Module 4: Ecology & Evolution

What is evolution?

Change in inherited traits of a population over generations. 

Darwin's 4 postulates of natural selection

1) Variation exists 

2) Variation is heritable 

3) Some traits improve survival/reproduction 

4) Those traits become more common over time 

Natural selection

Individuals with favorable heritable traits survive and reproduce more → those traits increase in the population. 

Genetic drift 

Random changes in allele frequencies — most significant in small populations. Includes Bottleneck and Founder Effects. 

Bottleneck Effect

Population crash → survivors hold only a fraction of original genetic diversity. 

Ex: northern elephant seals reduced to ~20 individuals. 

Founder Effect

Small group colonizes new area → limited alleles carried over. 

Ex: Amish community has elevated rates of rare genetic conditions. 

Gene flow

Alleles move between populations via migration. Reduces genetic differences; when it stops, populations can diverge. 

Mutation

Random change in DNA — the ultimate source of all new genetic variation. Most are neutral or harmful; rare ones are beneficial. 

Natural selection example

Peppered moths: soot darkened trees → dark moths survived more → dark form became dominant.

Genetic drift example

Cheetahs: ancient bottleneck left near-identical genetics → highly vulnerable to disease. 

Homologous structures

Same anatomy, different function — inherited from a common ancestor. 

Ex: human arm, whale flipper, bat wing. 

Analogous structures

Similar function, different evolutionary origin (convergent evolution). 

Ex: bird wings vs. insect wings. Does NOT indicate close relatedness

4 types of evidence for evolution

1) Fossil record 

2) Homologous anatomy 

3) Embryology 

4) Molecular biology (shared DNA) 

Transitional fossil

Shows intermediate features between ancestor and descendant. 

Ex: Tiktaalik — fish-to-land-vertebrate transition. 

Common evolution misconceptions

• Individuals evolve (NO — populations do) 

• Evolution has a goal (NO — no foresight) 

• Evolution explains origin of life (NO — that's abiogenesis) 

• 'Fittest' = strongest (NO — best suited to environment) 

• Humans evolved from chimps (NO — shared ancestor) 

Scientific theory vs. hypothesis

Hypothesis: testable explanation for one observation. 

Theory: broad framework backed by extensive evidence — NOT a guess. 

Ecology 

Study of how organisms interact with each other and their environment. 

Levels of ecological study

Organism → Population → Community → Ecosystem → Biosphere

The 10% Rule

Only ~10% of energy transfers to the next trophic level; ~90% lost as heat. Limits food chains to 4–5 levels. 

Producers, consumers, decomposers

Producers: make food via photosynthesis (plants, algae) 

Consumers: eat other organisms 

Decomposers: break down dead matter, recycle nutrients 

Carbon cycle (key points)

In: photosynthesis fixes CO₂ 

Out: respiration, decomposition, combustion 

Human impact: burning fossil fuels raises atmospheric CO₂ 

Nitrogen cycle (key steps)

1) Fixation: bacteria convert N₂ → NH₃ 

2) Nitrification: NH₃ → NO₃⁻ (usable by plants) 

3) Assimilation: plants/animals absorb nitrogen 

4) Denitrification: bacteria return N₂ to atmosphere 

Mutualism (+/+)

Both species benefit. 

Ex: bees + flowers; mycorrhizal fungi + plant roots 

Commensalism (+/0) 

One benefits, other unaffected. 

Ex: barnacles on whale skin 

Parasitism (+/−)

Parasite benefits, host harmed (rarely killed immediately). 

Ex: tapeworms, ticks 

Predation (+/−)

Predator eats prey. Drives coevolution — prey develop defenses, predators develop counter-adaptations. 

Competitive exclusion principle

Two species cannot share the exact same niche indefinitely — one outcompetes the other or they partition resources. 

What determines biome type?

Temperature + precipitation (the two key abiotic factors).

Tropical rainforest

High temp, very high rainfall, highest biodiversity on Earth, nutrient-poor soils. 

Tundra

Extremely cold, permafrost, very low rainfall, treeless, short growing season. 

Extremely cold, permafrost, very low rainfall, treeless, short growing season. 

< 25 cm rain/yr, extreme temp swings, drought-adapted plants, mostly nocturnal animals. 

Taiga (boreal forest)

Cold winters, moderate rain, dominated by evergreen conifers. Largest terrestrial biome by area. 

Human impacts on ecosystems

1) Habitat destruction 

2) Pollution 

3) Climate change 

4) Invasive species 

5) Overexploitation 

Biodiversity

The variety of life at all levels: genetic, species, and ecosystem diversity

3 levels of biodiversity

Genetic: variation within a species 

Species: number & abundance of species in an area 

Ecosystem: variety of habitats and communities 

Why biodiversity matters

• Ecosystem stability & resilience 

• Ecosystem services (clean water, pollination, carbon storage) 

• Medical/food value 

• Intrinsic & cultural worth 

HIPPCO — threats to biodiversity

H – Habitat loss (largest threat) 

I – Invasive species 

P – Pollution 

P – Population growth 

C – Climate change 

O – Overexploitation 

Current extinction rates

100–1,000× the natural background rate (~1–5 species/yr). Called the Sixth Mass Extinction — first driven by a single species.

Invasive species

Non-native species with no natural predators that outcompete or harm natives. 

Ex: kudzu, cane toads, zebra mussels 

Extirpation

Local extinction — species disappears from one area but survives elsewhere. 

Ex: wolves in Yellowstone (extirpated 1920s, reintroduced 1995) 

Habitat fragmentation

Large habitat broken into small isolated patches → reduces gene flow, increases inbreeding and extinction risk

Wildlife corridor

Strip of habitat connecting fragmented patches — allows movement, gene flow, and range shifts. 

Ex: Yellowstone to Yukon Initiative 

Captive breeding & reintroduction

Breed endangered species in captivity → release into wild. 

Ex: California condor: 27 birds (1987) → 500+ today 

Habitat restoration

Actively repairing degraded ecosystems: reforestation, wetland rebuilding, invasive species removal.

Eutrophication

Nutrient runoff → algal bloom → oxygen depletion → dead zone where fish can't survive. 

Endangered Species Act (ESA)

U.S. law (1973) protecting listed species from harm and requiring recovery plans. 

Ex: helped recover bald eagle, gray wolf.

CITES

International treaty regulating/banning trade in endangered species and their products. 180+ member countries. 

Genetic diversity

Variation in alleles within a species. High = more adaptable, more disease-resistant. Low = higher extinction risk.