CSCredit Sprint
Sign upStart free

Credit Sprint / Study guides / Biology Unit 7

Credit Sprint's Guide to Biology Unit 7: Natural Selection

AP® weight 13–20%CLEP® weight 14–18%

Populations evolve when allele frequencies change over generations. Natural selection acts on inherited variation, genetic drift and gene flow change populations too, and evidence from fossils, anatomy and molecules shows that all life shares common ancestors.

Modes of natural selection Trait value (e.g., beak depth)Original population (dashed)

What you need to know

  • Natural selection needs variation, inheritance, overproduction and differential survival and reproduction. Individuals don't evolve; populations do.
  • Fitness means reproductive success, not strength.
  • Directional selection favors one extreme; stabilizing selection favors the middle; disruptive selection favors both extremes.
  • Genetic drift is random change in allele frequency, strongest in small populations (bottleneck and founder effects). Gene flow moves alleles between populations.
  • Hardy–Weinberg equilibrium holds when there's no selection, no mutation, no migration, random mating and a very large population.
  • Evidence for evolution: fossils, homologous structures, vestigial structures, embryology, DNA and protein comparisons, and biogeography.
  • Homologous structures share ancestry; analogous structures share function from convergent evolution.
  • Phylogenetic trees show relationships. Taxa that share a more recent common ancestor are more closely related, and shared derived characters define clades.
  • Speciation: allopatric (geographic isolation) or sympatric (in the same area, for example by polyploidy). Prezygotic barriers stop fertilization; postzygotic barriers make hybrids weak or sterile.
  • Extinction opens niches; adaptive radiation follows. Earth's early conditions may have allowed simple organic molecules and RNA to form first.

Equations and rules

Hardy–Weinberg: p + q = 1
p² + 2pq + q² = 1 (homozygous dominant, heterozygous, homozygous recessive)
q² = frequency of the recessive phenotype

Exam traps

Watch forOrganisms don't develop traits because they need them. Variation must already exist.
Watch forRead a phylogenetic tree by the most recent common ancestor, not by how close two tips sit on the page.
Watch for2pq, not p × q, is the heterozygote frequency.

Key terms

  • Natural selection: Individuals with heritable traits that fit the environment survive and reproduce more.
  • Fitness: Reproductive success: how many genes an individual passes on.
  • Genetic drift: Random change in allele frequencies, strongest in small populations.
  • Bottleneck effect: A disaster shrinks a population, leaving a random, less diverse set of alleles.
  • Founder effect: A small group starts a new population with different allele frequencies.
  • Gene flow: Movement of alleles between populations through migration.
  • Hardy–Weinberg equilibrium: A population whose allele frequencies don't change, because no evolution is happening.
  • Homologous structures: Similar structures from a common ancestor, possibly with different functions.
  • Analogous structures: Similar functions from convergent evolution, not shared ancestry.
  • Clade: A common ancestor and all of its descendants.
  • Allopatric speciation: New species form after a geographic barrier separates populations.
  • Prezygotic barrier: Prevents mating or fertilization, such as different mating seasons.

Practice questions

Five of the 26 Unit 7 questions in Credit Sprint. Every choice is explained in the app.

1. Which is required for natural selection to occur?
  1. Organisms trying to change to fit their environment
  2. Heritable variation in a trait that affects survival or reproduction
  3. A large population with no mutations
  4. A constant environment
Show answer

Heritable variation in a trait that affects survival or reproduction. Without inherited variation there is nothing to select.

2. A storm kills 90% of a beetle population at random. The survivors' allele frequencies differ from the original population's. This is
  1. a bottleneck effect (genetic drift)
  2. gene flow
  3. Hardy–Weinberg equilibrium
  4. natural selection
Show answer

a bottleneck effect (genetic drift). Random survival changes frequencies.

3. Using the previous population (q = 0.3), what fraction is heterozygous?
  1. 0.42
  2. 0.21
  3. 0.49
  4. 0.09
Show answer

0.42. 2pq = 2 × 0.7 × 0.3.

4. On a phylogenetic tree, two species that share the most recent common ancestor are
  1. the oldest species
  2. the most different
  3. unrelated
  4. most closely related
Show answer

most closely related. That's the definition used on trees.

5. Polyploidy in plants can lead to
  1. gene flow between species
  2. allopatric speciation only
  3. the extinction of the parent species automatically
  4. sympatric speciation in a single generation
Show answer

sympatric speciation in a single generation. Polyploids often can't breed with the parent species.