Natural Selection, Population Growth, and Resource Challenges

Genetic Variation, Phenotypes, and Natural Selection

• Types of genetic variation

  • Harmful

  • Beneficial

  • Neutral

• Fates of each type under natural selection

  • Harmful variations → ultimately eliminated from populations

  • Beneficial variations → increase in frequency because they raise reproductive success

  • Neutral variations → largely ignored by selection; can drift or persist

• Crucial clarifications

  • Natural selection acts on the phenotype, not directly on the genotype

  • Selection has no forward-looking goal (no intent to “improve” a species)

Antibiotic Resistance: A Modern Illustration of Selection

• Medical protocol

  • Doctors routinely prescribe ~10-day antibiotic courses (e.g.
    Amoxicillin)

  • Symptomatic relief often occurs by day ≈3, tempting patients to stop early

• Evolutionary consequences of stopping early

  • Survivors are the most resistant strains → they reproduce, spreading resistance alleles

  • Over-prescription in the 1960s–70s (e.g.
    “penicillin shot in the butt” for nearly any ailment) accelerated this process

• Key takeaway: Finish the full course to minimize the probability that any resistant bacterium survives

• Analogy to pesticide resistance in agriculture

  • Mutant pests survive a pesticide → reproduce → next generation carries resistance → farmers escalate to ever stronger chemicals

  • Leads to a "chemical arms race" and motivates growth of (smaller-scale) organic farming

Darwin’s Reasoning: Competition & Overproduction

• Intellectual inputs

  • Charles Lyell’s Principles of Geology (deep time, slow change)

  • Thomas Malthus’ essay on human population growth (resource limits)

• Mental “rioting” → epiphany: species overproduce relative to resources

• Elephant thought experiment

  • Life span ≈ 100\,\text{yrs}

  • Breeding span from 30 to 90\,\text{yrs}

  • If every calf survived, after 750\,\text{yrs} a single pair would yield 19,000,000 elephants

  • Environments cannot support that magnitude → inevitable struggle & differential survival

• Concept name: geometric ratio of increase (a.k.a. exponential growth)

Human Population Growth

• Historical plateaus → rapid acceleration

  • Pre-agriculture ((<10,000\,\text{yrs}) ago): near-steady human numbers

  • Agricultural Revolution: sedentary farming → modest growth

  • Industrial Revolution (~1700 CE): medical, technological, and agricultural advances → exponential growth curve

• Current & projected numbers

  • 2025 ≈ 8\times10^{9} people (8 billion)

  • 2100 projection ≈ 1\times10^{10} people (10 billion)

• Carrying capacity debate

  • Many scholars argue we have already exceeded Earth’s sustainable carrying capacity

  • Result → intensified competition for finite resources (food, water, habitable land)

Coastal Concentration & Climate Migration

• ~2\times10^{9} people (≈70 % of global population) live within 50\,\text{km} of a coastline

• Warming planet → sea-level rise, storm surge, salt-water intrusion

  • Potential for mass mobilization ("environmental refugees")

  • Strain on inland infrastructure, economies, and water supplies

Water Scarcity as the Looming Limiting Resource

• Oil & coal are replaceable (at inconvenience); fresh water is not

• Signs of impending crisis

  • Increasing civil unrest in regions already water-stressed

  • Infrastructure inequities magnify the problem (aging pipes, poor distribution networks)

• Key idea: Water scarcity could become the primary driver of future geopolitical conflict

Colorado River Basin Case Study

• Dependents: 7 U.S. states + Mexico rely on the river

• Current problems

  • Extensive diversion → river reaches Mexico as a trickle

  • Heavy industrial/agricultural withdrawals lead to over-pumping of groundwater

  • Infrastructure decay compounds losses

• Drought of record (largest in ≈1200 yrs)

  • Reduced snowpack (primary source of Colorado’s water) endangers year-to-year supply

• Major reservoirs

  • Lake Powell and Lake Mead (largest U.S. drinking-water stores)

  • Water levels fell so low that hydroelectric power generation was nearly halted

  • One strong snow year offered temporary relief; full recovery remains elusive

• Food supply linkage: Large swaths of U.S. produce, especially from California, rely on Colorado River allocations; water insecurity threatens supermarket availability and prices

Ethical, Philosophical, and Practical Implications

• Medical ethics: Overuse/misuse of antibiotics today jeopardizes future generations’ ability to combat infections

• Agricultural ethics: Chemical arms race with pests raises questions about long-term soil & ecosystem health

• Climate justice: Coastal and water-scarce populations (often poorer) may bear disproportionate burdens of environmental change

• Geopolitical stability: Water scarcity and climate migration could precipitate conflict, requiring cooperative international frameworks

Conceptual Connections & Equations

• Exponential growth model (generalized): N(t) = N_0 e^{rt}

  • $N(t)$ = population at time $t$

  • $N_0$ = initial population

  • $r$ = intrinsic growth rate

• Carrying capacity (logistic model): \frac{dN}{dt} = rN\left(1-\frac{N}{K}\right)

  • $K$ = carrying capacity; when N \to K, growth slows

• Selection differential: Qualitatively discussed—beneficial alleles increase in frequency proportional to their fitness advantage

Study Tips / Integrative Thoughts

• Relate antibiotic resistance to core evolutionary principles: mutation, selection, inheritance, and time

• Map Darwin’s insights onto modern examples: elephant overproduction ↔ human population expansion

• Track how resource limitations (space, food, water) convert biological theory into social, economic, and political realities

• Practice interpreting exponential vs. logistic curves; be able to explain where humans currently sit on each trajectory