Lesson Explainer: Classification Biology

In this explainer, we will learn how to name and classify organisms and describe the importance of developments in biology to the classification system.

To you, my dearly beloved botanists, I submit my rules. If they seem to you worthy, let them be used by you also; if not, please propound something better! (Carl Linnaeus)

Carl Linnaeus sculpture-72

Figure1:AphotographshowingthestatueofCarlLinnaeus.

When we talk about biological classification, we are almost guaranteed to mention one name: Carl Linnaeus. Linnaeus was a Swedish botanist who, in the 1700s, developed a method of classifying and naming organisms that we still use today. However, scientific minds had been considering the problem of classifying organisms for millennia already. The famous Greek philosopher Aristotle had published work as early as the 4th century BCE separating all life into two categories: plant and animal. Aristotle further classified animals into two groups, blooded and bloodless animals, depending on whether or not they possessed red blood. Aristotle is also sometimes credited with classifying plants into trees, shrubs, and herbs, but this was actually the work of Theophrastus, one of Aristotle’s students.

The science of classification has progressed rapidly since then. Over one million different species of organisms have been identified on our planet so far. Some experts think there may still be 7 million more left to discover. The study of classification, also called “taxonomy,” allows us to efficiently gather and exchange information about different species as our scientific knowledge expands.

Fact: Carl Linnaeus (1707–1778)

Carl Linnaeus was a Swedish botanist who is most remembered for his contributions to taxonomy, specifically developing the system of binomial nomenclature.

Key Term: Biological Classification

Biological classification is the organization of species into groups, called taxa, based on meaningful similarities. These taxa are then ranked from general to specific.

Definition: Taxonomy

Taxonomy is the study of classifying and naming organisms.

Example 1: Defining Biological Classification

Which of the following is the correct definition of classification?

  1. Classification is the organization of living things into groups that have similar features.
  2. Classification is the organization of living things into groups that have different features.
  3. Classification is the organization of dead things into groups that have similar features.
  4. Classification is the organization of organisms into large, mixed groups.
  5. Classification is the organization of organisms into groups that inhabit the same area.

Answer

Biological classification is the organization of species into groups based on meaningful similarities. These groups are then divided into smaller, more specific subgroups. Each subgroup provides additional information about the species that are contained within it. Scientists classify organisms to make them easier to study and to more efficiently share information. The study of biological classification dates back to a time before biologists fully understood genetics. Initially, scientists classified organisms based only on physical characteristics. This is now referred to as artificial classification. Later, as genetic science became more advanced, organisms were classified based on their genetic and evolutionary relationships. We call this natural classification.

From this information, we can conclude that classification is the organization of living things into groups that have similar features.

The method of naming organisms that Carl Linnaeus developed is called “binomial nomenclature.” Binomial means “two names,” and nomenclature is a term that means “naming system.” Linnaeus was a botanist, so he studied plants. At the time, species of plants were often given long, Latin names that described various parts of their anatomy. Since there were many names involved, this was called “polynomial nomenclature” (“poly-” means “many”). For example, the herb commonly known in English as “catnip” had the polynomial name Nepeta floribus interrupte spicatus pendunculatis. This type of name is hard to remember, and each newly discovered species would have to be named individually. Using binomial nomenclature, each different species is given a name that consists of just two parts, so now catnip is simply called Nepeta cataria.

Definition: Binomial Nomenclature

Binomial nomenclature is the scientific system of naming species using their two most specific taxonomic ranks, for example, Homo sapiens.

Not only does binomial nomenclature simplify the naming of species, it also gives us a clue as to their classification. Linnaeus is also credited with popularizing the current system of taxonomic hierarchy. In this classification system, there are seven taxonomic ranks, ordered from most general to most specific. The seven taxonomic levels are: kingdom, phylum, class, order, family, genus, and species. More general taxa, or groups, each contain several of the more specific taxa. This means that every organism is a part of a species, that species is a part of a genus, that genus is a part of a family, and so on. Each grouping tells you something about the characteristics of the organism within it. The taxonomic hierarchy is illustrated in Figure 2.

The binomial name of a species consists of two terms. The first term is the name of the genus that the species belongs to. The second term is the designation given to the species itself. So, catnip belongs to the genus Nepeta and cataria is the species. Genus is a taxonomic rank more general than species. Several different, related species will share the same genus name, but the combination of the genus and species will be unique. It is similar to how you may share a family name with many different, related people, but the combination of your family and given name is what identifies you as an individual. Figure 3 illustrates the difference between a common name and a scientific name of an organism.

