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Evolution & Natural Selection: How Populations Change Over Time

Natural selection isn't 'survival of the strongest' — it's differential reproduction based on heritable variation. Master Darwin's mechanism, the evidence for evolution, and the famous Galápagos finches.

9 min TEKS 7A,7B,7C,7D,7E Biology

Darwin's idea in one sentence

Natural selection is the differential reproduction of individuals due to heritable variation in traits that affect survival and reproduction.

That’s a mouthful, but unpacking it gives you everything you need:

The four conditions for natural selection
  1. Variation — individuals in a population differ from each other.
  2. Heritability — those differences are passed from parent to offspring.
  3. Differential survival/reproduction — some variants reproduce more than others.
  4. Time — the favored traits become more common across generations.

What natural selection is NOT

  • It’s not “survival of the strongest.” A peacock’s tail is heavy and impractical, but it survives because peahens prefer it.
  • It’s not directed by any goal. Mutations are random; selection just filters them.
  • It’s not something individuals do. Populations evolve. Individuals just live or die.

The classic example: peppered moth

Across three generations, the dark-beetle frequency rises. That's natural selection in action — no labels needed.
Across three generations, the dark-beetle frequency rises. That's natural selection in action — no labels needed.

Across three generations, the dark variants come to dominate. Why? Predators (birds) spotted and ate the lighter ones more often. The dark beetles weren’t “trying” to evolve — they just had a heritable color trait that helped them survive long enough to reproduce.

Darwin's Galápagos finches

Charles Darwin observed 14 species of finches on the Galápagos Islands, all descended from a single mainland ancestor. Each species had a beak adapted to a different food source — small/sharp for insects, thick/strong for cracking seeds, long/curved for probing flowers.

Four finches, four beak shapes, one common ancestor. This is adaptive radiation.
Four finches, four beak shapes, one common ancestor. This is adaptive radiation.
Adaptive radiation
When one ancestor diversifies into many species, each adapted to a different niche or environment. The Galápagos finches are the textbook example.

Six lines of evidence for evolution

  1. Fossil record — layers show change over time (whale ancestors had legs).
  2. Homologous structures — same bone arrangement in human arm, bat wing, whale flipper. Common ancestry.
  3. Vestigial structures — appendix, whale pelvic bones — leftovers from ancestral function.
  4. Embryology — vertebrate embryos look strikingly similar early on.
  5. Biogeography — species on isolated islands resemble nearby mainland species.
  6. Molecular biology — closely related species share more DNA. The strongest modern evidence.

Vocabulary that often shows up

  • Adaptation — an inherited trait that increases survival/reproduction in a specific environment.
  • Speciation — one species splitting into two reproductively isolated species.
  • Genetic drift — random changes in allele frequency, especially in small populations.
  • Gene flow — allele exchange between populations via migration.

Check yourself

Quick check #1
Which of these is NOT necessary for natural selection to occur?
Quick check #2
Darwin observed 14 finch species on the Galápagos Islands, each with a different beak shape suited to a different food source. What evolutionary process is this?
The four Galápagos finches Darwin observed.

Practice with real CBE questions