How do biologists classify organisms and show their evolutionary relationships?
Explain the basis of the modern classification system, compare the domains and kingdoms, use dichotomous keys, and analyze relationships using phylogenetic trees and cladograms (Virginia 2018 Biology SOL BIO.6.a, BIO.6.b, BIO.6.c, and BIO.6.d).
A SOL-level answer on classification for the Virginia Biology EOC: the basis of the modern system, the three domains and the kingdoms, binomial nomenclature and the taxonomic hierarchy, using a dichotomous key, and reading phylogenetic trees and cladograms as evidence of common ancestry.
Reviewed by: AI editorial process; not yet individually human-reviewed
Have a quick question? Jump to the Q&A page
Jump to a section
What this topic is asking
Virginia Biology SOL standard BIO.6 asks you to investigate and understand the classification of organisms, using physical characteristics and molecular evidence. Its substandards cover the basis of the current system (BIO.6.a), comparing the domains and kingdoms (BIO.6.b), using and constructing dichotomous keys (BIO.6.c), and analyzing relationships with phylogenetic trees and cladograms (BIO.6.d). The EOC expects you to know how organisms are named and grouped, to place an organism using a key, and to read relatedness off a tree.
The basis of the modern system
This is why molecular data can move an organism from one group to another: when DNA shows that two organisms are more closely related than their appearance suggested, the classification is updated to match the evidence. Classification is a model that scientists revise as new evidence comes in.
Domains and kingdoms
The broadest level of classification is the domain. There are three:
- Bacteria and Archaea are both made of prokaryotic cells (no nucleus). Archaea includes many extremophiles that live in hot springs, salty lakes, and other harsh environments, and differs from Bacteria in important molecular details.
- Eukarya contains all organisms made of eukaryotic cells (with a nucleus). It is divided into kingdoms, commonly Protista (mostly single-celled eukaryotes), Fungi (decomposers such as molds and mushrooms), Plantae (multicellular producers), and Animalia (multicellular consumers).
A common EOC trap is to mix the levels: a domain is broader than a kingdom, so Animalia (a kingdom) sits inside Eukarya (a domain).
The hierarchy and naming
Below domain, organisms are sorted into nested groups, from broadest to most specific: domain, kingdom, phylum, class, order, family, genus, species. Each step down contains organisms that are more alike and more closely related.
The two-name system gives every species one precise, universal name, avoiding the confusion of common names that differ by region or language.
Dichotomous keys
A dichotomous key identifies an unknown organism through a sequence of paired, either/or choices about its characteristics. At each step you pick the statement that fits and follow it to the next pair, narrowing the possibilities until you reach the organism's name. "Dichotomous" means "divided in two," so every step offers exactly two options. The EOC may give you a short key and an organism's traits and ask which name you reach.
Phylogenetic trees and cladograms
The most common mistake is to judge relatedness by how close two labels sit on the page; instead, trace the branches back and find the shared branch point. The nearer that shared ancestor, the closer the relationship. Cladograms are often built from shared, derived characteristics and from molecular data, tying classification directly to the evidence for evolution.
Try this
Q1. List the levels of classification from broadest to most specific. [2]
- Cue. Domain, kingdom, phylum, class, order, family, genus, species.
Q2. Explain how a cladogram shows that two species are closely related. [2]
- Cue. Two species that share a more recent common ancestor (a branch point closer to their tips) are more closely related; the further back the shared branch point, the more distantly related they are.
Exam-style practice questions
Practice questions written in the style of VDOE exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
VA Biology SOL (2023 released style)1 marksWhich domain contains organisms that are all prokaryotic and includes species that live in extreme environments such as hot springs? (A) Eukarya. (B) Animalia. (C) Archaea. (D) Protista.Show worked answer →
A 1-point multiple-choice item on the three domains.
The correct answer is C. Archaea is a domain of prokaryotes, many of which live in extreme environments (extremophiles). A (Eukarya) is the domain of organisms with a nucleus. B (Animalia) and D (Protista) are kingdoms, not domains, so they cannot be the answer to a question asking for a domain.
VA Biology SOL (2024 released style)2 marksA student uses a cladogram to study four species. (a) State what a cladogram shows. (b) Explain how the student can tell which two species are most closely related.Show worked answer →
A 2-point item on reading a cladogram.
(a) 1 point: a cladogram (a type of phylogenetic tree) shows the evolutionary relationships among organisms, with branch points representing common ancestors.
(b) 1 point: the two species that share the most recent common ancestor (the branch point closest to their tips) are the most closely related; the further back you must go to find a shared branch point, the more distantly related two species are.
Markers reward reading relatedness from the position of the branch points, not from how close the labels sit on the page.
Related dot points
- Describe the evidence supporting the theory of evolution by natural selection, including the fossil record, comparative anatomy, embryology, molecular evidence, and biogeography (Virginia 2018 Biology SOL BIO.7.a).
A SOL-level answer on the evidence for evolution for the Virginia Biology EOC: the fossil record, comparative anatomy (homologous, analogous, and vestigial structures), comparative embryology, molecular and DNA evidence, and biogeography, and why independent lines that agree make the theory strong.
- Explain how the role of variation and mutations drives natural selection, producing adaptation and changing the heritable traits of a population over generations (Virginia 2018 Biology SOL BIO.7.b).
A SOL-level answer on natural selection for the Virginia Biology EOC: variation and mutations as the raw material, overproduction and competition, differential survival and reproduction (fitness), and how selection produces adaptation and shifts allele frequencies, with antibiotic resistance as the worked example.
- Explain how speciation occurs and the effects of reproductive isolation and geographic isolation on the formation of new species (Virginia 2018 Biology SOL BIO.7.c, BIO.7.d, and BIO.7.e).
A SOL-level answer on speciation for the Virginia Biology EOC: what a species is, how geographic isolation and then reproductive isolation split one population into two, the difference between prezygotic and postzygotic barriers, and how allele frequencies diverge until interbreeding is no longer possible.
- Describe the structure of bacteria as prokaryotic cells, how they reproduce, and the beneficial and harmful roles they play in other organisms and the environment (Virginia 2018 Biology SOL BIO.4.b, BIO.4.d).
A SOL-level answer on bacteria for the Virginia Biology EOC: prokaryotic structure, rapid asexual reproduction, and the beneficial roles (decomposers, gut bacteria, nitrogen fixation) and harmful roles of bacteria.
- State the cell theory and the evidence for it, and distinguish prokaryotic from eukaryotic cells and plant from animal cells (Virginia 2018 Biology SOL BIO.3.a).
A SOL-level answer on cell theory for the Virginia Biology EOC: the three parts of cell theory and its evidence, the difference between prokaryotic and eukaryotic cells, and how plant and animal cells compare.
Sources & how we know this
- 2018 Science Standards of Learning (Biology) — Virginia Department of Education (2018)
- SOL Practice Items (All Subjects) — Virginia Department of Education (2024)