In 1977, a drought hit Daphne Major, a small island in the Galápagos. Peter and Rosemary Grant had been measuring its medium ground finches for years. As small, soft seeds ran out, finches with deeper beaks were better at cracking the larger, harder seeds that were left. Many birds died. The survivors' offspring had deeper beaks, on average, than the birds before the drought. No finch changed its own beak; the population changed. Let's look closer at how that works.
Natural selection and fitness
Darwin's idea: competition for limited resources leads to differential survival. Individuals with more favorable phenotypes are more likely to survive and produce more offspring, so those heritable traits become more common in later generations.
- Fitness means reproductive success, not strength or speed.
- Selection acts on phenotypic variation. A variation can raise fitness in one environment and lower it in another.
- Environments change. Biotic and abiotic conditions shift, so the direction of selection can shift too.
Artificial selection
When humans choose which individuals breed, that's artificial selection. Cabbage, kale, broccoli, cauliflower and Brussels sprouts are all varieties of one species, Brassica oleracea, bred from one wild plant.
Population genetics
Evolution is also driven by random events:
- Mutation adds new genetic variation.
- Genetic drift changes allele frequencies by chance, with the strongest effects in small populations. The bottleneck effect happens when a population is cut to a few individuals; the founder effect happens when a small group starts a new, separate population.
- Gene flow (migration) adds or removes alleles and keeps populations from diverging.
A change in allele frequencies over generations is evidence that a population is evolving.
Hardy-Weinberg, briefly
The Hardy-Weinberg equations (p + q = 1 and p² + 2pq + q² = 1) describe a population that is not evolving. That requires a large population, no migration, no new mutations, random mating and no natural selection. No real population meets all five, which is why the model works as a null hypothesis.
Evidence of evolution and common ancestry
Fossils are dated by the rock layers they're in, by isotope decay such as carbon-14, and by geographical data. Homologous structures, like the forelimb bones of humans, whales and bats, and vestigial structures, like the small pelvic bones of whales, which no longer support hind legs, point to common ancestry. DNA and protein sequence comparisons add molecular evidence. All eukaryotes share membrane-bound organelles, linear chromosomes and genes with introns, which supports a common ancestor.
Evolution is still happening: think of antibiotic resistance in bacteria, pesticide resistance in insects, and emerging diseases.
Phylogeny
Cladograms and phylogenetic trees are testable hypotheses about relationships. Each node is the most recent common ancestor of the lineages branching from it. The out-group is the least closely related lineage. Shared derived characters group lineages, and molecular data are usually more reliable than morphology. Only a phylogenetic tree shows amount of change over time, calibrated by fossils or a molecular clock.
Speciation
By the biological species concept, a species can interbreed to produce viable, fertile offspring. Speciation happens when populations become reproductively isolated: allopatric when they're geographically separated, sympatric when they overlap. Prezygotic barriers (habitat, timing, behavior) block mating or fertilization; postzygotic barriers make hybrids inviable or sterile. Apple maggot flies (Rhagoletis) are often studied as possible sympatric speciation in progress.
Change can be gradual (gradualism) or come in bursts after long stasis (punctuated equilibrium). Divergent evolution and adaptive radiation produce diversity as new habitats open; convergent evolution gives unrelated groups, like sharks and dolphins, similar shapes.
Variation, extinction and the origin of life
Genetically diverse populations are more resilient, because some individuals are likely to withstand a new pressure. Populations with little diversity, such as California condors, are at risk of decline or extinction.
Earth formed about 4.6 billion years ago, was too hostile for life until about 3.9 billion years ago, and the earliest fossil evidence of life dates to about 3.5 billion years ago. The RNA world hypothesis proposes that RNA may have been the first genetic material, copied by base pairing, with no genetically encoded proteins acting as catalysts.
Common mistakes students make
- "Individuals evolve." Selection acts on individuals; populations evolve.
- Writing Lamarckian answers. Organisms don't develop traits because they need them. The variation was already there.
- Using "fitness" as a buzzword. Explain it as reproductive success.
- Mixing up isolating mechanisms. Say whether the barrier acts before or after a zygote forms.
Frequently asked questions
What is the difference between natural selection and genetic drift?
Natural selection changes allele frequencies because some phenotypes reproduce more. Genetic drift changes them by chance, regardless of phenotype.
How do you read a cladogram?
The more recent the node two groups share, the more closely related they are.
What is the difference between homologous and analogous structures?
Homologous structures come from a common ancestor; analogous structures look or work alike because of convergent evolution.
Is extinction part of AP Biology Unit 7?
Yes, through genetic diversity: populations with little variation are at greater risk of decline or extinction.
Try it
Say each answer before you flip the card. Then predict: what would you expect to see if a population of identical, cloned plants met a brand-new disease?
AP is a trademark registered by the College Board, which is not affiliated with Braineos and doesn't endorse it.