Every fact in the three evolution lessons is listed here with where it comes from. The numbers in [brackets] on the lesson pages link to this list.
How we rate sources
High: space agencies (NASA), peer-reviewed open textbooks (OpenStax, Rice University) and professional scientific societies (American Physical Society).
Medium-high: reputable science news reporting directly on a primary document.
Medium: single-author essays and popular history sites. We only use these for history details, and never on their own: each one is paired with a second source.
Wikipedia was not used as a source. Numbers are rounded (for example 9.8 m/s² for Earth's gravity). The text was drafted with help from AI (Claude) and then checked against these sources. Corrections: hello@echoesof.science.
Where the lessons simplify: the paper-moth and beak-tool activities are models: real predators, real birds and real seeds behave in more complicated ways, and a 'generation' in the class game takes seconds instead of a year. The peppered moth story is told in its classic form from Britannica (soot, lichens and birds); the 99-to-1 figure is a historical count from the Manchester area, not a worldwide value. The finch example uses the 1977 drought on one Galápagos island, as described in OpenStax Concepts of Biology; later years with different weather changed bill sizes in other directions. The selection calculations for ages 15–16 use made-up round numbers to show the method; they are not measured data. Antibiotic resistance is described as general education only: questions about medicines belong with a doctor or pharmacist.
All sources
[1] Biology 2e, 18.1 Understanding Evolution
OpenStax (Rice University), open textbook · Reliability: High
Type: Open university-level textbook
Charles Darwin and Alfred Russel Wallace independently described natural selection and presented their ideas together in 1858; Darwin's On the Origin of Species followed in 1859; natural selection follows from three observations: most characteristics are inherited, more offspring are produced than are able to survive, and offspring vary in their characteristics, so individuals with traits better suited to their surroundings are more likely to survive and reproduce; Darwin saw that the beak shapes of Galápagos finches matched their food: seed-eaters had thick, strong beaks and insect-eaters spear-like beaks; evidence for evolution comes from fossils (gradual changes in form over millions of years), anatomy (homologous structures such as the same bone layout in the limbs of humans, dogs, birds and whales; vestigial structures such as wings on flightless birds), embryology (vertebrate embryos briefly show gill slits and tails), biogeography (for example Australia's marsupials after long isolation) and molecular biology (the universal use of DNA, and more similar DNA in more closely related groups); misconceptions: a scientific theory is a well-tested, well-supported explanation, not a guess; individuals do not evolve, populations do; evolution is not goal-directed, and a trait that helps in one environment may be harmful in another.
[2] Concepts of Biology, 11.1 Discovering How Populations Change
OpenStax (Rice University), open textbook · Reliability: High
Type: Open introductory textbook
Natural selection can only take place if there is variation among individuals in a population, and the variation must have a genetic basis or selection will not lead to change in the next generation; in 1977 a drought on a Galápagos island changed the vegetation: small, soft seeds became scarcer than large, hard seeds, large-billed medium ground finches survived better than small-billed ones, and the average bill size in the next generation was larger, because bill size is inherited; an adaptation is a 'match' of the organism to its environment, and whether a trait is favourable depends on the environment, which can change; the wings of a hummingbird and an ostrich are homologous structures, inherited from a common ancestor with wings.
The peppered moth (Biston betularia) is a classic example of natural selection (industrial melanism); it has a light form and a dark form, controlled by genes; before 1848 the light form was the usual one; a dark moth was first noticed in Manchester, England, in 1848, and by 1898 dark moths outnumbered light ones 99 to 1; the dark form first arose by chance mutation; coal soot from the Industrial Revolution darkened tree trunks and killed pale lichens; on sooty trunks dark moths were harder for birds to see and light moths were easier to see, so birds ate more of the light moths.
US Centers for Disease Control and Prevention (CDC) · Reliability: High
Type: National public-health agency page
Antimicrobial resistance happens when germs such as bacteria and fungi develop the ability to defeat the drugs designed to kill them; resistance speeds up when antibiotics and antifungals put pressure on bacteria and fungi to adapt; the resistant germs survive, multiply and can spread; antimicrobial resistance is estimated to kill over one million people around the world each year.