Once scientists agreed on the characteristics of life, they realized life existed in millions of different forms. In fact, although less than 2 million different types of organisms have been identified, there are an estimated 10 million more that are yet to be located and named.
Clearly, in order for us to talk about different types of organisms, we need to name them. But a name isn’t enough. We humans also like to simplify our lives by creating order. So we look for patterns we can use for grouping things together.
So, scientists started a new branch of science called taxonomy. Taxonomy is the science of organizing, or classifying organisms with similar characteristics into groups.
As was just stated, taxonomists organize or classify life forms into groups based on the characteristics they share in common. Organisms that are very similar, that can mate with each other and produce offspring that are also able to reproduce are organized into a group also known as a species .
History of Taxonomy Initially, organisms were divided into two groups. What were these two groups? Animals Plants
The organisms in each of these groups then needed to be organized into smaller subgroups, once again based on shared characteristics.
The earliest documented taxonomists were Greeks alive in 400 B.C. Aristotle was noted for his classification of animals. He grouped organisms according to where they lived, such as in the air, on land or in water.
Theophrastus, who lived during the same era, focused on plants. He organized plants based on their stem structure, creating three separate groups. They were: herbs (soft stems) shrubs (multiple woody stems) trees (single woody stem)
However, as more and more organisms were discovered, more groupings were needed. In the 1600s, two Europeans, John Ray and Carolus Linnaeus, developed more thorough classification systems.
But it was Linnaeus who came up with the seven name classification system we still use in classifying organisms today. Ray was responsible for developing the groupings called the genus and species. Therefore, Linnaeus is known as the father of taxonomy .
The first most general level taxonomists use to categorize all life forms is the Kingdom level. The two domains of bacteria are usually listed as kingdoms. The domain Eukarya is divided into four distinct kingdoms of its own. LEVELS OF CLASSIFICATION
The 6 Kingdoms of Life Animal Plant Fungi Protista Archaebacteria Eubacteria
the type of cell structure of an organism how it gets the energy needed to live and how it moves how it reproduces Classification by kingdom depends largely on:
1) Prokaryotic cells have DNA that isn’t stored in a nucleus. They also lack specialized structures within the cell. Organisms belonging to the Archaebacteria and Eubacteria have prokaryotic cells. The prokaryotic cell is believed to be the first cell type to evolve. Two types of cellular structure are present in the six kingdoms.
2) Eukaryotic cells have a nucleus containing DNA They also have organelles, the “organs” of the cell (e.g. chloroplasts, mitochondria). Organisms belonging to the Protista , Fungi , Plantae and Animalia kingdoms have eukaryotic cells. Organelles are specialized structures that perform specific functions.
Now, within each kingdom, the life forms are broken into other categories based on their similarities. These categories are called Phylums . Two examples of animal phylums are: chordates (animals with backbones), arthropods (animals with jointed appendages and an exoskeleton
And within a phylum, organisms are grouped into Classes … … and organisms within classes are grouped into Orders . … and then into Families … … Genuses … … and finally, each type of organism has its own grouping called the Species .
Kingdom Phylum Class Order Family Genus Species Most general category Most specific category The Classification Scheme of Carolus Linnaeus
So each organism has a name with 7 different parts. Notice that the last two groups, the genus and species are italicized (the genus is capitalized). This is because these two groups make up an organism’s Scientific Name . For example, we humans are grouped like this: sapiens Animalia Chordata Mammalia Primates Hominidae Homo Kingdom : Phylum : Class : Order : Family : Genus : Species :
In reality, there are 100 different types of pine trees. Somehow, scientists need a better way to distinguish which type they are talking about. Why bother with the scientific name for an organism, rather than its common name? Well, for example, how many of us know what a pine tree looks like? For example, there’s Pinus banksiana , Pinus strobus , Pinus resinosa …
The current study of taxonomy relies not only on the structural similarities between organisms. More importantly, it focuses on the evolutionary relationships of organisms. The theory of evolution states that new species develop from significant changes in existing species, to the point where they are no longer able to interbreed with each other. CLUES TO THE PAST
Therefore, we can infer how closely related two organisms are by comparing the groupings they have in common. The more names in common, the closer the relationship. Animalia Arthropoda Insecta Lepidoptera Danaidae Danaus pleccipus Animalia Chordata Mammalia Carnivora Ursidae Ursus maritimus Plantae Angiospermae Dicotyledones Fabales Fabaceae Phaseolus vulgaris Kingdom : Animalia Phylum : Chordata Class : Mammalia Order : Primates Family : Hominidae Genus : Homo Species : sapiens Compare the names below. Which two organisms are most closely related?
