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.