Systematics (taxonomy) is the science that describes the diversity of organisms on Earth and sorts them into groups by similarity and relatedness. More than 2 million species are known to science today (an approximate figure), and they cannot be studied without order. Systematics gives each organism a precise “address”: which kingdom, phylum, class, order, family, genus and species it belongs to. A well-built system also reflects the evolutionary history of organisms — their phylogeny. Exams ask about the sequence of taxa, binomial nomenclature, species criteria and the features of the kingdoms of life.
§1. History of systematics
Aristotle (4th century BC) divided animals into those “with blood” and those “without blood” — one of the first attempts at classification.
C. Linnaeus (18th century, a Swedish scientist) in “Systema Naturae” arranged plants and animals in a hierarchy of groups and introduced binomial nomenclature. He grouped plants mainly by the number and structure of their stamens — an artificial system, because it relied on one or two features and ignored relatedness.
J. B. Lamarck divided animals into vertebrates and invertebrates.
After C. Darwin’s theory of evolution, a natural (phylogenetic) system developed: organisms are grouped by many features together and by common descent.
Modern systematics uses embryology, biochemistry and comparison of DNA sequences as well as anatomy.
Systematics also has great practical value: identifying pests and pathogens accurately, finding wild relatives of medicinal and crop plants, and listing rare species in the Red Book for protection are possible only if species are identified correctly. For example, without knowing the species of a pest insect you cannot choose the right control measures, and closely related species often contain the same medicinal substances.
§2. Taxonomic ranks
Systematic groups are called taxa. They form a hierarchy — each higher taxon contains one or more lower ones:
Species → genus → family → order → class → phylum (in plants — division) → kingdom.
The species is the smallest basic unit.
Several related species form a genus, related genera a family, families an order, orders a class, classes a phylum, phyla a kingdom.
A higher rank always contains the lower: one family includes several genera, not the other way round.
Intermediate ranks are sometimes used: subphylum (vertebrates), subclass (placental mammals), subfamily; above kingdoms there are empires (non-cellular and cellular life) and superkingdoms (prokaryotes and eukaryotes).
Animals: phylum and order; plants: division and order.
§3. The species and its criteria
A species is a group of individuals similar in structure, life processes and chromosome set, living in a certain area (range), which interbreed freely and produce fertile offspring.
Several criteria are used together to identify a species:
morphological — similar external and internal structure;
physiological — similar life processes, above all interbreeding with fertile offspring;
genetic — the same number, shape and structure of chromosomes (for example, 2n = 46 in humans);
biochemical — composition of proteins and nucleic acids;
ecological — place in the environment, diet, living conditions;
geographical — the area of distribution (range).
One criterion is not enough: there are sibling (cryptic) species that look almost identical but do not interbreed. A horse and a donkey can produce a mule, but the mule is sterile — so the horse and donkey are different species.
§4. Binomial nomenclature
C. Linnaeus gave each species a two-word Latin name:
The first word is the genus (capital letter), the second is the species epithet (small letter): Homo sapiens, Felis catus, Triticum aestivum.
This name is the same all over the world, which removes the confusion of local names: one plant can have dozens of names in different languages but only one scientific name. If two species names share the first word, the species belong to the same genus.
§5. The kingdoms of life
Non-cellular life forms — viruses: they multiply only inside a host cell and consist of nucleic acid and a protein coat.
Prokaryotes (no nucleus) — bacteria and cyanobacteria (blue-green algae): circular DNA in a nucleoid; no membrane-bound organelles; 70S ribosomes; murein cell wall.
Eukaryotes (with a nucleus) are divided into four kingdoms:
Plants — autotrophs (photosynthesis), cellulose wall, plastids, storage carbohydrate starch, unlimited growth, do not move.
Animals — heterotrophs, no cell wall, most move actively, store glycogen, limited growth.
Fungi — heterotrophs, chitin wall, store glycogen, unlimited growth, reproduce by spores, feed by absorption.
Protists — unicellular eukaryotes: amoeba, paramecium, euglena (some textbooks treat them as a subkingdom of animals).
Fungi combine plant and animal features, so they form a separate kingdom.
§6. Working out a systematic position
Exams often ask you to arrange given taxa from largest to smallest or the reverse. Method:
First find the kingdom and phylum (division) — they are the largest.
If there is a subphylum, place it after the phylum.
Then class → order → family → genus → species.
Order names often end in “-iformes” in animals and families in “-idae” (Columbiformes — order, Columbidae — family).
The systematic position of humans: kingdom Animals → phylum Chordata → subphylum Vertebrata → class Mammalia → order Primates → family Hominidae → genus Homo → species Homo sapiens.
Worked example
What can be said about Canis lupus (wolf) and Canis familiaris (dog)?
The first word of both names is the same — Canis.
The first word is the genus, so they belong to one genus.
The second words differ, so the species differ.
Answer: one genus, two different species.
Worked example
Arrange the taxa of a laughing dove from largest to smallest: 1) class birds; 2) phylum chordates; 3) species laughing dove; 4) order pigeons and doves (Columbiformes); 5) subphylum vertebrates; 6) family Columbidae.
The largest is the phylum: chordates (2).
Then the subphylum: vertebrates (5).
Class: birds (1); order: Columbiformes (4).
Family: Columbidae (6); species: laughing dove (3).
Answer: 2 → 5 → 1 → 4 → 6 → 3.
Worked example
A horse and a donkey produce a mule. Are the horse and the donkey one species?
The physiological criterion: members of one species interbreed and produce fertile offspring.
The mule is sterile — it cannot produce offspring.
So the horse and the donkey do not produce fertile offspring.
Answer: they are different species (but the same genus — Equus).
Check yourself
Which taxon is the smallest basic unit?
Which term replaces “phylum” for plants?
Which word in Homo sapiens names the genus?
Who introduced binomial nomenclature?
What is the fungal cell wall made of?
Is a family inside an order, or an order inside a family?
What is the storage carbohydrate of animals?
What are species that look alike but do not interbreed called?
Common mistakes
Swapping family and order — the order is larger.
Putting fungi in the plant kingdom.
Writing the species epithet with a capital letter — only the genus is capitalised.
Confusing cyanobacteria with eukaryotic algae — they are prokaryotes.
Identifying a species by one criterion only (for example, appearance).
Using “division” for animals and “phylum” for plants — it is the other way round.