Claim-Evidence Writing

Sample Claim: Genetic evidence supports common ancestry, with more closely related species sharing more DNA. Humans and chimps share 98% of DNA, indicating recent common ancestry, while humans and fruit flies share only 60%, indicating more distant ancestry.

Reasoning: DNA accumulates mutations over time; species that diverged recently have had less time to accumulate differences.

Correction: Evolution is not goal-directed. Organisms don't "try" to adapt. Random mutations occur; those that happen to improve survival get passed on. The environment selects, but organisms don't intentionally change.

Key concept: Variation occurs randomly; selection is non-random. Evolution has no foresight or intention.

Sample CER: Claim: Antibiotic resistance in bacteria is an observable example of evolution by natural selection. Evidence: Before antibiotic use, resistant bacteria were rare (less than 1%); after decades of antibiotic use, many bacterial populations show over 50% resistance. Reasoning: Antibiotics kill susceptible bacteria but resistant ones survive and reproduce. Since resistance is heritable, it increases in the population over generations - exactly what evolution by natural selection predicts.

Answer: "Fittest" doesn't mean strongest or fastest. Fitness means reproductive success - producing surviving offspring. A slow, weak animal that produces many surviving offspring is more "fit" than a strong one that dies without reproducing.

Key point: Evolutionary fitness is measured by genetic contribution to future generations, not physical attributes.

Sample response: The fossil record shows organisms changing over time, with simpler forms appearing in older layers and more complex ones later. Transitional fossils (like Tiktaalik) show intermediate features. Limitations: Fossilization is rare; soft-bodied organisms rarely fossilize; there are gaps. However, the pattern of change is consistent with evolutionary predictions.

Sample Claim: Island birds evolved from mainland ancestors through geographic isolation and natural selection. Evidence: Similar bone structure (homology) indicates common ancestry; differences in external features indicate adaptation to island conditions. Reasoning: When ancestral birds colonized the island, geographic isolation prevented gene flow, and different selective pressures caused divergence over time.

Correction: Humans did not evolve from chimps. Humans and chimps share a common ancestor that lived approximately 6-7 million years ago. Both lineages evolved separately since then. Chimps are our closest living relatives, not our ancestors.

Key concept: Evolution is a branching tree, not a ladder. Living species are cousins, not ancestors of each other.

Answer: Evolution (change in populations over time) is an observed fact - we see it in bacteria, fossils, and DNA. The theory of evolution (by natural selection, genetic drift, etc.) explains HOW it happens. In science, "theory" means a well-tested explanation, not a guess.

Key point: Scientific theories explain facts. Gravity is also both a fact and a theory.

Sample Claim: Geographic isolation can lead to unique evolutionary paths. Evidence: Australia's marsupials (kangaroos, koalas) evolved independently after Australia separated from other continents 45 million years ago, while placental mammals dominated other continents. Reasoning: Without gene flow between continents, different lineages evolved to fill similar ecological niches through convergent evolution.

Sample CER: Claim: Convergent evolution produces similar structures in unrelated organisms facing similar environmental challenges. Evidence: Birds (feathered wings), bats (membrane wings), and insects (chitin wings) all evolved flight independently - shown by different wing structures despite similar function. Bat wings have finger bones; bird wings have fused arm bones; insect wings are entirely different structures. Reasoning: Each group evolved wings separately because flight provides survival advantages (escape predators, find food). Similar selection pressures produced similar solutions through different developmental pathways, demonstrating that natural selection can produce analogous structures in unrelated lineages.