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Grade 7 / Middle 1 (age 12-13)

grade 7 Biological Classification System

More than two million living species have been named, so biologists sort them by grouping similar ones and then gathering those groups into bigger ones. Starting from the smallest group, the species, the ranks climb through genus, family, order, class and phylum to kingdom, and each step up takes in more organisms that resemble each other less closely. Every species also gets a scientific name made of its genus name and a specific epithet, so a scientist anywhere can point to the same organism with the same name. Move the pyramid one rank at a time and you can watch which group humans belong to at that rank and how many living species share it.

Classification Levels

Species
💡 Moving Up and Down the Ranks
①From narrow to broad the order is species → genus → family → order → class → phylum → kingdom, and kingdom is the widest unit. English speakers often read it the other way with 'King Philip Came Over For Good Soup'.
②Humans sit in genus Homo, family Hominidae, order Primates, class Mammalia, phylum Chordata and kingdom Animalia, so each step up adds less similar company: chimpanzees, then monkeys, then dogs, then fish.
③A species is a group whose members mate in the wild and produce young that can themselves reproduce. A horse and a donkey can produce a mule, but mules are sterile, so horses and donkeys are separate species.

Five Kingdoms

Five kingdoms

Five kingdoms sorted by nucleus, cell count and how food is obtained
🔍 Three Questions That Split the Kingdoms
①Is there a nuclear membrane? If not, it belongs to the prokaryote kingdom; if so, to one of the other four.
②Is the body simple? A nucleus but no real tissues or organs means the protist kingdom, which also holds many-celled forms such as spirogyra and kelp.
③How does it get food? Photosynthesis means plants, breaking down and absorbing means fungi, and eating means animals.
④Viruses are not made of cells, so they are placed in none of the five kingdoms.

Scientific Names & Binomial Nomenclature

Binomial nomenclature
Scientific name = genus + specific epithet
The system Linnaeus set out: written in Latin and in italics, with the genus capitalized and the epithet in lowercase.
📖 Why Scientific Names Exist
①The same organism has different common names in different countries and regions. A cat is gato in Spanish and neko in Japanese, but its scientific name is Felis catus everywhere.
②A shared genus means close relatives: the wolf Canis lupus and the dog Canis familiaris are both in genus Canis.
③The epithet never stands alone. 'sapiens' by itself names nothing; only together with its genus does it pick out one species.

Using a Dichotomous Key

Sample key

Dichotomous key for vertebrates
🧬 Following a Key
①Each fork asks about one trait only, and the answer splits two ways, yes or no.
②Choose traits every member of the group has, such as hair, feathers or gills, rather than size or color, which vary from one animal to the next.
③Follow the forks until there is nothing left to split; the name written there is the group the organism belongs to.

Exam Key Points

Summary
species → genus → family → order → class → phylum → kingdom · scientific name = genus + specific epithet
The ranks widen to the right, and a scientific name joins the names of the two narrowest ranks.
🎯 Exam Points
①There are seven ranks, species, genus, family, order, class, phylum and kingdom, and species is the narrowest.
②A species is a group that mates in the wild and produces fertile young (horse × donkey = sterile mule shows two species).
③Scientific names follow Linnaeus's binomial system: genus + specific epithet, in Latin italics.
④The five kingdoms are prokaryotes, protists, fungi, plants and animals, separated by nuclear membrane, body structure and how food is obtained.
⑤A dichotomous key splits by one trait at a time with yes/no answers to find the group an organism belongs to.

Examples and Unit Test

Example 1
Which of the five kingdoms holds single-celled organisms with no nuclear membrane, and which holds organisms that photosynthesize and have cell walls?
1
Check the nucleus first. Organisms whose cells lack a nuclear membrane belong to the prokaryote kingdom, whatever their other traits.
2
Organisms that have a nucleus, make their own food by photosynthesis and have cell walls and organs belong to the plant kingdom.
▸ No nuclear membrane, single-celled = prokaryotes / photosynthesis and cell walls = plants
Ask about the nucleus, then body structure, then how food is obtained, and the five kingdoms stop getting mixed up.
Example 2
What does a scientific name consist of, and why do we use one?
1
Under the binomial system Linnaeus set out, a scientific name is a genus name (capitalized) followed by a specific epithet (lowercase).
2
The same organism is called different things in different countries, so scientists everywhere use the scientific name to refer to one organism by one name.
▸ Genus + specific epithet (binomial nomenclature) / so that the whole world uses the same name
When two names share the first word, they share a genus, which also tells you they are close relatives.
Unit-test style
Which statement about the classification system is correct?
①Among the ranks, species is the broadest range
②A scientific name is made of a genus name and a species name
③The plant kingdom is prokaryotes without a nuclear membrane
④The fungus kingdom makes its own food by photosynthesis
⑤Scientific names are made differently in each country
▸ ② A scientific name is made of a genus name and a species name
1
Under binomial nomenclature a scientific name is made of a genus name (capitalized) and a specific epithet (lowercase), so ② is correct.
2
The broadest rank is the kingdom (①). The plant kingdom is made of eukaryotes with a nuclear membrane (③), and fungi cannot photosynthesize but get food by breaking matter down (④). Scientific names are shared worldwide (⑤).
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Plant Vegetative Organs
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Tectonic Activity

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