biological classification. It uses taxonomic units, known as taxa (singular taxon), organized in a hierarchical parent-child relationship.

As the table above illustrates, insects, people and plants are all eukaryotes (organisms whose cells contain complex structures enclosed within membranes). Insects and humans are further defined as animals (kingdom Animalia), but roses are plants (kingdom Plantae).

After Animalia, the honey bee and human go their own way. Like chimpanzees and blue whales, humans are both chordates (phylum Chordata) and mammals (class Mammalia). Humans and chimpanzees even belong to the same order and family (Primates > Hominidae), but blue whales are cetaceans (order Cetacea).

Rank

So now you’ve been introduced to the seven basic taxonomic levels - kingdom through species - illustrated again on the left. These are the taxonomic levels, or ranks defined by the international nomenclature codes. Domain has become more popular, though still used relatively infrequently, and it isn’t an official rank yet.

There are slight differences between zoology and botany. For example, the animal kingdom is divided into phyla, while plants are divided into divisions.

These taxonomic levels work pretty good for most of the more familiar living plants and animals. But as biologists learn more about the complexities of evolution and resulting relationships, they fine tune things by adding more taxonomic levels, like superorder, suborder, even magna order.

Things get still more complex when we start classifying prehistoric life along with living things. Your perspective can change dramatically when you see classification schemes that group dinosaurs with birds.

Species Names

The most important taxonomic rank is species. Species names differ from names given to other taxonomic ranks in that they consist of two words. Technically, it’s called binomial nomenclature - a fancy term for two names.

The first name is the genus name, the second the species name. For example, humans belong to the genus Homo, so our full scientific name is Homo sapiens. Our prehistoric ancestors have been given names like Homo habilis.

Taxonomic names are traditionally designated in Latin. For example, the American black bear’s taxonomic name is Ursus americanus, while an elk subspecies named for Theodore Roosevelt is known to science as Cervus canadensis roosevelti. Thus, some people refer to taxonomic names as “Latin names,” even though Greek words are sometimes used.

Species names can be confusing. When scientists reclassify a higher taxon - class, order, family, etc. - species names generally remain unchanged. But if a genus is renamed, or a species is placed in a different genus, then its species name changes.

Occasionally, multiple scientific names are mistakenly given to the same species. For example, one biologist might discover a new species of bird (a male), and someone else discovers a female of the same species. They look different, so they are given different names.

Species can also be divided into subspecies, which have three names. For example, the wolf’s species name is Canis lupus, but it has evolved into many varieties, or subspecies, over its vast range. These include the extinct Newfoundland wolf (Canis lupus beothucus) and Great Plains or buffalo wolf (Canis lupus nubilus). Domestic dogs, which are descended from wolves, are classified as Canis lupus familiaris, though they were once regarded as a separate species (Canis familiaris).

Analogy vs Homology

Before people were able to analyze organism’s DNA, biologists relied largely on intuition in gauging taxonomic relationships. The giant anteater looks a lot like arboreal anteaters. They all eat ants and live in tropical America, so they’re presumably related.

In fact, they are related. But what about pangolins - Old World ant-eating animals with a striking resemblance to New World anteaters? The latest research confirms that New World anteaters are more closely related to arboreal tree sloths than they are to pangolins.

To put it another way, related species tend to be similar in appearance, habits and distribution (though there are many exceptions), while unrelated species generally differ in appearance and habits. But, once again, there are some striking exceptions.

Meet two new words - homologous and analogous. Homologous traits are traits inherited from a common ancestor, while analogous traits are typically an example of convergent evolution. (For example, New World hummingbirds and Old World sunbirds evolved to fill a similar niche and are therefore very similar in appearance.)

Blue whales and humpback whales both have flippers because they share a common ancestor, which also had flippers. Thus, whale flippers are deemed homologous. Penguins and sea turtles also have flippers, even though they aren’t related to whales; their flippers evolved independently. Penguin flippers are therefore homologous when compared with other penguin flippers but analogous in relation to whale and sea turtle flippers.

Pop Quiz! The animal at the top - the one with the long tongue - is an arboreal tropical American anteater called a tamandua. Which of the four animals beneath it do you think it’s most closely related to - (from left to right) the pangolin, aardvark, sloth or echidna (aka “spiny anteater”)? (See the answer)

2. Relative Classification

(Above) The width of spindles indicate the number of families for each class of vertebrates over periods of millions of years. Spindle diagrams are typical for evolutionary taxonomy.
(Below) The same relationship, expressed as a cladogram
(Click either image to see a larger image.)

