Block 7 · Evolution · MCAS Reporting Category 3
Evolution and natural selection
Evolution was only about 6% of your class final, but it makes up 20% of MCAS — so it needs more attention than usual. This block covers natural-selectionProcess where organisms with helpful traits survive and reproduce more. (the four conditions), evidence for evolutionHow species change over time through inherited traits and natural selection. (DNA is strongest), convergent-evolutionUnrelated species evolving similar traits because they live in similar environments. vs common ancestry, and speciationThe formation of new species when populations become separated and evolve differently..
natural-selection
evolution
convergent-evolution
speciationWhat you need to know cold
- natural-selectionProcess where organisms with helpful traits survive and reproduce more. needs four things: variation, heritability, differential reproduction, and selection pressure.
- The best evidence for how closely two species are related: comparing DNA or amino acid sequences. Not fossils. Not anatomy. DNA.
- homologousStructures in different species that share the same origin but may have different functions. structures = same bone pattern, different function = common ancestor.
- convergent-evolutionUnrelated species evolving similar traits because they live in similar environments. = unrelated species look alike because they live in similar environments.
- speciationThe formation of new species when populations become separated and evolve differently. = populations get isolated → evolve differently → can no longer interbreed → two species.
- An adaptationA trait that helps an organism survive and reproduce in its environment. is a trait that helps survival. It develops over generations through natural selection.
- High-diversity populations survive environmental changes better than low-diversity populations.
- Mass extinctions happen when environments change faster than organisms can adapt.
homologous
adaptationThe Big Rule for this block
Natural selection needs four things. No exceptions.
Variation, heritability, differential reproduction, selection pressure. If the question asks "what does natural selection require?" or gives you a scenario and asks "is this natural selection?" — check for all four.
Key vocabulary in 8 languages
Words from this block. Use the row in your home language to help your memory. Many of these words are similar across languages because they come from Greek and Latin roots.
| English | Español | Português | Français | Italiano | Kreyòl | Tiếng Việt | العربية |
|---|---|---|---|---|---|---|---|
| evolution | evolución | evolução | évolution | evoluzione | evolisyon | tiến hóa | تطور(taṭawwur) |
| natural selection | selección natural | seleção natural | sélection naturelle | selezione naturale | seleksyon natirèl | chọn lọc tự nhiên | انتقاء طبيعي(intiqāʾ ṭabīʿī) |
| adaptation | adaptación | adaptação | adaptation | adattamento | adaptasyon | đặc điểm thích nghi | صفة تكيفية(ṣifa takayyufiyya) |
| speciation | especiación | especiação | spéciation | speciazione | espesyasyon | sự hình thành loài | انتواع(intiwāʿ) |
| homologous | homólogo | homólogo | homologue | omologo | omològ | cơ quan tương đồng | أعضاء متماثلة(aʿḍāʾ mutamāthila) |
| convergent evolution | evolución convergente | evolução convergente | évolution convergente | evoluzione convergente | evolisyon konvèjant | tiến hóa hội tụ | التطور التقاربي(at-taṭawwur at-taqārubī) |
| mutation | mutación | mutação | mutation | mutazione | mitasyon | đột biến | طفرة(ṭafra) |
All 7 rows use verified translations from the Quick Reference vocabulary table (Section 1). Vietnamese and Arabic were verified by GPT-5 and Gemini; Romance languages rely on cognate consistency.
The full picture
Evolution and natural selection — how species change over time
What this reading is about
evolutionHow species change over time through inherited traits and natural selection. was about 6% of your class final but makes up 20% of MCAS. This block gives it the attention it deserves. You need to understand four things:
- What natural-selectionProcess where organisms with helpful traits survive and reproduce more. is and the four conditions it needs.
- What counts as evidence for evolution — and which evidence is strongest.
- The difference between convergent-evolutionUnrelated species evolving similar traits because they live in similar environments. and common ancestry.
- How new species form (speciationThe formation of new species when populations become separated and evolve differently.).
Natural selection: the engine of evolution
natural-selectionProcess where organisms with helpful traits survive and reproduce more. is the main process that drives evolutionHow species change over time through inherited traits and natural selection.. It is not random — it is the environment "selecting" which traits work best. For natural selection to happen, four conditions must all be present:
- Variation — individuals in a population are different from each other (different colors, sizes, speeds, etc.).
- Heritability — those differences are genetic and can be passed from parent to offspring.
- Differential reproduction — some individuals survive longer and leave more offspring than others.
- Selection pressure — something in the environment (predators, disease, food scarcity, climate) favors certain traits.
When all four are present, the traits that help survival become more common in the population over generations. That is natural selection.
What happens when the environment changes?
If the environment changes, the traits that are "fit" may change too. A population with high diversity (lots of variation) has a better chance of surviving because some individuals may already have traits that work in the new environment. A population with low diversity is more vulnerable — if no individuals have the right traits, the whole population may decline or go extinct.
