Population: Group of individuals of same species in same area that can interbreed

Darwin's Theory of Natural Selection

Key observations:

1. Overproduction: More offspring than can survive
2. Variation: Individuals differ in traits
3. Heredity: Traits passed to offspring
4. Competition: Struggle for resources

Result: Natural Selection

• Individuals with advantageous traits survive and reproduce more
• Favorable alleles increase in frequency
• "Survival of the fittest" (reproductive success)

Mechanisms of Evolution

1. Natural Selection

Types:

Directional selection:

• One extreme favored
• Mean shifts one direction
• Example: antibiotic resistance, peppered moths

Stabilizing selection:

• Intermediate favored
• Reduces variation
• Example: human birth weight

Disruptive selection:

• Both extremes favored
• Increases variation
• Example: beak sizes in African seedcrackers

Sexual selection:

• Traits increase mating success
• May reduce survival (e.g., peacock tail)
• Examples: bright colors, large antlers, mating displays

2. Genetic Drift

Random changes in allele frequencies

• More effect in small populations
• Not related to fitness

Bottleneck effect:

• Population drastically reduced
• Survivors' alleles determine future
• Reduces genetic diversity
• Example: Northern elephant seals

Founder effect:

• Small group colonizes new area
• Limited genetic variation
• Example: Amish populations

3. Gene Flow (Migration)

• Movement of alleles between populations
• Increases genetic variation
• Can introduce new alleles
• Reduces differences between populations

4. Mutation

• Ultimate source of new alleles
• Random changes in DNA
• Provides raw material for evolution
• Usually neutral or harmful, rarely beneficial

Conditions for Hardy-Weinberg Equilibrium

Hardy-Weinberg: Population NOT evolving (allele frequencies constant)

Five conditions:

1. No mutations
2. Random mating
3. No gene flow (migration)
4. Large population (no genetic drift)
5. No natural selection

If conditions met: p² + 2pq + q² = 1 and p + q = 1

• p = frequency of dominant allele
• q = frequency of recessive allele
• p² = homozygous dominant
• 2pq = heterozygous
• q² = homozygous recessive

Real populations: Always evolving (conditions rarely met)

Evidence for Evolution

1. Fossil Record

• Shows change over time
• Transitional forms
• Age determined by radiometric dating

2. Comparative Anatomy

Homologous structures:

• Same structure, different function
• Common ancestry
• Example: vertebrate forelimbs

Vestigial structures:

• Reduced, no longer functional
• Evidence of evolutionary past
• Example: human appendix, whale pelvis

Analogous structures:

• Different structure, same function
• Convergent evolution (not common ancestry)
• Example: bird and insect wings

3. Molecular Biology

• DNA and protein similarities
• More similar = more recent common ancestor
• Universal genetic code
• Cytochrome c comparisons

4. Biogeography

• Geographic distribution of species
• Islands have unique species
• Related to continental species
• Example: Darwin's finches (Galápagos)

5. Direct Observation

• Bacterial resistance
• Pesticide resistance in insects
• Changes in populations over time

Speciation

Speciation: Formation of new species

Species: Group that can interbreed and produce fertile offspring

Reproductive isolation:

• Prezygotic barriers (before fertilization)
• Postzygotic barriers (after fertilization)

Allopatric speciation:

• Geographic separation
• Most common type

Sympatric speciation:

• No geographic separation
• Polyploidy in plants

Key Concepts

1. Evolution = change in allele frequencies over time
2. Natural selection favors advantageous traits
3. Three types: directional, stabilizing, disruptive
4. Genetic drift = random changes (bottleneck, founder effect)
5. Hardy-Weinberg describes non-evolving population
6. Evidence: fossils, anatomy, molecular, biogeography, observation
7. Speciation creates new species through reproductive isolation

Examples:

HIGH fitness: • Peacock with large tail: attracts many mates despite survival cost • Antibiotic-resistant bacteria: survives and reproduces in presence of antibiotic • Queen bee: produces thousands of offspring

LOW fitness: • Sterile organism (fitness = 0) • Organism that survives but never finds mate • Individual with genetic disease preventing reproduction

Inclusive Fitness: • Extended concept including relatives • Helping relatives reproduce (share genes) • Explains altruistic behavior • "Fitness = direct reproduction + indirect (through relatives)"

Key Principle: Evolution doesn't favor the "strongest" or "healthiest" - it favors those who leave the most viable offspring!

AFTER Clean Air Acts (1950s-present): Environment: • Pollution reduced • Lichens returned • Tree bark became lighter again

Moth populations: • Light moths increasing again • Dark moths decreasing • Populations reverting to pre-industrial frequencies • By 2000s: light form dominant again

Key Evidence:

1. Bernard Kettlewell's experiments (1950s) • Mark-recapture studies • Light moths survived better in unpolluted woods • Dark moths survived better in polluted woods • Birds observed preying on conspicuous moths

2. Geographic correlation • Dark moths common in industrial areas • Light moths common in rural areas • Cline (gradual change) between regions

3. Recent studies • Michael Majerus's work (2000s) • Confirmed selection by birds • Showed microhabitat selection by moths

4. Genetic studies • Gene for melanism identified • Dominant mutation • Arose ~1819

Significance: • Observable evolution in "real time" • Demonstrates natural selection • Shows evolution can be rapid (decades, not millions of years) • Reversible (evolution not one-way) • Human impact on evolution

Limitations: • Some details of original study questioned • More complex than originally thought • Multiple factors beyond bird predation • But core principle remains valid

Key Lesson: Natural selection responds to environmental changes, and evolution can occur rapidly when selection is strong!

