Systematics – order through relationship

Red water lily, a colour morph of
white water lily.

Linnaeus is sometimes called the father of systematics. In the discipline systematic biology the order in nature is essential. A classification of plants and animals that mirrors relationship is called a natural system. There are also artificial systems, for example when plants are grouped by colour of flowers. In such a flower colour system closely related tulips would be grouped in different colour groups and a red water lily would be referred to another group than a white water lily of the same species, even though they are only different colour morphs.

Linnaeus’ sexual system, based on the number and position of the stamens, was an artificial system. Closely related species could end up in different classes if they had different number of stamens.

To be related is to have the same origin. To search for the origin of species we have to know something about their history and the process called evolution. The fossils found tell us something about this, but far from all since they are too few. To find out how species are related to each other we have to study those living now and make some assumptions about how evolution has worked.

There are two ways to regard species when searching for relationships. The most common is to assume that the species most alike are most closely related. To measure the likeness one has to examine as many characteristics as possible. In more recent times another method called cladistics has been more widely used. With cladistic methods only similarities with a common origin are regarded. This method can sometimes show that species looking similar are not necessarily most closely related. One example is crocodiles and lizards. They show many similarities, but using cladistic methods we can see that crocodiles are more closely related to birds than to lizards.

Comparing species, mostly characteristics seen with the naked eye or a microscope are studied. In recent years also the DNA has been studied which provides more information to reveal relationships.

Cladistics reveal the tree of life

As long as there has been life on earth new species have sprung up and old species have gone extinct. A species can be divided into two by isolation of some individuals from the rest of the species. Through this isolation a new species with characters differing from the original species might be formed. In plants new species can also be formed through hybridisation.

Cladogram showing the development of the vertebrates.

The diversity of species we see today has originated through repeated division of species. A tree (cladogram) over the development of species can be made to show how this development has proceeded.

All species look different and have different qualities, characters. To make a cladogram the characters able to divide the species in two or more groups are searched for. When all those grouping characters have been compiled into a table they are run in a computer program that makes cladistic trees to show how the species are related to each other. The program calculates the simplest and most plausible tree.

Sometimes a species can look very different from its nearest relatives. This can be due to adaptations to extreme environments like for example to very dry habitats of a desert. In cladistic studies the characters shared by two or more species are informative, but not characters unique to one species. The shared characters in different constellations will be the basis for a tree and show what species are the closest relatives.

DNA reveals relationships

Coffee plant.

When two sex cells meet and are united they form a new cell and the new life begins. This first cell looks very similar in all species. After a while it develops into a specific organism, for instance a rabbit or a camel. The information in the cell's DNA directs the development of the cell. A rabbit's DNA tells that the rabbit should have long ears and a short tail but no humps on the back.

Cells build up all living things. In each cell there is DNA in long threads with shorter regions called genes. The DNA is inherited from generation to generation but can also change by chance. Changes in DNA are the bases for evolution and the origin of new species.

A new way to find out what species are most closely related is by comparing DNA with modern methods. The ideal situation would be to be able to compare all DNA, but this is too expensive and time consuming. Therefore only one or a few genes are compared when trying to show relationships. The ”building bricks” of the genes (DNA) can be separated on a gel where they are shown as bands.

The DNA from the coffee plant is seen as a pattern of bands on a gel.
Photo: Katarina Andreasen

To show how the species are related to each other, an evolutionary tree(cladistic tree) can be made by comparing the sequences of ”building bricks” in the DNA from the species studied. Such a tree most often gives the clearest information about relationship among species.

A system for all living organisms

Linnaeus divided the organisms into two realms. ”Plantae” was composed of land plants, fungi and algae and ”Animalia” contained the animals. As time went by the scientists realised that the organisms could not be divided into two parts, if a natural system was to be reached.

In 1969 the following system of realms that can be found in many textbooks was suggested:

Monera (bacteria)
Protista (unicellular organisms)
Plantae (plants)
Fungi (fungi)
Animalia (animals)

However, quite soon it was realised that also these groups were unnatural. Especially Protista, which was composed of a number of different, unrelated groups of organisms, did not fit in any of the other realms. Scientists have reached the conclusion that the most natural division of the organisms must be to group them after the way their cells are composed. One of the most recent suggestions how to divide the organisms looks like this:

Archaea (archae bacteria)
Bacteria (common bacteria)
Eucaryota (animals, plants, fungi and higher organisms)

Archae bacteria are some of the strangest organisms in the world. They can, for example, live in wells with a sulphur concentration and in submarine volcanoes. They differ much from the common bacteria, the Eubacteria, but both of these groups are characterised by having one simple cell lacking a nucleus. The rest of the world's organisms, Eucarya, have complicated cells with a nucleus and different cell organelles. To divide the Eucarya into natural groups is a great challenge to contemporary scientists, and this work is a continuing process.

The hierarchy among organisms

To understand the system within each realm one needs to know the hierarchy within the realm. Each level in the hierarchy has a scientific name. Above genus level some names have a specific suffix. The hierarchy of the wood anemone is as follows:

Level: Example: Suffix:
Realm Plantae
Division Anthophyta -phyta
Class Dicotyledones
Order Ranunculales -ales
Family Ranunculaceae -aceae
Genus Anemone
Species Anemone nemorosa

The hierarchy and naming looks a bit different for the housefly since it is an animal:

Level: Example: Suffix:
Realm Animalia
Phylum Arthropoda
Class Insecta
Order Diptera
Family Muscidae -idae
Genus Musca
Species Musca domestica

A species can be further divided into smaller groups. An abbreviation of the type of subgroup and an additional name is then added after the species name:

Level: Abbreviation:
Subspecies ssp.
Variety var.
Form f.

In the Swedish alpine area there is a subspecies of common sorrel named Rumex acetosa ssp. lapponicus. For animals the abbreviation is usually not provided. The narrow billed subspecies of Eurasian nutcracker is just written: Nucifraga caryocatactes macrorhynchus.

Last modified: 2021-12-07