Science with Spence

Scientists organize all of Earth's life forms into a hierarchy that begins with kingdom and works down into phylum, class, order, family, genus, and species. (To remember these categories, think "King Philip Crossed Over For Gold and Silver—a mnemonic referring to 16th-century Spanish exploration.) In this feature, step into the shoes of a taxonomist and classify three forms of life: a plant, an animal, and a...well, you'll need to figure that one out for yourself. Launch Interactive Try your hand at classifying three life forms and learn about taxonomy.

there are three basic shapes.

Spherical bacteria

little spheres or balls. They usually form chains of cells like a row of circles.

Scientists have found and described approximately 1.75 million species on Earth. Plus, new species are being discovered every day. From tiny bacteria to yeasts to starfish to blue whales, life's diversity is truly impressive! With such a diversity of life on Earth, how does one go about making sense of it all?

One way to make sense of it is by classification. Scientists put similar species into groups so that those millions of species do not seem so overwhelming. People rely on their knowledge of classification to understand what different species are like.

The figure at the left shows the three domains of life. The distance between groups indicates how closely related they are. Groups that are close together, like plants and animals, are much more closely related than groups that are far apart, like plants and bacteria. Do you see how the two types of microbes, Archaea and Eubacteria, are about as similar to one another as they are to animals? Recent studies have found that microbes are far more diverse than anyone had suspected.

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Generally-speaking, we humans have a desire to label and categorize things

We name objects because it makes our life easier. Let’s say you’re sitting on the sofa and you want your friend to pass the remote so you can see what else is on the TV – this process is rather difficult without names. A request like, “Please pass the thing on the thingy. I want to see what’s on the whats-am-a-jig”, is likely to meet with confusion. The request is easier for the other person to follow if things have names: “Please pass the remote on the coffee table. I want to see what’s on the TV”. Now, it’s true that you might be able to gesticulate at your friend until he or she either gets the idea, or misinterprets and takes offence, but what if you can’t see the person you need help from – charades doesn’t help then. Imagine that you’re sitting on the train going to work when you remember you forgot to get the pie out of the freezer to defrost in time for dinner; fortunately your partner has the day off and is at home. So, you phone up and ask “Can you get the thing out of the thingy so it’s thingy-ed in time for what’s-its-name?” 

So, the act of naming is a matter of convenience – whether the objects are pieces or furniture, bits of machinery, or animals we assign them names because it makes life a heck of a lot easier for us. We, for example, call a ‘fish’ with a cartilaginous skeleton and between five and seven pairs of gills a “shark”. This allows us to tell another person what animal we’re looking at or talking about. The use of a name certainly helps, but not without problems. Telling someone that you went diving with sharks while on holiday is kinda like saying you went out for dinner with some primates; it’s not quite as specific as we might want because there are lots of different ‘types’ of primates (and sharks). Consequently, to make our meaning as clear as possible, objects (be they animals, plants, bacteria, furniture, tools, etc.) are split into as narrow groups as possible and each group is given a name. So, for example, the group of ‘fish’ we call sharks gets further split up into different types of sharks based largely on how they look (their “morphology”), both internally (i.e. their skeleton, internal organs etc.) and externally (i.e. fins, gills, skin, colour etc.). Large groups are then split into smaller (i.e. more specific) ones and so on down the line until you have a group containing all the animals considered to be exactly the same in terms of the features we’re looking at (these can be morphological, genetic, ecological, biochemical, even behavioural): this is the species level (we’ll look at this in more detail later). Humans, chimpanzees, great white sharks, blackbirds, palmate newts and red squirrels are all examples of species. Some taxonomists opt to take the splitting below the species level and group animals into subspecies, infraspecies and forms (among others). Perhaps the extreme of this splitting is found in the human species, where every individual of the species is given his/her own name at birth. The problem is that this gets very complicated very quickly as the list of viable names soon runs out and leads to the confusing situation of several individuals with the same name – think how confusing it can be if there are two or three people in the office with the same name. Consequently, the branch of Science known as “Taxonomy” (from the Greek word taxis, meaning “order” or “rank” and –nomia, meaning “law”) is largely concerned with the grouping of organisms down to the species level.