Darwin's Theory of Evolution by Natural Selection:

Main Ideas:

1. Organisms produce more offspring than can survive
2. Individuals vary in their traits
3. Some variations are heritable
4. Individuals with advantageous traits survive and reproduce more
5. Over time, favorable traits become more common
6. Populations evolve (change in allele frequencies)

Key Observations: • Struggle for existence (Malthus's influence) • Overproduction of offspring • Limited resources • Variation within species • Much variation is heritable

Evidence Darwin Used:

1. Fossil Record • Extinct species similar to living ones • Succession of forms over time • Intermediate forms Example: Marine fossils on mountaintops

2. Biogeography • Species distributions match geological history • Similar environments, different species on different continents • Island species resemble mainland species Example: Galápagos finches similar to South American finches

3. Comparative Anatomy • Homologous structures (same structure, different function) • Vestigial structures (reduced, functionless) Example: Vertebrate forelimbs, human appendix, whale hip bones

4. Comparative Embryology • Similar embryonic development across vertebrates • Gill slits in all vertebrate embryos Example: Pharyngeal pouches in human embryos

5. Artificial Selection • Humans breed plants and animals for desired traits • Shows variation can lead to major changes Example: Dog breeds, crop varieties, pigeons

Modern Evidence (not available to Darwin): • Molecular biology (DNA/protein sequences) • Direct observation of evolution (bacteria, insects) • Experimental evolution • Genetics (understanding of inheritance)

Key Insight: Natural selection is differential reproductive success based on heritable variation!

Evolutionary Fitness: The relative ability of an organism to survive and pass its genes to the next generation. Measured by reproductive success.

Formal Definition: Contribution of an individual's genes to the next generation relative to other individuals in the population.

Key Components:

1. SURVIVAL to reproductive age
2. Ability to REPRODUCE
3. NUMBER of offspring produced
4. QUALITY/SURVIVAL of offspring

Difference from Common Usage:

Common usage: • Physical fitness • Health, strength, endurance • Athletic ability • "Being in shape"

Evolutionary usage: • Reproductive success • Number of surviving offspring • Genetic contribution to next generation • NOT about individual health per se

Crucial Distinctions:

1. Relative, not absolute • Fitness is always relative to others in population • Individual with 3 offspring has low fitness if average is 10 • Same individual has high fitness if average is 1

2. Reproductive success matters, not survival alone • Organism that lives 100 years but has no offspring: fitness = 0 • Organism that lives 1 year but has 1000 offspring: very high fitness Example: Salmon die after spawning but have high fitness

3. Timing matters • Having offspring young vs. old affects fitness • Earlier reproduction = genes spread faster

4. Quality AND quantity • Not just number of offspring • Offspring must survive to reproduce Example: 1000 eggs that all die vs. 10 offspring that survive

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!

Peppered Moth: Classic Example of Natural Selection

Background: • Two color forms: light (typical) and dark (melanic) • Color controlled by single gene • Moths rest on tree trunks during day • Birds prey on visible moths

BEFORE Industrial Revolution (pre-1800s): Environment: • Tree bark light-colored with lichens • Rural, unpolluted environment

Moth populations: • Light moths common (95%+) • Camouflaged against light bark • Dark moths rare (< 5%) • Dark moths visible to birds, heavily predated

Selection pressure: Birds preferentially eat dark moths

DURING Industrial Revolution (1800s-1950s): Environment: • Industrial pollution killed lichens • Soot darkened tree bark • Urban and industrial areas

Moth populations: • Dark moths increased dramatically • By 1895: ~98% dark in industrial areas • Light moths decreased • Light moths now visible against dark bark

Selection pressure: Birds preferentially eat light moths

Mechanism:

1. Variation: Light and dark forms exist
2. Heritability: Color genetically determined
3. Differential survival: Camouflaged moths survive better
4. Result: Population shifts toward better-camouflaged form

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!

Antibiotic Resistance: Evolution in Action

Mechanism of Evolution:

1. VARIATION • Random mutations create genetic diversity • Some bacteria have resistance genes by chance • Can also acquire resistance through horizontal gene transfer • Example: Gene for enzyme that breaks down antibiotic

2. SELECTION PRESSURE (Antibiotic Exposure) • Antibiotic introduced • Sensitive bacteria die • Resistant bacteria survive • Strong selection favoring resistance

3. DIFFERENTIAL REPRODUCTION • Resistant bacteria reproduce • Sensitive bacteria eliminated • Resistant bacteria have field to themselves • Rapid reproduction (20-minute generation time)

4. EVOLUTION • Population shifts to resistant strain • Can occur in days to weeks • Allele frequency changes dramatically • Population has evolved!

Example: MRSA (Methicillin-Resistant Staphylococcus aureus) • Resistant to methicillin and related antibiotics • Evolved through natural selection in hospitals • Now widespread and difficult to treat

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!

Genetic Drift: Random change in allele frequencies due to chance events. NOT based on fitness differences.

Difference from Natural Selection:

NATURAL SELECTION: • NON-random process • Based on fitness differences • Predictable direction (toward higher fitness) • More important in large populations • Adaptive (increases fitness) • Example: Antibiotic resistance spreads because it's beneficial

GENETIC DRIFT: • RANDOM process • NOT based on fitness (chance events) • Unpredictable direction • More important in small populations • Non-adaptive (doesn't necessarily increase fitness) • Example: Allele lost by chance, even if beneficial

Two Main Types of Drift:

1. BOTTLENECK EFFECT Occurs when population size drastically reduced by random event

Mechanism: • Disaster kills most of population • Survivors are random sample (not necessarily "best") • Rare alleles often lost • Genetic diversity reduced • Founding of new population from survivors

Example: Northern elephant seals • Hunted to ~20 individuals in 1890s • Recovered to 30,000+ today • But extremely low genetic diversity • All descended from ~20 survivors • Lost alleles cannot be recovered (except by mutation)

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.