Biology 30 · Populations
Populations and communities
This unit zooms out from one organism to whole populations. The maths is lighter than it looks — Hardy–Weinberg is two equations and a square root.
Hardy–Weinberg
The idea: for a gene with two alleles, p + q = 1 (allele frequencies) and p² + 2pq + q² = 1 (genotype frequencies). If nothing disturbs the population, those numbers stay the same generation after generation.
p is the frequency of the dominant allele and q the recessive. In the second equation, p² is homozygous dominant, 2pq is heterozygous, and q² is homozygous recessive.
Worked example
16% of a population shows a recessive trait. Find the allele frequencies and the percentage of carriers.
- Only homozygous recessive individuals show the trait, so q² = 0.16.
- Square root: q = 0.4.
- p + q = 1, so p = 0.6.
- Carriers are heterozygous: 2pq = 2 × 0.6 × 0.4 = 0.48, or 48%.
The five conditions
Equilibrium only holds with a large population, random mating, no mutation, no migration and no natural selection. Break any one and allele frequencies shift — that shift is evolution.
- Genetic drift is chance change, strongest in small populations.
- Founder effect: a few individuals start a new population carrying only some of the original alleles.
- Bottleneck: an existing population is suddenly cut down, and the survivors' alleles are not representative.
Watch out: using the percentage showing the trait as q. It is q². Take the square root first.
Measuring population change
The idea: four things change a population — births and immigration add, deaths and emigration subtract.
- Change in size: ΔN = (births + immigration) − (deaths + emigration)
- Per capita growth rate: cgr = ΔN ÷ N, where N is the starting size
- Density: D = N ÷ area (or volume)
A deer herd with 60 births, 20 deaths, 10 immigrants and 30 emigrants changes by (60 + 10) − (20 + 30) = +20. A population going from 400 to 460 has cgr = 60 ÷ 400 = 0.15.
Watch out: dividing by the final size. Per capita growth is always measured against where the population started.
Growth curves
The idea: with unlimited resources a population grows exponentially (J-curve). In real environments it slows and levels off at the carrying capacity (S-curve).
| r-selected species | K-selected species |
|---|---|
| Many offspring, little parental care | Few offspring, lots of parental care |
| Short lifespan, reproduce early | Long lifespan, reproduce later |
| Boom-and-bust numbers | Numbers stay near carrying capacity |
| Insects, mice, dandelions | Elephants, whales, humans |
What limits growth
Density-dependent factors get stronger as a population gets crowded: disease, competition for food, predation. Density-independent factors hit regardless of size: fire, flood, a late frost.
Communities and succession
The idea: species in a community affect each other, and whole communities change over time in a fairly predictable order.
| Relationship | Effect | Example |
|---|---|---|
| Mutualism | + / + | Bees and flowering plants |
| Commensalism | + / 0 | Barnacles on a whale |
| Parasitism | + / − | Tapeworm in a human |
| Predation | + / − | Lynx and snowshoe hare |
| Competition | − / − | Intraspecific (same species) or interspecific (different species) |
Predator–prey cycles
Hare numbers rise, so lynx have more food and their numbers rise too — but a little later, because raising young takes time. More lynx eat more hares, hares crash, and then the lynx crash. The predator curve always lags just behind the prey curve.
Succession
- Primary succession starts with no soil — bare rock after a glacier or lava flow. Pioneer species like lichens break rock down and build the first soil.
- Secondary succession starts where soil survived — after a fire, flood or abandoned farm. It is much faster.
- Both move toward a stable climax community.
Watch out: calling a burned forest primary succession. The soil is still there, so it is secondary.