Example 2: Using Binomial Nomenclature to Determine an Organism’s Species and Genus

The binomial naming system was developed by Carl Linnaeus. Each organism has a Latin name consisting of two parts, its genus and its species.

  1. The binomial name for a lion is Panthera leo. What is a lion’s genus?
  2. The binomial name for a Venus flytrap is Dionaea muscipula. What is a Venus flytrap’s species?
  3. The binomial name for humans is Homo sapiens. What genus do we belong to?

Answer

Part 1

Binomial nomenclature is a two-name naming system developed by 18th century botanist Carl Linnaeus. At the time, plants were given long, polynomial names that described certain parts of their anatomy. These names were inconsistent and hard to remember. In Linnaeus’s system, the binomial name for an organism consists of the genus name and the species name. The more general genus name is written first and the species name is written last. The scientific or binomial name is usually printed in italics and the genus name is capitalized.

Knowing this, we can conclude that a lion’s genus is Panthera.

Part 2

As explained in the answer to the previous part, the scientific name of an organism is a binomial name, which means that it is made up of two words, or names. The first word is the name of the organism’s genus and the second is the name of the organism’s species. So, a Venus flytrap’s species is muscipula.

Part 3

The third part of the questions asks about the genus that humans belong to. The scientific name of the human is Homo sapiens. As we know now from the answers to the previous two parts, the first word in the scientific name of an organism is the name of its genus. So, the genus that humans belong to is called Homo.

When Linnaeus proposed his system of classification, he divided life into two kingdoms: plants (which he called “vegetables”) and animals. Later, in 1975, a scientist named Robert Whittaker proposed five kingdoms among which all life on Earth can be classified. These kingdoms, shown in Figure 4, are Prokaryote (formerly called “Monera”), Plant, Animal, Fungus, and Protist.

Definition: Kingdom

Kingdom is one of the most general taxonomic ranks. A kingdom includes many different species that are divided into phyla, classes, orders, families, and genera.

Each kingdom contains a wide variety of different species that all share some general, common traits. The prokaryote kingdom includes all prokaryotic organisms, or single-celled organisms that do not have a nucleus to protect their DNA. The remaining 4 kingdoms all consist of eukaryotic organisms, or organisms made of cells that contain a nucleus.

Plants are multicellular autotrophs, meaning that they make their own food from sunlight. Animals are multicellular heterotrophs, meaning that they have to collect and eat food for energy. Fungi are also generally multicellular heterotrophs, but they are stationary like plants instead of moving around like animals. Finally, protists are the eukaryotic organisms that do not seem to fit squarely into any of the other 3 kingdoms. They can be multicellular or unicellular, autotrophs or heterotrophs. A summary of the traits of the 5 kingdoms is shown in Table 1.

A kingdom is a general taxonomic rank that contains many different species. A species is a group of closely related organisms that can reproduce together and bear fertile offspring. For example, all domestic dogs belong to the same species, even though their appearances vary widely. A domestic dog is able to breed with a wild wolf and their offspring will be fertile, meaning that it can have offspring of its own. This tells us that domestic dogs and wolves are actually the same species. In contrast, some closely related organisms can reproduce together, but their offspring will be infertile. A common example is a mule, which is the offspring of a horse and a donkey, as shown in Figure 5. Mules are infertile, meaning that they cannot produce offspring of their own. This is because horses and donkeys belong to the same genus, but are different species.

Definition: Species

Species is a group of organisms with similar characteristics that can breed together to produce fertile offspring.

Example 3: Defining the Term Species

Which of the following best describes a species?

  1. A species is a group of similar organisms that can reproduce.
  2. A species is a group of organisms that had a common ancestor in the last millennium.
  3. A species is a large group of organisms that inhabit the same ecological space.
  4. A species is a group of organisms that can breed to produce fertile offspring.
  5. A species is a group of organisms that are genetically identical.

Answer

In our current system of taxonomy, species is the smallest and most specific rank. The taxonomic ranks include, from largest to smallest, kingdom, phylum, class, order, family, genus, and species. A species is a group of closely related organisms. This means that they tend to have very similar traits with limited variations. But a genus also contains closely related organisms. How do scientists know that organisms are closely related enough to be considered a species? Well, closely related organisms are able to reproduce together. Yet some organisms, such as a lion and a tiger, are able to reproduce together but are different species. However, the offspring of a lion and a tiger will not be able to reproduce and have offspring of their own. A species includes organisms that are not only able to generate offspring, but also their offspring can have offspring.