This is why genetic variation matters. It is the raw material for natural selection. Without variation, there is nothing to select.
Evidence for evolution: which is strongest?
Scientists use several types of evidence to support evolution and to figure out how species are related:
| Type of evidence | What it tells us | How strong? |
|---|---|---|
| DNA / amino acid sequences | Species with more similar DNA are more closely related. | STRONGEST — most precise and direct. |
| Anatomy (homologousStructures in different species that share the same origin but may have different functions. structures) | Same bone pattern, different function = common ancestor. | Good |
| Embryology | Early embryos of related species look very similar. | Good |
| Fossils | Show what ancient organisms looked like; show transitions over time. | Good |
Homologous structures vs convergent evolution
This is a common MCAS distinction. Two species can look similar for two very different reasons:
| Homologous structures | Convergent evolution | |
|---|---|---|
| Why they look similar | Same ancestor | Same environment |
| DNA comparison | DNA is similar | DNA is different |
| Internal structure | Same bone/tissue pattern | Different internal structure |
| Example | Human arm and bat wing (same bones) | Dolphin and ichthyosaur (both streamlined, but mammal vs reptile) |
The test: if two species look alike, check their DNA. Similar DNA = common ancestor (homologousStructures in different species that share the same origin but may have different functions.). Different DNA = convergent-evolutionUnrelated species evolving similar traits because they live in similar environments. (similar environment pushed them to look alike independently).
Speciation: how new species form
speciationThe formation of new species when populations become separated and evolve differently. happens when one species splits into two. The usual path:
- A population gets separated by a barrier (river, mountain, ocean).
- Each group faces different environments and different selection pressures.
- Over many generations, the groups become so different they can no longer interbreed.
- Two species now exist where one used to.
This is why islands and isolated habitats often have unique species found nowhere else — populations arrived, were cut off, and evolved separately.
Mass extinctions
When the environment changes faster than organisms can adapt, populations decline. A mass extinction happens when a large-scale environmental change (asteroid impact, volcanic eruptions, rapid climate shift) kills off many species at once. After a mass extinction, surviving species often diversify rapidly to fill the empty niches — this is called an adaptive radiation.
Why this matters for MCAS
Evolution questions make up about 20% of MCAS Biology. The most common question types are:
- "What four things does natural selection need?" → variation, heritability, differential reproduction, selection pressure.
- "Best evidence for relatedness?" → DNA / amino acid sequences.
- "Two unrelated species look alike — why?" → convergent-evolutionUnrelated species evolving similar traits because they live in similar environments. (similar environments).
- "How do new species form?" → speciationThe formation of new species when populations become separated and evolve differently. (isolation + different selection pressures + time).
- Scenario questions: "Environment changes — what happens to the population?" → best-adapted survive; diversity decreases; population may shift.
If you can answer those five question types, you have the evolution portion of MCAS covered.
Pictures to recognize on the test
| The picture shows… | The answer is… |
|---|---|
| A table comparing DNA or amino acid sequences between species, with percentages or counts of differences. | Evidence for relatedness. More similar DNA = more closely related. |
| Four forelimbs (human arm, bat wing, whale flipper, cat leg) with the same bone pattern highlighted. | Homologous structures. Same origin, different function = common ancestor. |
| Two unrelated animals that look similar (dolphin and ichthyosaur, or bird wing and bat wing). | Convergent evolution. Similar environments → similar shapes, but NOT related. |
| A population graph showing diversity decreasing after an environmental change. | Natural selection in action. Best-adapted survive; diversity drops. |
| A branching tree diagram (phylogenetic tree / cladogram) connecting multiple species. | Evolutionary relationships. Closer branches = more closely related species. |
| A series of fossils showing gradual change over time (e.g., horse evolution, whale ancestors). | Fossil evidence for evolution. Species change over geologic time. |
Pattern rules
| If the question says… | Pick… |
|---|---|
| "Best evidence for how closely two species are related." | Comparing DNA or amino acid sequences. Always. Not fossils, not anatomy. |
| "High-diversity population faces a new environment." | Best-adapted survive — diversity decreases. The population shifts toward the favored trait. |
| "Two unrelated species look alike — why?" | Convergent evolution. Similar environments select for similar traits. |
| "What makes natural selection happen?" | Variation + heritability + differential reproduction + selection pressure. All four. |
| "How do new species form?" | Speciation. Populations get isolated, evolve separately, can no longer interbreed. |
| "Why do dolphins and ichthyosaurs look alike?" | Convergent evolution — same aquatic environment, not same ancestor. |
| "What causes mass extinctions?" | Environmental changes organisms cannot adapt to fast enough. |
| "Where do new alleles come from?" | Mutations. Mutations are the ultimate source of genetic variation. |
Where to practice
Practice the Block 7 — Evolution full review test on Pear Assessment. You can retake it as many times as you want — the questions and answer choices shuffle each time, so every attempt feels a little different. Try it without looking at this page first. If you get stuck, come back, look up the pattern, then try again.