A hospital ward uses a new antibiotic. Before treatment, roughly 1 in $10^9$ bacteria in a patient carry a resistance allele by chance mutation. Explain how a resistant strain can dominate within weeks.

1. Variation: random mutations produce rare resistant bacteria.
2. Selection pressure: the antibiotic kills non-resistant bacteria.
3. Survival and reproduction: resistant bacteria survive and divide every $\sim 20$ minutes.
4. Inheritance: resistance is encoded in DNA, so daughter cells inherit it.
5. After $24$ hours, a resistant lineage that started from one cell can exceed $10^{20}$ cells.

Key idea: natural selection does not create the mutation — it merely amplifies any variant that already survives and reproduces better in the current environment.

Before the Industrial Revolution, most peppered moths in England were light-coloured, camouflaged on pale lichen-covered trees. As factories blackened the trees with soot, the frequency of dark moths rose from below $1\%$ to over $95\%$ within about $50$ years. Explain using the four conditions.

1. Variation: both light and dark forms existed (genetic polymorphism).
2. Selection pressure: birds preyed more on the now-conspicuous light moths against sooty bark.
3. Reproduction: dark moths survived to lay more eggs.
4. Heritability: colour is genetic, so offspring were also dark.

When clean-air laws reversed the pollution, the light form recovered — a second wave of selection in the opposite direction.

Compare the wings of a bat and the wings of a butterfly. Are they homologous or analogous? What does this tell us about evolution?

1. Bat wings have the same bone layout as a human arm (humerus, radius, ulna, fingers) — evidence of shared ancestry. They are homologous to human arms.
2. Butterfly wings are made of chitin with no bones. They evolved independently for the same function (flight).
3. Bat and butterfly wings are analogous: similar function, different origin. This is an example of convergent evolution.

On the Galapagos islands, one ancestral finch species radiated into $\sim 15$ species with different beaks. Explain this using allopatric speciation.

1. A small founding population reached different islands.
2. Each island had different food sources (large seeds, insects, cactus flowers).
3. Isolation limited gene flow between islands.
4. Natural selection favoured different beak shapes on each island.
5. Over thousands of generations the populations became distinct species — ground finches, cactus finches, tree finches, warbler finches.

Key idea: geographic isolation plus different selection pressures drives species to diverge from a common ancestor.

Humans and chimpanzees share a common ancestor about $6$ million years ago; humans and gorillas about $8$ million; humans and orangutans about $14$ million. Which species is most closely related to humans, and how does DNA confirm this?

1. The most recent common ancestor is with chimpanzees.
2. DNA similarity with chimps is about $98.8\%$, with gorillas $\sim 98\%$, with orangutans $\sim 97\%$.
3. More DNA similarity $\leftrightarrow$ more recent common ancestor.