Ecology, classification, evolution and early-life hypotheses
Model population change, classify from evidence, and separate natural selection from hypotheses about life's origins.
What you will tackle
- Use food-web and population models
- Classify organisms with a supplied key
- Explain natural selection through inherited variation
- Distinguish early-life hypotheses from later evolution and observations
Populations interact
A population is one species in a defined place; a community includes interacting populations. Food-web arrows commonly point from a food source to its consumer, showing energy transfer. Births and immigration add individuals; deaths and emigration remove them. Resources, predation and other limiting factors can change population growth. Matter cycles through ecosystems, while energy transfers and ultimately dissipates as heat.
Classification and change
Taxonomy groups organisms using evidence of similarities and relationships. A dichotomous key uses paired observable choices; a classification is a model supported by features and, where available, genetic evidence. Natural selection requires variation, a heritable component and differences in reproductive success. Populations can change over generations; an individual does not evolve a required trait simply by trying.
Origins require a different question
Evolution explains changes in populations after reproduction and inheritance exist. Origin-of-life research investigates how early chemical systems could have acquired life-like properties. Laboratory production of an organic molecule under specified conditions supports chemical possibility; it does not demonstrate that the same historical route occurred or that a living organism was produced. Unresolved questions and competing hypotheses remain explicit.
Build your notes, then practise
Work on paper before opening an explanation. Guided tasks provide a starting point; independent tasks remove the scaffolding; application tasks ask you to justify a method or interpretation in context.
Guided practice
Guided 1
A population starts at 100; births=20, deaths=8, immigration=5, emigration=7. Find its final size. In grass → rabbit → fox, explain what the arrows mean. A supplied key says ‘backbone present → vertebrate; absent → invertebrate’: classify a rabbit.
Show a starting hint
Keep entry and exit processes distinct.
Compare with the explained answer
- 100 + 20 − 8 + 5 − 7 = 110.
- The arrows indicate transfer from food to consumer: grass feeds rabbit, rabbit feeds fox.
- A rabbit has a backbone and follows the vertebrate branch of this simple key.
Answer / response criteria: 110; food-to-consumer energy transfer; vertebrate.
Independent practice
Independent 1
Beetles vary in inherited colour; birds catch more visible beetles before they reproduce. Explain how colour frequencies may change without individuals deciding to change. If rabbit numbers fall, must fox numbers immediately rise? What would an experiment producing organic molecules establish about early life?
Compare with the explained answer
- Less-visible inherited variants can leave more offspring, changing colour frequencies over generations.
- The food-web relationship does not imply an immediate fox increase; reduced prey can constrain predators, with delays and other food sources relevant.
- Producing organic molecules demonstrates a possible chemical process under those conditions, not proof of the historical origin of life.
Answer / response criteria: Differential reproduction changes frequencies; no necessary immediate fox increase; chemical possibility is distinct from historical proof.
Application practice
Application 1
A study finds drought-tolerant plants more common after repeated dry seasons. Propose evidence distinguishing heritable population change from temporary individual adjustment. Explain how a new genetic comparison could revise an older feature-based classification, and why neither result alone explains the first origin of life.
Compare with the explained answer
- Compare inherited traits across generations under common conditions to separate heritable differences from immediate environmental responses.
- Genetic evidence can reveal relationships not captured by a superficial shared feature, motivating a revised grouping.
- These questions concern existing organisms and inheritance; they do not reconstruct the emergence of the first reproducing systems.
Answer / response criteria: Use generational/common-condition evidence; classification can change with new relationship evidence; origins remain a separate question.
What can you explain now?
A study finds drought-tolerant plants more common after repeated dry seasons. Propose evidence distinguishing heritable population change from temporary individual adjustment. Explain how a new genetic comparison could revise an older feature-based classification, and why neither result alone explains the first origin of life.
Compare with the explanation
Use generational/common-condition evidence; classification can change with new relationship evidence; origins remain a separate question.
Explain the reasoning before comparing. This review records your self-assessment rather than an official score or mastery decision.
After trying it yourself, choose your next review. This is your self-assessment.
Your review choice appears on Today. Sign in to sync it across devices.
Source and review status
Coverage follows the named requirements and examples in the 2026–2027 ACT test guide and subject descriptions. That guide explicitly does not list every possible topic. The broader coded College and Career Readiness Standards are also mapped to conceptual teaching and practice; this does not calibrate a learner to a score band. The separate scientific-background context list and reading-complexity rubric have their own stated limits.
Original teaching and explained self-check practice are available. Specialist review and difficulty calibration remain pending. Essays require self-check or teacher review and are not automatically marked or awarded official scores.
Official source specification / guide