2.3 Classification of Living Things

Learning Objectives

• Explain what makes up a scientific name.
• Explain what defines a species.
• List the information scientists use to classify organisms.
• List the three domains of life and the chief characteristics of each.

Introduction

When you see an organism that you have never seen before, you probably put it into a group without even thinking. If it is green and leafy, you probably call it a plant. If it is long and slithers, you probably call it as a snake. How do you make these decisions? You look at the physical features of the organism and think about what it has in common with other organisms.

Scientists do the same thing when they classify, or put in categories, living things. Scientists classify organisms not only by their physical features, but also by how closely related they are. Lions and tigers look like each other more than they look like bears. It turns out that the two cats are actually more closely related to each other than to bears. How an organism looks and how it is related to other organisms determines how it is classified.

Guided Learning

Linnaean System of Classification

People have been concerned with classifying organisms for thousands of years. Over 2,000 years ago, the Greek philosopher Aristotle developed a classification system that divided living things into several groups that we still use today, including mammals, insects, and reptiles.

Carl Linnaeus (1707-1778) (Figure below) built on Aristotle’s work to create his own classification system. He invented the way we name organisms today. Linnaeus is considered the inventor of modern taxonomy, the science of naming and grouping organisms.

^[1]

In the 18th century, Carl Linnaeus invented the two-name system of naming organisms (genus and species) and introduced the most complete classification system then known.

Linnaeus developed binomial nomenclature, a way to give a scientific name to every organism. Each species receives a two-part name in which the first word is the genus (a group of species) and the second word refers to one species in that genus. For example, a coyote's species name is Canis latrans. Latrans is the species and canis is the genus, a larger group that includes dogs, wolves, and other dog-like animals.

Here is another example: the red maple, Acer rubra, and the sugar maple, Acer saccharum, are both in the same genus and they look similar ( Figure below). Notice that the genus is capitalized and the species is not, and that the whole scientific name is in italics. The names may seem strange, but they are written in a language called Latin.

^[2]

These leaves (left and center) are from one of two different species of trees in the Acer, or maple, genus. One of the characteristics of the maple genus is winged seeds (right).

Modern Classification

Modern taxonomists have reordered many groups of organisms since Linnaeus. The main categories that biologists use are listed here from the most specific to the least specific category (Figure below).

^[3]

This diagram illustrates the classification categories for organisms, with the broadest category (Kingdom) at the bottom, and the most specific category (Species) at the top.

For a fun way to learn about classification, watch the Classification Rap.

Difficulty Naming Species

Even though naming species is straightforward, deciding if two organisms are the same species can sometimes be difficult. Linnaeus defined each species by the distinctive physical characteristics shared by these organisms. But two members of the same species may look quite different. For example, people from different parts of the world sometimes look very different, but we are all the same species (Figure below).

So how is a species defined? A species is group of individuals that can interbreed with one another and produce fertile offspring; a species does not interbreed with other groups. By this definition, two species of animals or plants that do not interbreed are not the same species.

^[4]

These children are all members of the same species, Homo sapiens.

Domains of Life

Let’s explore the least specific category of classification, called a domain.

All of life can be divided into three domains, which tell you the type of cell inside of an organism:

1. Bacteria: Single-celled organisms that do not contain a nucleus
2. Archaea: Single-celled organisms that do not contain a nucleus, and have a different cell wall from bacteria
3. Eukarya: Organisms with cells that contain a nucleus

Archaea and Bacteria

Archaea and Bacteria (Figure below and Figure below) seem very similar, but they also have significant differences.

Similarities:

• Both do not have a nucleus.
• Both have small cells.
• Both are one-celled.
• Both can reproduce without sex by dividing in two.

Differences:

• Cell walls of Bacteria and Archaea are made of different material.
• Archaea often live in extreme environments like hot springs, geysers, and salt flats while bacteria can live almost everywhere.

^[5]

The Group A Streptococcus organism is in the domain Bacteria, one of the three domains of life.

^[6]

The Halobacterium is in the domain Archaea, one of the three domains of life.

Eukarya

All of the cells in the domain Eukarya keep their genetic material, or DNA, inside the nucleus. The domain Eukarya is made up of four kingdoms:

1. Plantae: Plants, such as trees and grasses, survive by capturing energy from the sun, a process called photosynthesis.
2. Fungi: Fungi, such as mushrooms and molds, survive by "eating" other organisms or the remains of other organisms.
3. Animalia: Animals survive by eating other organisms or the remains of other organisms. Animals range from tiny ants to the largest dinosaurs (reptiles) and whales (mammals), including all sizes in between. (Figure below).
4. Protista: Protists are not all descended from a single common ancestor in the way that plants, animals, and fungi are. Protists are all the eukaryotic organisms that do not fit into one of the other three kingdoms. They include many kinds of microscopic one-celled organisms, such as algae and plankton, but also giant seaweeds that can grow to be 200 feet long (an alga protist is shown in Figure below).

Plants, animals, fungi, and protists might seem very different, but remember that if you look through a microscope, you will find similar cells with a membrane-bound nucleus in all of them. The main characteristics of the three domains of life are summarized in Table below.

^[7]

The Western Gray Squirrel is in the domain Eukarya, one of the three domains of life.

^[8]

This microscopic alga is a protist in the domain Eukarya.

Viruses

We have all heard of viruses. The flu and many other diseases are caused by viruses. But what is a virus? Based on the material presented in this chapter, do you think viruses are living?

The answer is actually “no.” A virus is essentially DNA or RNA surrounded by a coat of protein (Figure below). It is not a cell and does not maintain homeostasis. Viruses also cannot reproduce on their own; they need to infect a host cell to reproduce. Viruses do, however, change over time, or evolve. So a virus is very different from any of the organisms that fall into the three domains of life.

^[9]

These “moon lander” shaped viruses infects Escherichia coli bacteria.

Review

• Scientists have defined several major categories for classifying organisms: domain, kingdom, phylum, class, order, family, genus, and species.
• The scientific name of an organism consists of its genus and species.
• Scientists classify organisms according to their evolutionary histories and how related they are to one another by looking at their physical features, the fossil record, and DNA sequences.
• All life can be classified into three domains: Bacteria, Archaea, and Eukarya.

Vocabulary

Archaea

        A domain of rather rare single-celled prokaryotes, usually found in extreme environments.

bacteria

        A large domain of common prokaryotic microorganisms.

binomial nomenclature

A formal system of naming species, consisting of two parts, genus and species.        

classify

The processes of organizing objects into various categories based on their characteristics.         

domain

        The largest of the taxonomic subdivisions.

Eukarya

        The domain of life consisting of eukaryotes.

genus

A taxonomic group found above species and below family and also forms the first part of a scientific name.

species

A group of individuals that can interbreed with one another and produce fertile offspring, also the basic unit of the classification system, forming the second part of a scientific name.

taxonomy

        The science of classifying and naming organisms.