Topics in Depth
The Theme of History of Taxonomy in Taxonomy
Standing in line at the grocery store, we get the opportunity to see what latest boy band member is dating [insert new TV star's name here]. While deciding between brands of gum, you take a moment to decide if you ship the couple or not. Do they belong together? Probably not. It's probably best to just think about the gum.
Taxonomy is all about relationships, and relationships create groups. Taxonomists look for what one organism has in common with another and try to figure out the relationship between them. Based on that relationship, the organisms are grouped together in ways that help us make sense of the world.
In order for an organism to be properly classified, we need to know a lot about it. What we know about the world around us has been increasing greatly over the last 200 years. As modern technology has flourished, our ability to understand the relationships between organisms has improved dramatically.
It is part of human nature to analyze the world around us. Way back in the 300s B.C. lived the great Greek thinker named Aristotle. Most people would call him a philosopher, and he definitely was, but he was a man of many interests and talents. He didn't just think about things in the abstract, he carefully and thoughtfully observed the world around him. What he saw was a hierarchy of organisms, which he called the "Ladder of Nature". Different organisms showed different levels of complexity and abilities to thrive.
Aristotle's vision of the hierarchy of beings, his "Ladder of Nature."
Since there were no microscopes back then, he had no idea that little one-celled organisms exist. Among the organisms that he could see, he saw two major groups that he called kingdoms: plants and animals. Plants were usually green and stationary. They could reproduce and grow. However, their environments determined whether or not they thrived. For example, without rain, most plants shrivel up and die.
Aristotle thought of animals as higher-level organisms because they move around to search for food or to escape predators, and they experience sensation. These characteristics allow them to survive better than plants. They can leave an environment that is no longer providing for their needs. For example, zebras can look for a new source of water if the nearest watering hole dries up.
Aristotle categorized humans as the highest level of organism. In addition to possessing all of the characteristics of animals, humans also have the ability to think and choose and create.
Within the animal kingdom, Aristotle further divided animals into categories based on their anatomical and physiological similarities and differences. The highest level divided animals into those with and without blood. These two categories line up very well with the ones we use today to distinguish animals with bony backbones (vertebrates) from those without (invertebrates). Animals with blood were then subdivided into those that gave birth to live babies (mammals, including humans) versus those that laid eggs (birds and fish). Animals without blood were subdivided into insects, mollusks, and crustaceans, which were then divided into those with and without shells.
Aristotle was so insightful in the way in which he categorized organisms that many of his categories are still used today, 2,300 years later.
For many years, there was no common way of naming organisms. Each scientist had his own naming style based on the characteristics of the organisms that he thought were most important. Since Latin was the language of scholars, most organisms were given a group name (such as vegetable, flower, or herb) and then a descriptive name that could be long. For example, the wildflower species Plantago media used to be called Plantago foliis ovato-lanceolatus pubescen tibus, spica cylindrica, scapo tereti.
The situation improved considerably when a Swiss botanist (a scientist who studies plants) named Gaspard Bauhin began to make the names of plants less descriptive. However, the diversity on Earth is difficult to organize, and there is always room for improvement. One hundred years later, Bauhin's work would be picked up and improved upon by a Swedish botanist who understood this difficulty and decided that names could be short and random as long as they were unique and everyone knew which organism they named.
Nicknames usually poke a little fun at someone's personality, physical attributes, or habits: Babe, The Flying Tomato, Big Baby. Carl von Linné was no exception. Most people think of him as the father of modern taxonomy, but they know him by a different name. It is the Latin form of his name that he used for all of his academic writings: Carolus Linnaeus.
Linnaeus was a Swedish botanist who lived in the 18th century. He gave himself the huge task of creating a uniform system for naming all living organisms. At the time, Latin was the universal language of scholars. Because he wanted to fit in with the cool kids, Latin is the language he used. He created a hierarchal classification of organisms with 6 levels, or taxa. He started with the category of greatest diversity and worked his way down to the smallest category: Kingdom, Class, Order, Family, Genus, and Species. (Beware! The category Phylum was not included in Linnaeus's lineup.)
At the very top of the tree were 3 Kingdoms: Minerae (Minerals. Don't worry. He didn't think minerals were living organisms; he just liked to organize things. Give the man a break.), Plantae (plants), and Animalia (animals). Within each kingdom were several classes; within each class were several orders; within each order were several families; within each family were several genera (the plural spelling of genus); within each genus were several species. Every organism (and rock) belonged to a kingdom, class, order, family, genus, and species.
Much of Linnaeus' classifications have changed as we have learned more about the life around us. Even so, we still think he was great. First of all, we give him credit for applying Aristotle's hierarchical concept to many more organisms at a much more detailed level. By 1758, Linnaeus had named and classified 4,400 species of animals and 7,700 species of plants. Reality TV wasn't around in those days, so he had plenty of free time.
Most significantly, though, he decided that each type of organism should be identified with only two words: the name of the genus and species to which it belonged. This naming system is called the binomial system of nomenclature and we still use it today. He was so well known for his binomial nomenclature that he was sometimes called Princeps botanicorum (the prince of botanists).
Big Baby doesn't sound so bad, after all.
Darwin Does It Again
What is remarkable about all of the early taxonomists is that they had only crude ways of comparing creatures for the sake of classification. They were also operating with a bland understanding of living things. Taxonomists believed that there was a set number of species on Earth and that the species did not change over time. When they put two groups of organisms into the same taxon, they did so just for the sake of organizing things. They thought that the two groups happened to share some similar characteristics. They didn't think that the two groups were related to each other. Enter Darwin. Not this Darwin.