When we begin to look closely at other organisms, we will notice that we have more in common than we originally thought.
So, even though we may seem very different because of a few seemingly major differences, we actually are very similar to one another. For example, we humans have a head, a torso, and appendages (arms, legs). But so do lions, tigers, bears, and many other types of animals. Plus, we all have eyes, ears, a mouth, teeth, etc. Also, we all have to eat other organisms in order to stay alive. And there are so many more similarities between us.
The Tools of Taxonomy In their search for evolutionary relationships, taxonomists use a variety of research methods. 1) Comparative Anatomy Comparing the structural characteristics (e.g. skeletal structure) of organisms has been the common method of grouping organisms.
2) Comparative Embryology As an organism develops after the fertilization of an egg, the embryo undergoes a variety of stages of development. The more closely related two organisms are, the more embryonic stages they share in common.
With advances in technology, scientists can also now compare the DNA of organisms. DNA is an amazing type of molecule found in every cell of every organism. DNA molecules contain the chemical code that give the instructions for making every structure in our bodies. 3) Biochemical Information
As a result, we have now found that organisms which may not look similar to us are actually more closely related to us than we thought. By comparing the code in different organisms’ DNA, scientists can now identify more accurately the relationships between organisms. Chimpanzees and humans have 99% of their DNA that is identical! Bananas and humans have 50% of their DNA that is identical!!
4) Cytological (cellular) Information Scientists also compare the cell structures of different organisms. For example, organisms that have prokaryotic cells are grouped into one of the two bacterial kingdoms.
5) Behavioral Information Even behavioral differences can play a role in the grouping of organisms. For example, the mating behaviors of two seemingly similar beetles may be different enough that it prevents the two groups from interbreeding.
IDENTIFYING ORGANISMS In order to make it easier for us to identify organisms, scientists have created guides known as taxonomic keys that are based on the physical characteristics of organisms. A taxonomic key consist of a series of paired statements about a group of organisms. 1a. Leaves are evergreen…2 b. Leaves are deciduous…4 2a. Leaves are needle-like…3 b. Leaves are scaley… Arbovitae alba 3a. Needles in bundles of 2… Pinus resinosa b. Needles in bundles of 5… Pinus strobus 4a. Leaves arranged opposite of each other on stem…5 b. Leaves alternate along stem…8
You read the first two statements and select the most appropriate one. You then go to the next numbered set that the statement leads to. 1a. Leaves are evergreen…2 b. Leaves are deciduous…4 2a. Leaves are needle-like…3 b. Leaves are scaley… Arbovitae alba 3a. Needles in bundles of 2… Pinus strobus b. Needles in bundles of 5… Pinus alba 4a. Leaves arranged opposite of each other on stem…5 b. Leaves alternate along stem…8 Let’s say you have a tree you want to identify. Taking a branch in hand (see diagram below), you start at the beginning of a plant taxonomic key. You continue doing this until you get to the scientific name of the plant.
You will be using the skills necessary to read a taxonomic key as we begin to identify certain types of organism. Keys like this aren’t limited to biology. In fact, you will find similar techniques used in instruction and repair manuals, as well as other areas in your life.
We have now completed our introduction to Taxonomy.
In the next few weeks, we will be practicing the skills and techniques of taxonomists as we study different types of organisms.
We will primarily be using comparative anatomy, to identify these organisms, with the aid of taxonomic keys.