18.1 Understanding Evolution

Natural selection

Individuals with favorable heritable traits out-reproduce the rest, shifting the whole population

Darwin's three principles

Traits are inherited; more offspring born than resources support; offspring vary heritably

Whose population essay gave Darwin and Wallace the "not enough resources" piece?

Thomas Malthus — an economist, not a biologist

Adaptation

Heritable trait that boosts survival/reproduction in the environment you're in right now

Two sources of genetic variation

Mutation (brand-new alleles) and sexual reproduction (new combos of old ones)

Big leaves were great, then the climate dried out. What happened?

Selection flipped — small leaves conserve water and won instead. "Favorable" is never permanent

Divergent evolution

One ancestor, two lineages drifting apart in form

Convergent evolution

Unrelated species independently landing on the same solution

Bat wing vs. insect wing

Convergent — same job, completely different starting structures

Homologous structures

Same bone layout inherited from a common ancestor (your arm, a bird's wing, a whale's flipper)

Vestigial structures

Leftover parts doing nothing — whale hind-leg bones, cactus leaves, flightless bird wings

Analogy / homoplasy

Looks alike because of shared pressures, not shared ancestry

Why do human embryos have gill slits and a tail?

Embryo development is highly conserved — it's a receipt from our vertebrate ancestors

Proteaceae growing in Australia, southern Africa, AND South America

Biogeography — their ancestors covered Gondwana before it split

Molecular evidence

DNA is universal, and how similar two sequences are tracks how closely related the species are

"Evolution is just a theory"

In science a theory is tested and verified, like gravity. The everyday "theory" is really a hypothesis

Did the Grants' finches grow smaller beaks during their lifetimes?

No — the population average shrank as small-billed birds out-survived the rest. Individuals develop; populations evolve

Does evolution explain how life started?

No — only how populations change and diversify once life already exists

Did antibiotics create resistance in bacteria?

No — the resistance gene was already there at low frequency; the drug just kills everything else