DAT Review · Biology
Speciation and Ecology
On this page 7 sections
In 30 seconds
- Allopatric vs sympatric speciation is a classic comparison — geographic barrier vs reproductive isolation in the same location. Be able to identify each from a scenario.
- Prezygotic vs postzygotic barriers — the DAT loves listing barriers and asking you to classify them. Prezygotic = before fertilization; postzygotic = after fertilization.
- Ecological hierarchy (organism → population → community → ecosystem → biome → biosphere) and population growth models (exponential vs logistic, carrying capacity K) appear frequently.
- Symbiotic relationships (mutualism ++, commensalism +0, parasitism +−) and energy pyramids (~10% transfer) are memorization items.
- Biogeochemical cycles — especially the nitrogen cycle (N-fixation → nitrification → assimilation → ammonification → denitrification) — are high-yield.
The college version
Core Review
Speciation
Speciation is the formation of new and distinct species through evolution. A species is typically defined by the biological species concept: a group of actually or potentially interbreeding natural populations that are reproductively isolated from other such groups. Speciation occurs through two primary pathways:
Allopatric speciation ("different homeland") occurs when a geographic barrier (mountain range, river, ocean, habitat fragmentation) physically separates a population. Gene flow stops, and the two subpopulations evolve independently through mutation, drift, and selection. Over time, they diverge genetically to the point where they can no longer interbreed even if reunited. Example: Darwin's finches on the Galápagos Islands — geographic isolation on different islands led to adaptive radiation into distinct species.
Sympatric speciation ("same homeland") occurs without geographic separation. Reproductive isolation evolves within a single, freely interbreeding population, usually through a genetic change that alters mating preferences, habitat use, or chromosome number. Polyploidy (especially in plants) is a common sympatric mechanism — an error in meiosis produces offspring with extra chromosome sets that can only breed with other polyploids. Example: wheat (Triticum aestivum) arose through hybridization and polyploidy events.
Reproductive Isolating Mechanisms
These barriers prevent gene flow between populations and maintain species boundaries.
Prezygotic barriers (before zygote formation):
| Barrier | Description | Example |
|---|---|---|
| Habitat | Populations occupy different habitats in same area; rarely encounter each other | Garter snakes: one species lives in water, another on land |
| Temporal | Breed at different times of day, season, or year | Skunk species: Spilogale gracilis mates in late summer; S. putorius mates in late winter |
| Behavioral | Courtship displays, mating calls, or other signals differ | Blue-footed vs red-footed boobies: different courtship dances |
| Mechanical | Reproductive structures are incompatible | Snail species with shells spiraling in opposite directions cannot align genital openings |
| Gametic | Sperm cannot fertilize egg (biochemical incompatibility) | Sea urchins release gametes into water; species-specific proteins prevent cross-fertilization |
Postzygotic barriers (after zygote formation):
| Barrier | Description | Example |
|---|---|---|
| Hybrid inviability | Hybrid zygote fails to develop or dies before reaching reproductive age | Leopard frog hybrids: embryos of some crosses do not complete development |
| Hybrid sterility | Hybrid is viable but sterile (cannot produce gametes) | Mule: sterile offspring of horse × donkey |
| Hybrid breakdown | First-generation hybrids are viable and fertile, but their offspring (F₂) are inviable or sterile | Some cotton species: F₁ is fertile; F₂ dies as seeds |
Population Ecology
Population growth models:
- Exponential growth: dN/dt = rN. Occurs when resources are unlimited. Produces a J-shaped curve. r = intrinsic rate of increase; N = population size. Rare in nature except during colonization or after a catastrophic decline (recovering population).
- Logistic growth: dN/dt = rN((K−N)/K). Resources are limited. Produces an S-shaped curve that levels off at the carrying capacity (K) — the maximum population size the environment can sustain indefinitely.
Regulation of population size:
- Density-dependent factors: Effect intensifies as population density increases. Examples: competition for resources, predation, disease, accumulation of waste. These tend to stabilize populations near K.
- Density-independent factors: Effect is unrelated to population density. Examples: natural disasters (fire, flood, drought), severe weather events, habitat destruction. These cause population crashes regardless of density.
Predator-prey dynamics often show coupled oscillations. As prey populations increase, predator populations follow (with a lag). High predator numbers drive prey down, which eventually causes predator decline, allowing prey to recover — repeating the cycle. Classic example: snowshoe hare and lynx cycles (~10-year period).
Community Ecology
Symbiotic relationships:
| Relationship | Species A | Species B | Example |
|---|---|---|---|
| Mutualism | + | + | Bees and flowering plants; mycorrhizal fungi and plant roots |
| Commensalism | + | 0 | Barnacles on whales; cattle egrets following livestock |
| Parasitism | + | − | Tapeworms in host intestine; Plasmodium (malaria) in humans |
Ecosystems and Energy Flow
Trophic levels: Producers (autotrophs) → Primary consumers (herbivores) → Secondary consumers (carnivores) → Tertiary consumers → Decomposers.
Energy pyramid: Only approximately 10% of the energy at one trophic level is converted to biomass at the next level. The rest is lost as metabolic heat (respiration) and waste. This explains why food chains rarely exceed 4–5 trophic levels and why top predators are the rarest organisms.
Biogeochemical cycles — Nitrogen cycle (most heavily tested):
- Nitrogen fixation: Atmospheric N₂ → NH₃ (ammonia) or NH₄⁺ (ammonium). Carried out by nitrogen-fixing bacteria (Rhizobium in legume root nodules, Azotobacter in soil, cyanobacteria in aquatic systems).
