Study Guide

Taxonomy - Systematics

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Guess how many different species of living critters there are in the world today. Taxonomists know of about 1.8 million, but they think that there may be several million more that haven't been identified yet. That's a lot of critters to keep track of. How do scientists know how to arrange them all? The study of the diversity of organisms and their relationships is called systematics. (It takes a whole system to keep them organized.)

Classifying Organisms

A Rose by Any Other Name Would Smell as Sweet: What's in a Name?

When you looked around your classroom on the first day of school, what stood out was probably everyone's differences. You probably noticed many different colors of skin, hair, and eyes. You saw kids of different heights and weights, with noses and ears of different shapes, and maybe you thought that we are each really different from each other. And it is true that each person is totally unique and unrepeatable, but, at the genetic level, individual human beings are extremely similar to each other and only differ by 0.1%!

We are all part of the same species of organisms called Homo sapiens. Homo means "man;" sapiens means "wise." Homo sapiens is our full species name; sapiens is our species identifier. All species names are always written as if they were Latin (sometimes called Neo-Latin). Using a Latin naming scheme provides unity among scientists all over the world; we all use the same language and the same names.

If you know a little Latin, you can learn a lot about an organism based on its species name and classifications. For example, there's a beetle named Agra schwarzeneggeri. What might we expect in this beetle? Rippling biceps, like those sported by Arnold? Yup. Neo-Latin at its best.

Agra is a genus of beetles that is made up of nearly 600 different species, including the species Agra vation.

Some names tell us about the organism; others hint at a story about the scientist that chose the name. What is most important is that everyone uses the same name for each unique species. Shakespeare knew that "a rose by any other name would smell as sweet." However, if some people call it a rose and others call it a mosquito, chaos would ensue.

A Rose of Any Other Color Smells as Sweet: What's in a Characteristic?

You're backpacking up volcanoes in the Galapagos Islands and you come upon a pink iguana. (Truth is stranger than fiction, people.) You are quite sure that this is a lizard that has never been named before. That means you get the great privilege—and challenge—of properly categorizing it and giving it a unique name. Where do you begin?

Your buddy says, "Well, it's pink. That is a very important part of its identity. There aren't many animals that are pink. Flamingos are the only other ones that I can think of. Maybe it should be classified with the flamingos."

Nope. Not a good plan. Just because a characteristic is very visible or very unique does not mean that it is very important to the organism's identity. If we created a classification system based primarily on color, we would have to put pink iguanas, pink flamingos, and pink roses in the same group. Beyond their color, we all know that those 3 organisms are very different. Detailed characteristics like color only factor into taxonomic classification at the more detailed levels of the hierarchy, especially at the species level.

For more on the pink iguana story, check out the Pink Iguana section at the end of the page.

Color isn't a good starting place for taxonomists, but there are other characteristics that organisms share that are very crucial to their survival. These characteristics can tell us more about their place in the tree of life. Why? Evolution.

Taxonomy is about grouping similar organisms together, and evolutionary adaptation is what creates new similarities and preserves old ones. Similarities express evolutionary relatedness and relatedness is what makes us group organisms together.

Since evolution works through the survival of the fittest, the characteristics that most affect survival are the ones that matter when classifying organisms. For example, body structure , habitat, diet, and mode of reproduction all tell us more about an organism than whether or not it is pink. Genes, directly or indirectly, guide all of these characteristics. Genes are what allow them to be passed on to future generations and are affected by natural selection.

Characteristics to Assess for Proper Classification

1. Cell type is probably the most fundamental characteristic to assess for proper classification. Whether or not an organism is prokaryotic or eukaryotic is crucial to its classification at the domain level. Which structures are inside and outside of the cell is also important. Even technical details, like which molecules are found in the organism's cell wall, can give important clues. Most detectives can get away with just a magnifying glass; scientists need a microscope for their sleuthing. For sure, Sherlock Holmes would have made a good taxonomist.

Related to cell type is body organization. How complex is the organism? Organisms can sometimes be single celled. Others exist as single cells but can sometimes form groups in order to work together for survival. Animals are complicated, beginning as a single cell and developing into an entity with organ systems that are made up of organs, which are made up of tissues, which are made up of individual cells all working together to maintain the health of the organism.

