Evolution
8th Grade Science · 5 lessons
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Lessons
- 1Natural Selection Needs Three Conditions at OnceNatural selection requires variation between individuals, heritability of that variation, and a difference it makes to survival or reproduction. Remove any one and nothing evolves — which is why a bodybuilder's muscle, acquired within a lifetime and not heritable, changes no population at all. Selection also never creates the variation it acts on; mutation does that, and selection only changes how common existing variants become.Lesson
- 2Antibiotic Resistance Is Selection You Can WatchAntibiotics do not produce resistant bacteria. Rare resistant mutants are already present in a large population, and the antibiotic kills everything else, leaving them to inherit the whole environment. With generation times near twenty minutes, a strain can go from rare to dominant within days. Finishing a prescribed course matters for exactly this reason: stopping early leaves the partly-resistant survivors alive and hands them the population.Experiment
- 3Four Independent Records of Common AncestryFour separate lines of evidence point to the same branching tree: homologous skeletons built from the same set of bones in different proportions, embryos that pass through shared developmental stages, vestigial structures with no current function, and DNA sequences whose differences grow steadily with the time since two lineages split. Any one line could be argued about individually. The force of the case is that four records gathered by different methods produce the same tree.Lesson
- 4Selective Breeding Uses the Same Mechanism, and Has a CostArtificial selection works through exactly the mechanism natural selection uses; only the agent choosing which variants reproduce is different, and because the pressure is intense it works fast — maize was bred from teosinte in a few thousand years. The cost is narrowed variation. Breeds pushed hard toward one trait carry a burden of inherited disease, and a population with little genetic variation has little to draw on when conditions change, which is why crop gene banks exist.Lesson
- 5Speciation Requires a Barrier to Gene FlowOne population becomes two species when gene flow between them stops and selection then pulls the isolated groups apart. The barrier is often geographic — a new river, an island, a rising mountain range — and once separated the groups accumulate different mutations and face different pressures. They count as separate species when they can no longer produce fertile offspring, which is why horses and donkeys, whose mule offspring is sterile, are classified as two species rather than one.Lesson
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