- Nitrification: NH₄⁺ → NO₂⁻ (nitrite) → NO₃⁻ (nitrate). Nitrosomonas oxidizes ammonium to nitrite; Nitrobacter oxidizes nitrite to nitrate. Plants absorb nitrate.
- Assimilation: Plants incorporate nitrate and ammonium into amino acids, nucleic acids. Animals obtain nitrogen by consuming plants or other animals.
- Ammonification: Decomposers (bacteria, fungi) break down organic nitrogen (dead organisms, waste) back into NH₄⁺.
- Denitrification: NO₃⁻ → N₂ gas, returning it to the atmosphere. Carried out by denitrifying bacteria (Pseudomonas) in anaerobic conditions.
Other cycles tested: Water cycle (evaporation, transpiration, condensation, precipitation, runoff); Carbon cycle (photosynthesis fixes CO₂; respiration and combustion release CO₂; long-term storage in fossil fuels and limestone).
Biomes
| Biome | Key Features |
|---|---|
| Tundra | Permafrost, low vegetation (mosses, lichens), very cold, low precipitation |
| Taiga (boreal forest) | Coniferous trees, long cold winters, short summers |
| Temperate deciduous forest | Distinct seasons, broadleaf trees shed leaves in autumn |
| Grassland | Dominated by grasses, moderate rainfall, rich soil (prairies, steppes, savannas) |
| Desert | Very low precipitation, extreme temperature swings, specialized flora/fauna |
| Tropical rainforest | Highest biodiversity, warm year-round, very high precipitation |
| Freshwater | Lakes, rivers, ponds, wetlands; low salinity |
| Marine | Oceans, coral reefs, estuaries; high salinity; largest biome by area |
Common Traps
- Confusing allopatric (geographic) with reproductive isolation alone. Allopatric = physical barrier first, then reproductive isolation evolves.
- Mixing up prezygotic barriers. "Gametic isolation" sounds postzygotic because it involves gametes, but it occurs before fertilization.
- Forgetting that density-dependent and density-independent factors are about population density, not species type.
- Thinking the energy pyramid's 10% rule means 10% of calories transfer — it's 10% of biomass/energy, not a fixed calorie number.
Eli explains
The same idea, in plain words
Explain it like I’m 10
How do you get two species from one? Picture a river splitting a forest in half. Squirrels on the left side can't reach squirrels on the right. Over thousands of years, they change in different ways — one group gets darker fur, the other gets bushier tails. Eventually, even if the river dries up, they can't mate with each other anymore. That's allopatric speciation! Now picture a forest where some birds start singing a different song — birds that sing the new song only mate with each other. Eventually, they become separate species without ever being physically separated. That's sympatric speciation! In ecology, picture the food chain as a pyramid of hamburgers. A cow eats 10,000 calories of grass but only turns 1,000 into cow meat. You eat the cow and only get 100 calories stored as you. That's why there aren't many lions — the energy runs out at the top!
Key takeaways
- Allopatric = geographic barrier; sympatric = same location, reproductive isolation. Polyploidy = sympatric.
- Prezygotic = before fertilization (habitat, temporal, behavioral, mechanical, gametic). Postzygotic = after fertilization (hybrid inviability, sterility, breakdown).
- Logistic growth levels off at K. Exponential growth has no ceiling (J-curve).
- 10% energy transfer between trophic levels — explains why top predators are rare.
- Nitrogen cycle in order: fixation → nitrification → assimilation → ammonification → denitrification. Know the bacteria for each step.
- Mutualism ++, Commensalism +0, Parasitism +− — know examples.
Check yourself
3 review questions from the chapter. Try each one, then open the answer.
Two species of frogs live in the same pond. Species A breeds in early March; Species B breeds in late May. Despite overlapping habitat, they never interbreed. What type of reproductive isolating mechanism is this?
Show answer
Temporal isolation, which is a prezygotic barrier. The two species breed at different times, so gametes never have the opportunity to meet. This prevents zygote formation entirely.
A deer population on an island grows from 50 to 500 individuals in 10 years but then stabilizes around 500 for the next 20 years. What ecological concept best explains the stabilization, and what type of growth curve does this represent?
Show answer
The stabilization reflects the population reaching its carrying capacity (K) — the maximum number of individuals the island's resources can sustain. This pattern represents logistic growth, producing an S-shaped curve. The early rapid increase (approaching exponential) slows as density-dependent factors (food limitation, disease, competition) intensify, and the population oscillates around K.
In an ecosystem, grass captures 100,000 kJ of solar energy per year via photosynthesis. Approximately how much energy would be available to secondary consumers (animals that eat herbivores)?
Show answer
Primary producers (grass): 100,000 kJ. Primary consumers (herbivores): ~10% → 10,000 kJ. Secondary consumers (carnivores eating herbivores): ~10% of 10,000 → approximately 1,000 kJ. This follows the 10% rule of energy transfer between trophic levels, with the remaining 90% lost as metabolic heat and waste at each step.
Study tools & related lessonsYou’ll learn to · Related
You’ll learn to
- Distinguish allopatric and sympatric speciation, giving examples of each.
- Classify reproductive isolating mechanisms as prezygotic or postzygotic.
- Describe exponential and logistic population growth models and the concept of carrying capacity.
- Identify density-dependent vs density-independent factors regulating populations.
- Trace energy flow through ecosystems, including the 10% rule and trophic levels.
- Diagram the nitrogen cycle and name the key bacterial conversions.
Sources & references
- OpenStax Biology 2e, Chapter 18: "Evolution and the Origin of Species"; Chapter 44: "Ecology and the Biosphere"; Chapter 46: "Ecosystems"
- NCBI Bookshelf: "Speciation"
This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.
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