Texas STAAR Biology Reporting Category 3 (Biological Evolution and Classification): a complete overview of evidence, natural selection, speciation, taxonomy, and cladograms

What Reporting Category 3 actually demands

Reporting Category 3, Biological Evolution and Classification, ties together how species change over time and how we organize them. It is about a fifth of the test points. The recurring themes are patterns (in fossils, structures, sequences, and cladograms) and cause and effect (how selection and isolation change populations). Many items in this category are diagram-based: a cladogram, a table of molecular differences, or a described scenario you must reason about.

This guide ties together the matching dot-point pages, each with its own practice questions: evidence for evolution, natural selection and adaptation, mechanisms of genetic change, taxonomy and classification, and cladograms and phylogeny.

The evidence for evolution

Several independent lines of evidence support evolution. The fossil record shows life has changed over long spans of time, with older forms in deeper layers. Homologous structures (the same bone arrangement used for different functions, like the forelimbs of humans, whales, and bats) point to a common ancestor. Vestigial structures are reduced remnants of features useful in ancestors. Molecular evidence (DNA and protein similarities) shows that more closely related species have more similar sequences. The strength of the case is that all these lines agree.

Natural selection and adaptation

Natural selection needs four conditions: variation, heritability, overproduction (a struggle to survive), and differential survival and reproduction. Over generations the helpful trait becomes more common, so the population becomes adapted. The variation comes from mutation and sexual reproduction; selection only acts on what already exists. A key idea: the environment does not create the helpful variation on demand; it selects among existing variants. Antibiotic resistance and the peppered-moth and finch-beak examples all follow this logic.

Genetic change and speciation

Beyond selection, populations change through mutation (new alleles), gene flow (alleles moving between populations), and genetic drift (random change, strongest in small populations). Speciation happens when part of a population becomes reproductively isolated (often by a geographic barrier) and the groups change independently until they can no longer interbreed. Natural selection is non-random; drift is random.

Classification and cladograms

Organisms are classified in a hierarchy from broadest to most specific: domain, kingdom, phylum, class, order, family, genus, species, across the three domains (Bacteria, Archaea, Eukarya). The lower the shared level, the closer the relationship. A cladogram shows these relationships as a branching tree: a node is a common ancestor, and organisms that share a more recent node are more closely related. To compare two organisms, trace back to where their branches meet.

Check your knowledge

A mix of recall and reasoning questions covering Reporting Category 3. Attempt them under timed conditions, then check against the solutions.

1. Explain why homologous structures are evidence of a common ancestor. (2 marks)
2. State whether more or fewer molecular differences indicate a closer relationship. (1 mark)
3. State the four conditions required for natural selection. (2 marks)
4. Explain why the environment does not create a helpful mutation on demand. (2 marks)
5. State the difference between gene flow and genetic drift. (2 marks)
6. Explain how a geographic barrier can lead to speciation. (2 marks)
7. List the taxonomic levels from broadest to most specific. (2 marks)
8. State what a node on a cladogram represents. (1 mark)
9. On a cladogram, two species share a recent node and a third branches near the base. Which two are more closely related? (1 mark)