How do scientists organize the diversity of life into a system that reflects how organisms are related?
Describe how organisms are classified using a hierarchical taxonomic system based on shared characteristics, and use the levels from domain to species (TEKS Biology, Reporting Category 3; patterns; systems and system models).
A TEKS-level answer on classification for the Texas STAAR Biology EOC: the hierarchical taxonomic levels from domain to species, the three domains, binomial nomenclature, and how shared characteristics group organisms.
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What this topic is asking
The Biology TEKS ask you to describe how organisms are classified using a hierarchical system based on shared characteristics, and to use the levels from domain to species. For STAAR Reporting Category 3 you need the order of the levels, the idea that organisms sharing a lower (more specific) level are more closely related, and the basics of naming species. This is a patterns and systems and system models topic.
The taxonomic hierarchy
Because the levels are nested, the lower (more specific) the level at which two organisms are grouped together, the more characteristics they share and the more closely related they are. Two organisms in the same genus are very closely related; two that share only a kingdom are distantly related. This is the single most tested idea in classification, and it links directly to evolution: classification reflects how organisms are related by common ancestry.
The three domains
At the highest level, all life is divided into three domains:
- Bacteria. Prokaryotes (no nucleus), common and widespread.
- Archaea. Prokaryotes that are biochemically distinct from bacteria; many live in extreme environments.
- Eukarya. All organisms with eukaryotic cells (nucleus and membrane-bound organelles): protists, fungi, plants, and animals.
The domain level captures the deepest divisions in the history of life, which is why it sits above kingdom.
Naming species: binomial nomenclature
A scientific name is more precise than a common name (which can vary by region or language) and shows relationships, because organisms in the same genus share the first part of the name.
Classification reflects relationships
Modern classification groups organisms by shared characteristics, including structure and increasingly molecular data (DNA and protein similarities). Because shared features are usually inherited from a common ancestor, the classification tends to reflect evolutionary relationships: organisms grouped closely are closely related. This is the link between classification (this topic) and the evidence for evolution, and it is why the same molecular data used to study evolution is used to classify organisms and to build cladograms.
Try this
Q1. List the taxonomic levels from broadest to most specific. [2]
- Cue. Domain, kingdom, phylum, class, order, family, genus, species.
Q2. Two organisms share the same order but are in different families. A third organism is in the same family as the first. Which two are more closely related? [1]
- Cue. The first and third, because they share the more specific level (family).
Exam-style practice questions
Practice questions written in the style of TEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
STAAR Biology (2023 released style)1 marksWhich list shows the taxonomic levels in the correct order from broadest to most specific? (A) Species, genus, family, order. (B) Domain, kingdom, phylum, class. (C) Kingdom, domain, phylum, class. (D) Genus, species, family, order.Show worked answer →
A 1-point multiple-choice item on the taxonomic hierarchy.
The correct answer is B. From broadest to most specific the order is domain, kingdom, phylum, class, order, family, genus, species. A and D go from specific toward broad, and C puts kingdom before domain, which is the wrong order (domain is the broadest level).
Domain is broadest; species is most specific.
STAAR Biology (2024 SCR style)2 marksTwo organisms are placed in the same genus, while a third organism is in the same kingdom but a different phylum. Explain which two organisms are more closely related and why. Support your answer with reasoning.Show worked answer →
A 2-point short constructed response on relatedness and taxonomic level.
Full credit (2 points): the two organisms in the same genus are more closely related, because the lower (more specific) the shared taxonomic level, the more characteristics the organisms share and the more recently they share a common ancestor. The third organism only shares the kingdom (a very broad level) and differs at the phylum level, so it is more distantly related.
Partial credit (1 point): identifies the same-genus pair as closer without explaining that lower shared levels mean closer relationship. The science is scored.
Related dot points
- Interpret cladograms and phylogenetic trees to determine evolutionary relationships based on shared derived characteristics and molecular evidence (TEKS Biology, Reporting Category 3; patterns; systems and system models).
A TEKS-level answer on cladograms for the Texas STAAR Biology EOC: how to read a cladogram or phylogenetic tree, what nodes and branches represent, how shared derived traits group organisms, and how to judge relatedness.
- Analyze and evaluate the evidence for evolution, including the fossil record, homologous and vestigial structures, and molecular (DNA and protein) similarities (TEKS Biology, Reporting Category 3; patterns; cause and effect).
A TEKS-level answer on the evidence for evolution for the Texas STAAR Biology EOC: the fossil record, homologous and vestigial structures, and molecular similarities, and how each line points to common ancestry and change over time.
- Recognize the factors that influence the genetic makeup of populations and lead to speciation, including mutation, gene flow, genetic drift, and reproductive isolation (TEKS Biology, Reporting Category 3; cause and effect; patterns).
A TEKS-level answer on the mechanisms of genetic change for the Texas STAAR Biology EOC: mutation, gene flow, and genetic drift as sources of change in a population, and how reproductive isolation leads to speciation.
- Explain how natural selection acts on heritable variation to produce adaptation in populations over time, and identify the conditions required for it to occur (TEKS Biology, Reporting Category 3; cause and effect; stability and change).
A TEKS-level answer on natural selection for the Texas STAAR Biology EOC: variation, overproduction, the struggle to survive, differential survival and reproduction, and how this leads to adaptation and change in populations over time.
- Describe the levels of ecological organization from organism to biosphere, and distinguish the biotic and abiotic factors that make up an ecosystem (TEKS Biology, Reporting Category 5; systems and system models; patterns).
A TEKS-level answer on ecological organization for the Texas STAAR Biology EOC: the levels from organism to population, community, ecosystem, biome, and biosphere, and the difference between biotic and abiotic factors.
Sources & how we know this
- Texas Essential Knowledge and Skills for Science (Biology) — Texas Education Agency (2024)
- STAAR Biology Assessed Curriculum — Texas Education Agency (2024)