That all changed dramatically in 1859, when Charles Darwin published his book The Origin of Species. At the time, Darwin's theory of evolution was radically different from the way most people saw the world. Today, evolution is generally accepted among scientists from every field.
Darwin saw that all populations of organisms exhibit variations among individuals. Some traits provide advantages for surviving in a particular environment. Others create disadvantages. As an environment changes, the individuals that happen to be best equipped for those changes thrive and reproduce abundantly while the individuals that are poorly adapted die off. Over time, the original traits of a population shift, creating a population whose characteristics are better suited to their new environment. Over hundreds and thousands of years, these small changes accumulate until the existing population is so different from the original one that they can no longer reproduce with each other, indicating the evolution of a new species. Thus, every currently existing species is related to a previously existing one.
Although Darwin's theory did not immediately alter taxonomic classifications, it drastically altered the significance of taxa. If two groups of organisms shared similar characteristics and were placed in the same taxon, a hypothesis was automatically generated: the organisms are probably related to each other evolutionarily; they probably shared a common ancestor.
While Darwin's theory was essentially correct, he didn't understand how it actually worked. That was for someone else to figure out.
Around the same time, a monk named Gregor Mendel was working hard to understand how characteristics are inherited from one generation of organisms to another. We now recognize him as the father of modern genetics. He didn't know about DNA but he knew that there had to be some entity inside reproductive cells from the male and the female parents that carried information. The combination of that information in the new organism determined its characteristics in predictable ways.
Although Darwin and Mendel were unaware of each other's work (although we're sure they would have been BFFs), Mendel's theory of genetics provided Darwin's theory of evolution with the information it was lacking. Not only are genes the vehicle for the inheritance of traits—mutations in genes provide populations with new variations in their traits.
Because genes are the fundamental unit of inheritance and variation in individuals, the genetic sequences of closely related individuals are similar. The more closely related two individuals are, the more similar their genetic sequences will be. That is why paternity testing makes for excellent television.
In the realm of taxonomy, scientists eventually learned to use genetic sequences to determine how closely related two different kinds of organisms are evolutionarily.
As you have seen, science is a constantly developing body of knowledge. As more and more organisms were discovered and classified, taxonomists realized that some of Linnaeus' taxa no longer fit what they knew. For example, with the invention of the microscope, tiny organisms were discovered that had never been seen before. Some were green like plants but swam like tiny animals; others created patches of bright yellow "fuzz" that is lovingly referred to as "dog vomit slime mold". Others were single-celled bacteria in all kinds of different shapes, sort of like pasta varieties. In 1866, the German biologist Ernst Haeckel suggested that it was time for a new kingdom, and Kingdom Protista was born. There had only been the Kingdoms Plantae and Animalia since the time of Aristotle 2000 years earlier. The system was way overdue for an update.
Dog vomit slime mold, a protist. Image from here.
The next big development in cell biology didn't result in new taxa right away, but was even more fundamental than the introduction of protists. In 1937, a French marine biologist named Edouard Chatton realized that all cells could be divided into two categories based on whether or not they had a nucleus. Cells without a nucleus were called prokaryotes, meaning "before nucleus;" those with a nucleus were called eukaryotes, meaning "true nucleus." Prokaryotes are always single-celled (unicellular) organisms. Eukaryotes can be unicellular or multicellular. Like people.
In 1969, the scientist R.H. Whittaker proposed two new kingdoms. He thought that "plants," like mushrooms, that don't photosynthesize, don't reproduce through seeds, and get their energy by absorbing it from other (dead or living) creatures should be separated from the rest of the Kingdom Plantae. Whittaker introduced Kingdom Fungi. He also proposed Kingdom Prokaryotae to include all of the prokaryotes, which were all considered to be bacteria.
To recap, by 1970 we had five kingdoms: Animalia, Plantae, Protista, Fungi, and Prokaryotae.
In the mid-1990s, an entirely new level was added to the hierarchy at the very top. This happened because scientists learned that the Kingdom Prokaryotae contained two very different types of organisms. Using genetic comparisons, a scientist named Carl Woese suggested that there were "true" bacteria and "ancient" bacteria. The "ancient" bacteria had different structures than "true" bacteria, had very different ways of producing energy, and often could withstand extreme environments, like those found in the super-hot, super-deep ocean vents.
Woese suggested that there are three major branches in the tree of life: "ancient" bacteria, "true" bacteria, and everything else (eukaryotes). This idea didn't catch on until Carol J. Bult genetically confirmed twenty years later. We now call these three major branches Domains. Domains are broader than kingdoms. The domains are Archaea, Bacteria, and Eukarya. The Kingdom Prokaryotae was kicked to the curb. It was divided into two kingdoms (Archaea and Bacteria), instead. The Kingdoms Protista, Fungi, Plantae, and Animalia are all neatly fined under the Domain Eukarya.
That, dear Shmoopers, is the basic hierarchy that we use today. When taxonomists need another level, they don't usually add a new term, like "domain," they just call it a "sub-whatever." Sometimes there are categories like subphyla, subclasses, and subwoofers. Just kidding on the subwoofers. To remember the order of the taxa, make up a mnemonic device like this one: Dear Karen Put Carrots On the Front Garage Steps.
Every species has a particular representative that is used by scientists as the model citizen. If you find a beetle that you've never seen before and want to decide if it's a new species, analyze all of the beetle models and see if yours is unique. If so, name it something awesome. We're thinking Beetleus shmoopeum. These models are called lectotypes. Do humans have a lectotype? Yup. Carolus Linnaeus has been named the lectotype for the species Homo sapiens.
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