From the tiniest of microbes to the largest of deep sea squids, Planet Earth is brimming with living things. Naturally, as a species whose primary focus is to make sense of the world around us, humanity has spent centuries trying to place the life around us into categories. By doing so, scientists past and present have hoped to learn a little more about how it all connects—and where it all started.
Unfortunately, classifying the millions of known species into neat little categories that make sense with what we know about evolution and geology has been one huge, sometimes frustrating puzzle. Throughout history, researchers have developed and scrapped dozens of hypotheses to uncover the truth about our planet’s “family tree.” Fortunately, these trial-and-error efforts have left us with the relatively reliable classification system we use today: that of domains, kingdoms, phyla, classes, orders, families, genera, and species. For the AP Biology Exam, you’ll need to know how some of these classifications work, as well as understand the characteristics of each kingdom.
Before we get into the kingdoms themselves and discuss how our current methods of classification break down, let’s go back in time and have a refresher on the history of naturalists’ attempts to categorize living things.
Classifying Life: A Brief History
One of the real dynamos of classification was an 18th-century Swedish botanist, zoologist, and physician named Carl Linnaeus. Linnaeus is often referred to as the father of modern taxonomy, as he spent a great deal of time naming and classifying various species (the word taxonomy refers to this systematic naming/classification of species). Throughout his education and career, Linnaeus made several expeditions to various regions of Sweden, during which he collected as many samples of local plants and animals as he could for study.
Ultimately, Linnaeus’s work led him to develop the nested hierarchy of classification that we use today. As mentioned in the introduction to this AP Biology Crash Course, the levels of this hierarchy are as follows:
• Domain (e.g. Eukarya/Eukaryota)
• Kingdom (e.g. Animalia)
• Phylum (e.g. Chordata)
• Class (e.g. Aves)
• Order (e.g. Passeriformes)
• Family (e.g. Turdidae)
• Genus (e.g. Turdus)
• Species (e.g. Migratorius)
We won’t be delving into all of this hierarchy in this particular AP Bio Crash Course, but we will look at the top two levels: domain and kingdom.
This classification system also led to the Latin-based method of naming that we now call binomial nomenclature. Under Linnaean taxonomy, each unique type of organism is identified by a two-word name consisting of the organism’s genus and species. An American Robin, per the examples that are given above, is identified by its binomial name, Turdus migratorius. Its genus is Turdus, while its species is migratorius.
Throughout the years, the number and identity of these classifications have shifted based on the research of the times. In Linnaeus’s work, he initially included a separate category for minerals, but eventually, this was dropped. In the mid-1800’s, the kingdom Protista was developed for organisms that didn’t quite fall into the plant or animal categories.
When the electron microscope was invented in the 1920s, it became clear to scientists that there was a significant set of differences between eukaryotic and prokaryotic organisms. As a result, some bacteria and blue-green algae were placed into their own kingdom called Monera. Over 40 years later, in 1969, Fungi were added as their own kingdom as well.
In the 1990s, scientists again considered reworking the existing kingdoms, proposing eight kingdoms with more unique distinctions between some small organisms and bacteria. In the end, it was decided that the best classification system for current information was in the six kingdoms that we use today: Animalia, Plantae, Fungi, Protista, Archaea, and Bacteria.
Before we get to the kingdoms, it’s important to start at the very top of the classification system: the three domains of life.
Imagine, for a moment, that you’ve been tasked with grouping all living things on the planet into categories. Not only have you just been handed possibly the worst project ever, but you likely have no idea where to start. You start grouping things that look alike and act alike, but things quickly get murky. Ultimately, you might find yourself initially splitting organisms into high-level groups like the current three domains: Archaea, Bacteria, and Eukarya.
Note: occasionally, Archaea and Bacteria are grouped together into the Prokaryota group. For this Crash Course, however, we will look at each domain separately.
If you have a keen eye for language, you might notice that Archaea looks and sounds much like the word archaic, meaning old or ancient. This is relatively fitting, as many of the types of organisms in this domain are extremophiles—creatures that thrive in what other life might consider extreme environments—which might have done well in theearliest atmospheres of Earth. Organisms like halophiles (which live in environments of extreme salinity) and thermophiles (which live in extreme temperatures) are categorized into Archaea.
You’ll see later that Archaea is not just a domain, but a kingdom as well.
As the name implies, the Bacteria domain comprises allbacterial life on the planet, including cyanobacteria (bacteria that are blue-green in color and create their own energy from photosynthesis) and heterotrophic bacteria (bacteria that use organic carbon as food to create energy). These bacteria may also have played a role in the beginnings of life on earth, helping to shift the environment toward one that could support diverse life.
Like Archaea, Bacteria are also its own kingdom.
The domain Eukarya (sometimes called Eukaryota)covers all the familiar eukaryotic organisms we’re used to, including fungi (Fungi), plants (Plantae), animals (Animalia), and protists (Protista). As you’ll see in the following section, the Eukarya domain accounts for the vast majority of kingdoms and is therefore the domain that modern science has been most thoroughly able to describe.
Now, let’s take a look at the specifics of each kingdom. As you can see in the diagram below, each kingdom is expected to have branched off common beginnings. The current predominant idea is that protists and plants were the first eukaryotes, having developed from Archaea and Bacteria. Then, following Protista, fungi and animals evolved.
Image Source: Wikimedia Commons
This is just what it sounds like: the kingdom into which all animals—from tiny rodents to enormous whales and squid—are categorized. Animalia covers both vertebrate and invertebrate animals, and is therefore the kingdom that contains humans.
