MA High School Biology MCAS Module 5 evolution and biodiversity: a complete overview of natural selection, the evidence, common ancestry, speciation, and biodiversity

What Module 5 actually demands

Module 5 is evolution, the Biological Evolution reporting category (HS-LS4) at about 20 percent of the test. It tells one story: natural selection, acting on heritable variation over long periods, produces adaptation, new species, and the diversity of life, and many independent lines of evidence show this has happened. The MCAS tests it heavily with data and argument: a beetle-color scenario, a protein-difference table, a phylogenetic tree, a changing environment. The crosscutting concepts are cause and effect, patterns, and stability and change.

This guide ties together the matching dot-point pages, each with its own practice questions: natural selection, evidence for evolution, common ancestry and phylogeny, speciation and population genetics, and biodiversity and classification.

Natural selection

Natural selection is the process by which individuals with advantageous heritable traits survive and reproduce more, so those traits become more common over generations. It needs variation, a selection pressure, differential survival and reproduction, and inheritance. The original variation comes from mutation. The key subtlety the MCAS rewards: selection acts on populations, changing the proportions of traits, not on individuals. Antibiotic resistance is the textbook example and shows selection happening fast.

The evidence for evolution

Evolution rests on multiple independent lines of evidence: the fossil record (change over time, transitional forms), homologous structures (the same forelimb bones in human, whale, and bat, suggesting common ancestry), embryology (similar early development), and molecular biology (fewer DNA or protein differences mean a more recent common ancestor). Because the independent lines all agree, the conclusion is strong. The MCAS often gives a protein-difference table and asks you to rank relationships.

Common ancestry and phylogeny

A phylogenetic tree or cladogram shows relationships through common ancestry. Each branch point is a common ancestor. Species sharing a more recent branch point are more closely related; species branching off earlier are more distantly related. What matters is the branching pattern, not how close two tips are drawn. Trees are built from shared characteristics and molecular data.

Speciation and allele frequencies

Speciation usually begins with reproductive isolation (often a geographic barrier), then divergence as the groups face different selection pressures, until they can no longer interbreed to produce fertile offspring. At the population level, evolution is a change in allele frequencies: when a selection pressure favors a trait, the alleles for it become more common. The peppered moth shows allele frequencies tracking the environment, shifting when pollution darkens and then lightens the bark.

Biodiversity and classification

Biodiversity is the variety of life: the number of species and the genetic variety within them. Higher biodiversity generally makes an ecosystem more stable, because more species means more alternatives if one declines. Human activities (habitat destruction, pollution, overhunting, invasive species) reduce it. Classification sorts organisms into a hierarchy by shared characteristics and molecular data, reflecting evolutionary relationships: closely grouped organisms share a more recent common ancestor.

Check your knowledge

A mix of recall, data, and application questions covering Module 5. Attempt them under timed conditions, then check against the solutions.

1. State the four conditions needed for natural selection. (2 marks)
2. Explain why natural selection acts on populations rather than individuals. (2 marks)
3. Define a homologous structure and state what it suggests. (2 marks)
4. Explain how a protein-difference table shows which species are most closely related. (2 marks)
5. State what a branch point on a phylogenetic tree represents. (1 mark)
6. Explain how you decide which two species on a tree are most closely related. (2 marks)
7. State what must happen for two populations to become separate species. (2 marks)
8. Explain what happens to the frequency of an allele when its trait becomes advantageous. (2 marks)
9. Define biodiversity. (2 marks)
10. Explain why higher biodiversity tends to make an ecosystem more stable. (2 marks)