GED Science
Evolution & Natural Selection
Natural selection is a filter, not a wish. Individuals in a population vary; the environment kills or starves some variants more than others; the survivors breed and pass their versions along. Over generations the population shifts. No individual ever changes itself.
The words first
- variation
- existing differences between individuals in a population
- adaptation
- an inherited trait that helps survival or reproduction
- natural selection
- the environment filtering which variants breed more
- fitness
- how much an individual reproduces — not gym strength
The method
THE FOUR-STEP FILTER
1. VARY beetles are green AND brown (already, by chance)
2. STRUGGLE birds hunt the beetles
3. SELECT green shows against brown bark → green eaten more
4. INHERIT survivors (mostly brown) breed → next generation browner
WHAT SELECTION IS NOT
✗ "the beetles decided to turn brown" (no choosing)
✗ "one beetle changed color to survive" (individuals don't change)
✗ "they needed brown, so they evolved it" (need creates nothing)
✓ brown ALREADY existed; it just survived better
EVIDENCE THE TEST CITES
fossils in dated layers · homologous limb bones ·
antibiotic-resistant bacteria (selection speeded up)
Worked all the way through
The problem
A doctor prescribes antibiotics; a few bacteria in the patient happen to carry resistance. Why can the infection come back stronger?
- Step 1 — Find the variation.Most bacteria are killable; a few, by chance, already resist the drug.
- Step 2 — Apply the filter.The antibiotic kills the killable ones. The resistant few survive.
- Step 3 — Let the survivors breed.Bacteria reproduce fast, so the next population descends almost entirely from resistant ancestors.
- Step 4 — State the outcome without the trap words.The drug didn't CREATE resistance and no bacterium changed itself — the drug filtered out everything except the resistance that already existed.
On the board
variation: a few already resistant
step 1
antibiotic = the filter
kills the rest
survivors reproduce fast
new population: mostly resistant
drug selected, didn't create
the key sentence
Answer
The antibiotic killed the vulnerable bacteria, and the already-resistant survivors repopulated
A second one, in a different form
Why do whales have finger-like bones inside their flippers?
same bone layout as human hands, bat wings
homologous structures
best explanation: shared ancestor
evidence FOR common descent
Answer
They inherited the bone pattern from a common ancestor with land mammals
The usual mistake
Saying an animal 'adapted itself' during its lifetime. Needing a trait does not create it.
Check yourself
Reread your answer and strike any verb where an organism wants, tries, decides, or needs. Selection has no intentions — if your explanation needs one, it is the wrong explanation.
Practice it
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