Like Animalia, Plantae is exactly what it sounds like: the kingdom that comprises Earth’s various plant life. This kingdom does not include things like photosynthetic bacteria or fungi, which may exhibit some plant-like behaviors. Plantae is strictly for plants. It’s pretty straightforward.
Here’s where organisms like mushrooms, yeasts, and molds make their home. The Fungi kingdom consists of various fungal organisms—both those that feed us and those that make us ill.
Protista is the kingdom of microscopic single-celled organisms known as protists. Common examples of protists include organisms like the Amoeba, the Paramecium, and the Plasmodium.
This kingdom contains organisms that are capable of handling extreme temperatures, salinities, and other over-the-top environments. Because these organisms can thrive in such severe situations, it is often thought that Archaea may represent life as it was in Earth’s earliest, harshest environments.
Another obvious name, Bacteria is the kingdom that covers all bacterial life on planet Earth. Well-known examples of bacteria include Escherichia coli (often abbreviated as E. Coli) and Salmonella enterica, both of which may cause food poisoning.
Kingdoms AP Biology Exam Review & Practice
Now that we’ve completed the Kingdoms AP Biology Crash Course, let’s take a quick look at what we’ve learned:
• Carl Linnaeus was a Swedish botanist and physician whose dedication to the taxonomy of life led not only to the prototype of the hierarchy we use today, but also to the current naming convention known as binomial nomenclature.
• Binomial nomenclature is a two-word Latin naming convention for organisms that consists of an organism’s genus and species name, e.g. Turdus migratorius.
• The three domains of life are Archaea, Bacteria, and Eukarya. Archaea consist of small organisms that thrive in extreme environments, Bacteria consist of bacterial organisms, and Eukarya consists of plants, animals, fungi, and protists.
• The six kingdoms of life are Animalia, Plantae, Fungi, Protista, Archaea, and Bacteria. Kingdom Animalia contains animals (both vertebrate and invertebrate).Kingdom Plantae consists of plants.Kingdom Fungi includesthings like mushrooms, mold, and yeast.Kingdom Protista covers single-celled protists (like Paramecia).Kingdom Archaea contains extremophiles, and Kingdom Bacteria is home to bacterial organisms.
Well, that about wraps up this Kingdoms AP Biology Crash Course. Think you’re ready for whatever the AP Bio Exam has to throw at you? Give the following practice question a try:
Q: List the six kingdoms and explain the types of organisms in each.
A: Animalia: all animals, vertebrate, and invertebrate; Plantae: all plants; Fungi: mushrooms and other fungus organisms; Protista: single-celled organisms known as protists, such as Paramecia; Archaea: small organisms that thrive in extreme environments, like halophiles and thermophiles; Bacteria: all bacteria, including cyanobacteria and heterotrophic bacteria.
The protist kingdom is a classification that includes a diverse group of eukaryotic organisms. Typically, protists reproduce asexually via mitosis and range from unicellular to multicellular organisms. In the protist kingdom, there are two main groups: protozoa (which are generally heterotrophic) and algae (which are generally autotrophic). Organisms in the protozoa group include things like amoebas, slime molds and paramecium while common organisms in the algae group include green algae, brown algae, diatoms and euglena.
eukaryotesasexual mitosisprotozoaalgaeamoebasslimemoldsparameciumgreen algaediatomsbrown algaeeugina
When studying classification, it's kind of annoying to teach it because when life started, it forgot to make a textbook, to make things nice and simple and easy for us and instead of just having maybe one kingdom or two kingdoms or three kingdoms, it winds up having this mess and when people started studying they said, "Okay, there's plants and animals" and other people came along and said, "What about things like fungi? They're not really plants 'cause they don't make their own food" and then people eventually came up with a five kingdom system. Well, unfortunately, that is falling by the wayside too but we're left by this mess called the protist. They used to be all gathered together into one big happy kingdom called the protist kingdom but now everybody's saying, "But they're not one big kingdom, they're actually this whole mess of lots of other kingdoms but we're still like to talk about them," so I'm going to go through some of the basics of the protist. The protist can be kind of subdivided into two major groups but with those groups all share is that they're eukaryotes and they reproduce mainly asexually using mitosis. That doesn't mean that they can't do meiosis, some of them do but they usually reserve meiosis which is the correct pronunciation.
They use that primarily for extremely for extreme circumstances because that's sexual reproduction and they figure, "Hey, if I'm reproducing asexually life is good for me so if I call myself asexually, life will be good for my descendants." It's only when life starts to get bad they say, 'My genes aren't good enough, maybe I can try a new combination. It's time to have some sex.' They range in size from unicellular, individual single-celled organisms to multicellular, things like kelp is lumped together originally into the protist or kingdom and they're huge, they can be over 150 feet long.
Now, the two major groups are the protozoa and their name means early animals and they're in general are heterotrophic and then there's the algae which are generally autotrophic. Heterotrophic means that you hunt down other things and then you consume them for their food. Autotrophic means that you generally make your own food but what you'll find and this is one of the reasons why this grouping hasn't really worked out so well is that some of the protozoa, yeah, they'll eat other things but they may also do some photosynthesis and sometimes some of them will eat some of the algae and then use them as reserve photosynthetic organisms inside of themselves while continuing to eat other things. It's kind of freaky weird, so let's take a look at the protozoa.
They are very diverse organization, they include things ranging from amoeba to slime molds to things called paramecium that use little cilia to move around, while within the algae, wide range of things ranging from the green algae which are the ancestors to our modern land plants to the glass cell-walled diatoms which are a major photosynthetic organism in our oceans to things as big and huge and multicellular as the brown algae.
So as you can see the protist kingdom is huge and that's why nobody really uses it anymore except as a matter of convenience.