Some purists don’t recognize taxa at all. Many champion an alternative to the traditional rank-based biological classification called phylogenetic systematics, which aims at postulating phylogenetic trees (trees of descent), rather than focusing on taxonomic units.

The best-known form is cladistics. Cladists believe that taxa (if recognized) must always correspond to clades, united by apomorphies (derived traits) which are discovered by a cladistic analysis. The results of cladistic analyses are often represented as cladograms, like the one pictured on the right.

Monophyly, etc.

Imagine traveling back in time and selecting a plant or animal species, then selecting all the groups and species that later descended from it. Such a group is called a clade or a monophyletic group. (A less commonly used synonym of monophyly is holophyly.) Monophyletic groups are typically characterized by shared derived characteristics (synapomorphies).

Just as a paramedic isn’t quite a doctor, so is a paraphyletic group not quite a monophyly. Rather, a paraphyletic group consists of a common ancestor and most of its descendants (usually all but one).

Like a paraphyletic group, a polyphyletic group contains only some of a common ancestor’s descendants. However, the missing descendants don’t form one or more monophyletic groups, unlike a paraphyletic group’s missing descendants.

In the illustration above, the yellow circle highlights a group of animals called sauropsids. All are descended from reptiles, making the group monophyletic. In the second picture, we take away birds (Aves), leaving a paraphyletic group - the traditional reptiles. If we highlight the warm-blooded animals (mammals and birds), we have a polyphyletic cladogram. This example might be less confusing if we understand some of the group names. Actinopterygii is just another name for fish (minus sharks and rays, lampreys and hagfish). Tetrapods (Tetrapoda) are animals descended from a four-legged ancestor - amphibians, reptiles, birds and mammals (or all the vertebrates except fish). Amniotes (Amniota) are all the vertebrates with eggs that can be laid on land (or hatched in the mother’s body) instead of in water; i.e., all the vertebrates except amphibians and fish.

Confused? You might think of monophyly as a family portrait of two grandparents (common ancestors) with all their children and grandchildren. If we ask one grandchild - or perhaps all the grandchildren who are boys - to step aside before taking another portrait, we now have a photo of a paraphyly.

If we take a photo of six children with two different sets of parents, we have a picture of a polyphyly.

In cladistics, the key word is relativity. Instead of saying birds belong to the class Aves, period, a cladist is likely to say “birds are a group of animals that evolved from the Archosauria, as did crocodilians.” In other words, crocodiles are more closely related to birds than they are to turtles, even though birds are more similar to mammals in so many ways.

3. Life Science Identifiers

An even newer trend takes us from the relativity of cladograms back to a system that seems even more rigid than the traditional kingdom-species scheme. And as anyone who has struggled with taxonomy - which sometimes seems wishy washy with it’s frequent reclassifications and name changes - can tell you, rigid isn’t necessarily bad.

So welcome globally unique identifiers in the form of Life Science Identifiers (LSID) for all biological names.

The familiar wolf can now be catalogued with the (LS)ID number 6850415, rather than the “scientific name” Canis lupus. Of course, Canis lupus is easier to remember and is even a little more romantic than a sterile numeral.

On the other hand, there are hundreds of thousands of scientific names, the vast majority of which you will never even see, let alone remember. Moreover, many of these names are much longer and far more confusing than Canis lupus.

LSID’s reduce the significance of spelling errors and allow authors to cite names unambiguosly in electronic media. Three large nomenclatural databases (referred to as nomenclators) have already embraced LSID’s - Index Fungorum, International Plant Names Index and ZooBank. Other databases that publish taxonomic rather than nomenclatural data, have also started using LSIDs to identify taxa, notably Catalogue of Life.

On closer inspection, LSID’s don’t really represent a new classification scheme; rather, they simply offer an alternative to the familiar binomial nomenclature, or “scientific names.” Thus, LSID’s ought to work just fine with both traditional taxonomy and cladistics.

Ready for the bad news? Common sense suggests that the experts ought to collaborate on a single system of LSID’s. Instead, there are two or more separate systems. So while some groups recognize 6850415 as the LSID for the wolf, others use a different numeral - or an insanely long string of alphanumeric characters.

4. Pop Groups

If you’re feeling overwhelmed by taxonomy, biological classification or whatever it’s called, you might enjoy an alternate classification embraced by GeoZoo: popular groupings.

Examples include marine mammals, waterfowl and cereal grains.

