Evolution
High-Yield Summary
- Evolution = a change in allele frequencies within a population over time. Occurs at the POPULATION level across generations — an individual does not evolve during its lifetime.
- Natural selection (Darwin): heritable traits improving survival/reproduction become more common over time. 4 observations: overproduction, variation, selection, adaptation. Acts on PHENOTYPE; the resulting allele-frequency shift IS evolution.
- 3 modes of natural selection: stabilizing (favors intermediate, extremes selected against, distribution narrows), directional (favors one extreme, distribution shifts), disruptive (favors both extremes, middle selected against, distribution becomes bimodal).
- Modern evolutionary synthesis unites Darwin + Mendelian genetics: mutation introduces new variation, recombination reshuffles existing variation (crossing over + independent assortment), selection acts on the result.
- Inclusive fitness: evolutionary success includes both direct fitness (own offspring) and indirect fitness (helping relatives who share alleles reproduce).
- Punctuated equilibrium (Eldredge & Gould, 1972): long periods of stasis interrupted by brief bursts of rapid change — not constant gradual change.
- Speciation = formation of a new species via reproductive isolation. Prezygotic isolation prevents fertilization (temporal, behavioral, mechanical, gametic). Postzygotic isolation occurs after fertilization (hybrid inviable or sterile, e.g. mule).
- 3 patterns: divergent evolution (common ancestor → different traits → homologous structures), convergent evolution (unrelated species → similar traits from similar pressures → analogous structures), parallel evolution (related lineages independently evolve similar traits after diverging).
Modes of Natural Selection
| Mode | Effect |
|---|---|
| Stabilizing | Favors intermediate phenotypes; extremes selected against; distribution narrows, average stays the same |
| Directional | Favors one extreme; distribution's peak shifts toward it; average phenotype changes |
| Disruptive | Favors both extremes; middle selected against; distribution becomes bimodal, may split the population |
Divergent vs. Convergent vs. Parallel Evolution
| Pattern | Result |
|---|---|
| Divergent | Common ancestor → different traits in different environments → homologous structures (shared origin) |
| Convergent | Unrelated species, similar niches/pressures → similar traits independently → analogous structures (no shared origin, similar function) |
| Parallel | Related lineages, already diverged, independently evolve similar traits under similar pressures |
Prezygotic vs. Postzygotic Reproductive Isolation
| Prezygotic | Postzygotic |
|---|---|
| Prevents fertilization from occurring at all | Occurs after fertilization has taken place |
| Temporal, behavioral, mechanical, gametic incompatibility | Hybrid inviable (doesn't survive) or sterile (e.g., the mule) |
Key Terms
- Inclusive fitness
- Evolutionary success counting both an individual's own offspring and indirect contributions through helping relatives (who share alleles) reproduce.
- Punctuated equilibrium
- Model where evolution proceeds via long stasis periods punctuated by rapid bursts of change, rather than constant gradual change.
- Homologous structures
- Structures with shared evolutionary origin, potentially different functions (product of divergent evolution).
- Analogous structures
- Structures with similar function but no shared ancestral origin (product of convergent evolution).
Common MCAT Trap
- Natural selection acts on PHENOTYPE; evolution itself is defined as the resulting change in ALLELE FREQUENCY — a question testing 'what does selection act on' wants phenotype, not genotype directly.
- Homologous structures (shared ancestry, divergent evolution) vs. analogous structures (shared function only, convergent evolution) — a classic trap is bat wings vs. insect wings (analogous) vs. bat wings vs. human arms (homologous).
- Mutation introduces NEW variation; recombination only reshuffles EXISTING variation — don't credit recombination with creating novel alleles.
- An individual organism does not evolve — evolution is a population-level, multi-generational phenomenon.
Quick Recall
What are the 4 core observations underlying natural selection?
A trait distribution becomes bimodal after selection pressure. What mode of selection is this?
Why is a mule (horse × donkey) an example of postzygotic reproductive isolation?
Bat wings and bird wings serve a similar function but evolved independently in unrelated lineages. What kind of structures are these, and what evolutionary pattern produced them?
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