Factors Accelerating Resistance:

1. Overuse of antibiotics • Unnecessary prescriptions • Agricultural use (livestock) • Increases selection pressure

2. Incomplete treatment • Patients stop taking antibiotics early • Kills most bacteria but not all • Survivors often partially resistant • Selects for resistance

3. Horizontal gene transfer • Plasmids carry resistance genes • Transfer between different bacterial species • Speeds evolution of resistance • Example: Conjugation, transformation, transduction

4. High mutation rate • Large population sizes (billions of bacteria) • Short generation time • Increases probability of resistance mutation

Public Health Concerns:

1. Untreatable infections • "Superbugs" resistant to multiple antibiotics • Limited or no treatment options • Increased mortality Example: XDR-TB (extensively drug-resistant tuberculosis)

2. Longer, more expensive treatment • Need for stronger, newer antibiotics • Longer hospital stays • Higher healthcare costs • More side effects from stronger drugs

3. Complications in medical procedures • Surgery depends on antibiotics to prevent infection • Chemotherapy patients immunocompromised • Organ transplant recipients need antibiotics • These become riskier without effective antibiotics

4. Global spread • Resistant bacteria spread worldwide • International travel • Trade and food supply • "Post-antibiotic era" possible

5. Evolutionary arms race • Bacteria evolve resistance • We develop new antibiotics • Bacteria evolve resistance to those • Cycle continues, but we're losing ground • Rate of resistance evolution > rate of new drug development

Strategies to Combat Resistance:

1. Antibiotic stewardship • Use only when necessary • Complete full course of treatment • Right drug, right dose, right duration

2. Reduce agricultural use • Don't use antibiotics as growth promoters • Reserve certain antibiotics for human use only

3. Infection prevention • Hygiene and sanitation • Vaccination • Reduce need for antibiotics

4. New drug development • Research new antibiotics • Alternative treatments (phage therapy, etc.)

5. Combination therapy • Multiple antibiotics simultaneously • Harder for bacteria to develop resistance to all

Evolutionary Insight: • We cannot "defeat" evolution • Bacteria will always evolve • Must work WITH evolutionary principles • Slow selection for resistance, don't eliminate it

Key Principle: Antibiotic resistance is not just a medical problem - it's an evolutionary problem requiring evolutionary solutions!

Other examples: • Cheetahs (low genetic diversity from ancient bottleneck) • Florida panthers • Many endangered species

1. FOUNDER EFFECT Occurs when few individuals establish new population

Mechanism: • Small group colonizes new area • Founders carry only subset of original genetic variation • New population not representative of source • Rare alleles may become common (or vice versa) by chance

Example: Amish populations • Small number of founders • Certain genetic diseases more common • Ellis-van Creveld syndrome (dwarfism) • One founder carried rare allele • Now much more common in Amish than general population

Other examples: • Galápagos finches (initial colonization) • Hawaiian Drosophila species • Island populations in general

When is Drift Most Important?

1. SMALL POPULATIONS • Random sampling error larger in small samples • One random death removes higher % of alleles • Drift overwhelms selection Rule: Drift important when N < 100 (effective population size)

2. NEUTRAL ALLELES • When alleles have equal fitness • No selection to oppose drift • Drift is only force acting • Molecular clock based on neutral drift

3. NEWLY FORMED POPULATIONS • Colonization events • After bottlenecks • Limited genetic variation

4. ISOLATED POPULATIONS • No gene flow to counter drift • Island populations • Fragmented habitats

Consequences of Drift:

1. Loss of genetic variation • Random alleles lost • Even beneficial alleles can be lost • Reduced evolutionary potential

2. Fixation of alleles • Random allele eventually reaches 100% • Other alleles lost • Time to fixation depends on population size

3. Population differentiation • Different populations drift in different directions • Populations become genetically distinct • Can contribute to speciation

4. Can override selection • In small populations, drift stronger than weak selection • Slightly beneficial alleles can be lost • Slightly harmful alleles can increase

Mathematical Relationship: • Strength of drift ∝ 1/N (inversely proportional to population size) • Large population: drift weak, selection dominates • Small population: drift strong, can overwhelm selection

Conservation Implications: • Small endangered populations lose genetic diversity • Inbreeding increases • Reduced ability to adapt • "Extinction vortex" • Need to maintain large population sizes

Key Principle: Evolution is not just natural selection! Random processes (drift) also shape genetic variation, especially in small populations.