Marine mammals include all aquatic mammals that live in the sea, regardless of their taxonomic classification. They include cetaceans (whales, porpoises and dolphins), pinnipeds (seals and sea lions), sirenians (manatees and dugong) and marine otters. The polar bear is commonly classified as a marine mammal as well, though it isn’t truly aquatic.

Some pop groups are divided into subgroups. For example, the group food plants includes the subgroups fruit, vegetables and cereal grains. It also overlaps with the group domesticated plants, which is in turn a subgroup of the group domestic species.

Some species belong to multiple groups that don’t overlap so neatly. Thus, the platypus might be considered a member of the groups semi-aquatic mammals, venomous animals (a group including a few other mammals as well as a number of herptiles, fish and invertebrates) and amniotes (a group that includes animals that lay hard-shelled eggs - echidnas, birds and reptiles). Similarly, the snowy owl might be placed in the groups birds of prey and nocturnal birds.

Obviously, the pop group classification scheme isn’t hard core science. It’s very imprecise. Yet popular plant and animal groups can be much easier for the novice to understand. In addition, pop groups can be very educational.

Not all pop groups are really popular (i.e. familiar), but most are easy to understand.

What am I?

What you’ve read is just an introduction to biological classification. In fact, there are many formal and informal ways to classify living things. Consider the table below.

It features five vertebrates - the wolf, ostrich, grass snake, bull frog and king salmon. All have jaws and are therefore classified with the gnathostomata. In fact, all living vertebrates have jaws - except for lampreys and hagfishes, which are borderline vertebrates. However, there were many jawless fishes millions of years ago.

All mammals, birds, reptiles and amphibians are further classified as tetrapods - animals descended from four-legged ancestors. However, some tetrapods have lost their limbs. Thus, only the wolf and frog qualify as quadrupeds - animals that actually have four legs.

Mammals, birds and reptiles aren’t as closely tied to the water as amphibians because they produce a different kind of egg. These animals are classified as amniotes. Living amniotes can be subdivided into sauropsids and theraspids, which can be distinguished by their skull structure. Birds are descended from reptiles and can therefore be classified as sauropsids. Theraspids are all extinct except for one important group - mammals.

We could go on and on, creating classification schemes based on anatomical features, evolutionary relationships or whatever we want to focus on.

If you’re wondering where humans fit in the above table, just look at the wolf column. Everything in that column applies to humans except we aren’t quadrupeds. Rather, we’re bipeds, using two legs for locomotion. Other bipeds include kangaroos, kangaroo rats and many birds. In fact, that sounds like a good pop group category. ;)

Kingdoms

Wow, were you expecting to be hit over the head with so much information about classification, taxonomy or whatever it’s called? If it’s any consolation, biological classification is an endless wrestling match even for scientists.

If you’re a serious biology student, you’All benefit from learning a little about taxonomy, because it really is a huge part of not just biology but everyday language. If you understand taxonomy, you’all understand why zoologists call llamas New World camels and “flying lemurs” are neither.

If you want to learn more about the classification of plants, animals or various vertebrate classes, check out the links on the left. In the meantime, let’s finish this article at the beginning with a brief look at kingdoms.

Inanes recognized two kingdoms of living things, Animalia and Vegetable. Since his time, biologists have renamed Vegetable. to Plantae (plants) and created more kingdoms to accommodate smaller, more primitive life forms - most of which were unknown before the invention of the microscope.

Kelp can probably be considered the biggest truly marine vegetation, yet it’s a type of brown algae (kingdom Chromista), possibly more closely related to microscopic amoebas than to plants.

Today, six kingdoms are commonly recognized. It isn’t hard to guess that kingdoms Animalia, Plantae and Fungi cover animals, plants and fungi (which include many species that are commonly regarded as plants).

Fungi are structurally much simpler than more complex plants and don’t have green leaves; in fact, they don’t have leaves at all.

The other three kingdoms are pretty much restricted to very tiny (many of them microscopic) and relatively simple life forms. In fact, bacteria and protozoans are unicellular, as are many chromistans.

Protozoan include the famous amoeba. Some protozoan cause human diseases, including malaria and dysentery. Kingdom Chromista includes algae, diatoms and kelp and is sometimes classified with Protozoa. Most are aquatic and have chlorophyll.

Of course, everyone has at least a vague idea of what bacteria are; they’re the microscopic “germs” that cause disease - though bacteria also do a lot of good things.

Series: Life 101 (12 articles)