So, a species is a group of organisms that can breed to produce fertile offspring.

Members of the same species are more closely related than members of the same genus or members of the same kingdom. When we say that organisms are “closely related,” we mean they have more DNA in common, which means they share more similar traits. For example, chimpanzees are more closely related to gorillas than they are to chickens, shown in Figure 6. This is because chimpanzees have much more DNA in common with gorillas than they do with chickens. This is also the reason that members of the same species produce fertile offspring, but members of different species do not. Because of our understanding of evolution, we know that being more closely related also means that two species share a more recent common ancestor.

Classification based on physical traits is referred to as artificial classification. Artificial classification is the basis of systems devised by both Linnaeus and Whittaker. At the time, scientists did not have a true understanding of genetics. Today, we know that our DNA carries many genes and that these genes control our traits. Genes also show us evolutionary relationships between different species that cannot be determined from physical characteristics alone. Classification based on genetic similarity and evolutionary relationships is called natural classification. Modern taxonomy uses natural classification for its basis.

Definition: Natural Classification

Natural classification refers to the classification of organisms based on genetic analysis and evolutionary relationships.

Definition: Artificial Classification

Artificial classification refers to the classification of organisms based only on observable physical characteristics.

Example 4: Contrasting Natural and Artificial Classification

Biological classification systems have changed over time. Previously, biologists used artificial classification to group organisms, but natural classification methods are now much more common.

  1. Which of the following describes artificial classification?
    1. Grouping organisms together using observable characteristics
    2. Using DNA sequencing to group organisms based on evolutionary links
  2. Which of the following describes natural classification?
    1. Grouping organisms together using observable characteristics
    2. Using DNA sequencing to group organisms based on evolutionary links

Answer

Part 1

Biological classification is the organization of species into groups based on meaningful similarities. These groups are then divided into smaller, more specific subgroups. Scientists classify organisms to make them easier to study and to more efficiently share information. The study of biological classification dates back to a time before biologists fully understood genetics or the study of DNA and how it affects the traits of organisms. Initially, scientists classified organisms based only on physical characteristics. This is now referred to as artificial classification. We call it “artificial” because physical characteristics do not always guarantee that organisms are closely related or that they are not. Later, as genetic science became more advanced, organisms were classified based on their genetic and evolutionary relationships. We call this natural classification. It is called “natural” because it is based on the actual relationships between organisms, verified using genetic analysis instead of guesses based on observations.

According to this information, the statement that describes artificial classification is “grouping organisms together using observable characteristics”.

Part 2

Referring back to the brief explanation and comparison in the previous part, we will understand that natural classification looks into the evolutionary relationships between organisms rather than only focusing on the similarities and differences between organisms in terms of physical characteristics and appearance. Also, scientists would apply the field of genetics to study the genetic material (DNA) of organisms to better understand these evolutionary relationships and produce more accurate classifications.

Given this information, it is safe to conclude that the statement that describes natural classification is “using DNA sequencing to group organisms based on evolutionary links.”

One important development from the advent of genetic analysis is the use of an 8th taxon that is even more general than kingdom. In 1977, a scientist named Carl Woese used genetic analysis to examine ribosomal RNA which led him to determine that archaebacteria, formerly classified as bacteria, in fact belonged in a separate taxonomic group. Using this phylogenetic information, scientists have since divided all the organisms on the planet into three domains: Bacteria, Archaea, and Eukarya (Eukaryota), as displayed in Figure 7. Based on genetic analysis, scientists have learned that prokaryotes fall into two distinct groups: bacteria and archaea. All the kingdoms within Eukarya are understood to share a more recent common ancestor with archaea than with bacteria.

Taxonomy is an area of science that is continually progressing. Scientists use biological classification to share information about known species, identify new organisms, and classify organisms that have already gone extinct. It is a science that allows us to efficiently compare and contrast the traits of various organisms, organize knowledge, and share information. While the overall system Linnaeus developed is still used, many aspects have changed over time as we learned more from genetic analysis.

Key Points

  • Classification is the organization of living organisms into groups based on shared or similar features.
  • Organisms are named by their genus and species according to the system of binomial nomenclature.
  • The 5 kingdoms of life are Plant, Animal, Fungus, Protist, and Bacteria/Prokaryote (or Monera).
  • The three domains are Eukaryota, Bacteria, and Archaea, and these were discovered by genetic analysis.
  • Classification is useful to scientists as it allows them to identify new species and group them, predict characteristics of organisms belonging to certain groups, and understand more about extinct species.

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