AP Biology: Darwin’s Theory – Natural Selection, Descent with Modification, Common Ancestry

Darwin’s Theory – Natural Selection, Descent with Modification, Common Ancestry

Concept Summary

• Natural Selection: Differential survival and reproduction of individuals due to heritable traits, driving adaptive evolution. Significance: Primary mechanism of evolution.
• Descent with Modification: Species change over generations through inherited variations, leading to diversity. Significance: Explains unity (shared ancestry) and diversity (adaptations) of life.
• Common Ancestry: All organisms share a single ancestor, evidenced by homologous structures, DNA, and fossils. Significance: Foundation of phylogenetic trees.
• Adaptation: Heritable trait that enhances survival/reproduction in a specific environment. Significance: Result of natural selection, not goal-directed.
• Fitness: Relative reproductive success of an individual, measured by offspring contribution to the next generation. Significance: Context-dependent (environment-specific).

Core Questions

WHAT (definitional)

Q: What is natural selection? A: A process where individuals with advantageous heritable traits survive and reproduce more successfully, increasing those traits’ frequency in a population. Trap/Clarification: Natural selection acts on individuals, but evolution occurs in populations; individuals do not "evolve."

Q: What is homology? A: Similarity in traits (e.g., bone structure) due to shared ancestry, not function. Trap/Clarification: Analogous structures (e.g., bat vs. insect wings) arise from convergent evolution, not common ancestry.

WHY (causal/explanatory)

Q: Why is variation essential for natural selection? A: Without heritable variation, there are no differential traits for selection to act upon. Trap/Clarification: Variation must be genetic (not acquired) to drive evolution; Lamarck’s "use/disuse" is incorrect.

Q: Why is common ancestry important for biology? A: It explains nested patterns of shared traits (e.g., DNA code, cell structures) and unifies all life under one evolutionary framework. Trap/Clarification: Shared traits-identical traits; divergence occurs over time (e.g., human vs. chimp DNA is ~98% similar but functionally distinct).

HOW (process/application)

Q: How does natural selection change allele frequencies? A: Beneficial alleles increase in frequency as individuals with them reproduce more; deleterious alleles decrease. Trap/Clarification: Selection acts on phenotypes, but evolution requires changes in genotype frequencies (e.g., recessive alleles may persist in heterozygotes).

Q: How do you identify homologous structures? A: Compare anatomical/molecular traits across species for shared developmental origins (e.g., vertebrate limb bones) despite different functions. Trap/Clarification: Vestigial structures (e.g., whale pelvis) are homologous but non-functional; don’t confuse with analogous traits.

CAN (conditions/possibilities)

Q: Can natural selection produce perfect organisms? A: No; selection works on existing variation, is constrained by trade-offs (e.g., cheetah speed vs. bone strength), and environments change. Trap/Clarification: "Survival of the fittest" is misleading—"fitness" is relative, not absolute.

Q: Under what conditions does descent with modification occur? A: When heritable variation exists, differential reproduction occurs, and traits are passed to offspring over generations. Trap/Clarification: Modification-progress; evolution has no "goal" (e.g., tapeworms lost digestive systems—an adaptation, not a "degeneration").

Quick Facts & Traps

• Fact: Natural selection requires 1) heritable variation, 2) overproduction of offspring, 3) differential survival/reproduction, 4) time.
• Trap: "Individuals adapt to their environment."-Reality: Populations evolve; individuals acclimate (non-heritable changes).
• Fact: Fitness is measured by relative reproductive success, not strength/size (e.g., a sterile peacock with bright feathers has zero fitness).
• Trap: "Evolution explains the origin of life."-Reality: Darwin’s theory addresses diversification of life, not abiogenesis.
• Fact: Convergent evolution (e.g., shark/dolphin body shapes) produces analogous traits, not homologous ones.
• Trap: "All traits are adaptations."-Reality: Some traits are byproducts (e.g., blood color) or neutral (e.g., human chin shape).

Rapid-Fire True/False

• Statement: Natural selection is the only mechanism of evolution. Answer: FALSE Why the common mistake happens: Overemphasis on selection; other mechanisms (e.g., genetic drift, gene flow) also drive evolution.

• Statement: If a trait is common in a population, it must be adaptive. Answer: FALSE Why the common mistake happens: Ignores neutral traits, genetic drift, or historical constraints (e.g., human appendix).

• Statement: All descendants of a common ancestor share the same DNA sequences. Answer: FALSE Why the common mistake happens: Mutations accumulate over time; shared ancestry-identical genomes (e.g., humans and bacteria share ~7% of genes).