2. Body structures like feathers, scales, skin, and bones are your next big organism indicator. Body structures are extremely important for the survival of an organism in its environment. They tell us a lot about how two different kinds of organisms are related. If they both have bones, for example, they probably both evolved from a common ancestor with bones. We can also compare the shape, size, and number of bones to find similarities. All mammals have three little bones in their inner ear, indicating that they are probably related.

3. Habitat gives us more information. A single-celled critter residing in a hot vent on the ocean floor probably belongs to the Domain Archaea. Organisms that live in very unique environments are suited to survive in those environments in unique ways. Life in the desert vs. life in Antarctica vs. life at high altitudes in the Andes all present populations with particular challenges that they must adapt to—or else go extinct.

4. Diet is often related to habitat. Organisms need to be able to obtain energy and molecular building blocks from their surroundings, whatever they may be. Some organisms are self-sufficient and are able to make organic molecules on their own out of inorganic molecules. Most of them get their energy from the light of the sun. Other organisms have to get their organic molecules and energy from other organisms, either by living in them (as a parasite), eating them, or breaking them down after they have died to absorb their nutrients.

5. An organism's mode of reproduction also tells us a lot. Reproducing asexually by splitting in two is more primitive than reproducing sexually. It does not create as much genetic diversity and does not require any specialized cells or organs. Organisms that do reproduce sexually have many different ways of doing so. The fertilization of the eggs might take place outside of the female if the male and female deposit the sperm and eggs in the water. Fertilization might take place within the female, after which she lays eggs on land with a hard or leathery shell. Fertilization and the development of the young might take place within the female, culminating in a live birth. Those are just the 3 most common options. Don't ask about the others.

6. Speaking of development, scientists have observed that animals with similar developmental patterns are related to each other evolutionarily, too. This has given rise to a whole field of biology nicknamed EvoDevo, or evolutionary developmental biology. The body structures and other adaptations of adult animals have to develop over a period of time. At first, they don't look like their future selves. They are in a smaller, simplified form; the differences visible in adulthood haven't developed yet. In fact, it can sometimes be difficult to distinguish embryos of different species from each other.

7. The last characteristic that scientists compare is the behavior of organisms. Because behavior can often help or hinder survival, it is also selected for evolutionarily. How social an animal is with other members of its species is one pattern to look for. Some fish, for example, always swim in schools because there is safety in numbers. Certain species of sharks, however, tend to roam through the ocean on their own, only meeting up to breed.

Mating rituals, preening, care for young, aggression, sleeping habits and reactions to danger are other types of behavior that scientists monitor. All of these give them clues about which organisms are related.

Since each of these seven characteristics is genetically determined, an organism's genome (or entire genetic sequence) provides a major tool for analyzing relationships. The genome of each organism is the raw material for evolution. It is where mutations occur, which add variety to a population, which allows for competition and natural selection.

Pink Iguanas

What should we call our pink iguana? We've actually already solved most of our classification problem by calling it an iguana. All iguanas are part of the family Iguanidae, the order Squamata, the class Reptilia, the phylum Chordata, and the kingdom Animalia. We know it's an iguana because of the unique characteristics that this lizard shares with all other iguanas. They don't share their colors in common but they share similar body structures, habitats, diets, and ways of reproducing. These characteristics are much more important to the identity of an organism than color.

We have our pink iguana classified down to its family. Does it need a new genus or is it part of an existing one? There are only eight living genera of iguanas, and two of those live in the Galapagos. One genus (Amblyrhynchus) lives in the water. We know that genus is not it. We found this iguana at the top of a volcano, so it might be part of the land dwelling Conolophus genus. When we compare it to other Conolophus iguanas, especially at the genetic level, we see their similarities and decide that this is the right classification. Furthermore, we determine that this iguana is its own distinct species.

Now for the fun part. We get to choose the species identifier. Back to reality—since we didn't make up this story, we won't get to make up a name. Dr. Gabriele Gentile first discovered this iguana in 1986. She named it after her daughter Martha. Our newly discovered pink iguana is known around the world as Conolophus marthae. Would you like to have a new iguana named after you?

Brain Snack

Hilarious species names: an excellent excuse to become a taxonomist.

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