6.1 Natural selection and population genetics
Natural selection, explained by Charles Darwin and Alfred Russel Wallace, rests on four observations: individuals differ in inherited ways; organisms produce more young than can survive; some traits improve the chance of surviving and reproducing; and these traits become more common in later generations. Evolution is a change in a population's allele frequencies over generations — populations evolve, not individuals.
Natural selection is not the only mechanism. In small populations, allele frequencies can change by chance (genetic drift); a disaster that shrinks a population (a bottleneck) greatly reduces its variety. Migration brings in and takes away alleles (gene flow), and mutation creates new ones. If gene flow between populations stops, they can drift apart over time and form new species (speciation).
6.2 Hardy–Weinberg equilibrium
For a gene with two alleles, if the dominant allele's frequency is p and the recessive allele's is q, then p + q = 1. If the population is large, mating is random, and there is no mutation, migration or selection, genotype frequencies stay the same each generation: AA = p², Aa = 2pq, aa = q². This "non-evolving population" model is used to test whether real populations are evolving.
Because the proportion of recessive-phenotype individuals gives q² directly, the proportion of carriers (2pq) can be calculated from how often an inherited condition occurs. This is used in public-health screening.
Formulap + q = 1 · p² + 2pq + q² = 1
6.3 Energy, matter and populations
In an ecosystem, energy flows one way but matter is recycled. Only about 10% of the energy passes from one feeding level to the next; the rest is lost as heat through respiration or stays in parts that aren't eaten. That is why food chains rarely exceed 4–5 levels. In the nitrogen cycle, Rhizobium bacteria in the roots of legumes turn N₂ from the air into ammonia (nitrogen fixation); nitrifying bacteria turn ammonia into nitrate, and denitrifying bacteria return nitrogen to the air.
With unlimited resources, a population grows exponentially (a J-curve). In reality resources are limited: as the population approaches the largest number the environment can support (the carrying capacity, K), growth slows, giving an S-shaped logistic curve. The growth rate is highest when the population is at half of K.
Key terms
- Genetic drift
- Chance changes in allele frequencies in small populations.
- Carrying capacity (K)
- The largest population an environment can support in the long term.
- Nitrogen fixation
- Bacteria turning N₂ from the air into ammonia.
- Speciation
- New species forming from populations whose gene flow has stopped.
End-of-unit problems
Solve each one yourself first, then open the solution.
In a population in Hardy–Weinberg equilibrium, 9% of individuals show a recessive trait. Find p, q and the proportion of heterozygous carriers. How many carriers would you expect in a town of 10,000?
Show the solution
q² = 0.09 → q = 0.3; p = 1 − 0.3 = 0.7. Carriers 2pq = 2 × 0.7 × 0.3 = 0.42 (42%). 10,000 × 0.42 = 4,200 carriers. Homozygous dominant: p² = 0.49.
In an ecosystem, producers fix 20,000 kJ of energy per square metre per year. Using the 10% rule, how much reaches the tertiary consumers? Use the result to explain why food chains can't be long.
Show the solution
Primary consumers 2,000 kJ, secondary 200 kJ, tertiary 20 kJ. Energy falls tenfold at every level, so after a few levels there isn't enough left to support another predator population; chains therefore usually stop at 4–5 levels.
Fifty fish are released into a lake with a carrying capacity of 800. At what population size will the population grow fastest? Why does this matter for fisheries management?
Show the solution
In logistic growth, the rate is highest at K/2: 800 ÷ 2 = 400 fish. If fishing keeps the population near 400, it produces the most new individuals each year and the catch stays sustainable ("maximum sustainable yield").
A storm carries just 10 birds to an island, where they found a new population. An allele that is rare on the mainland has a frequency of 30% on the island. Which mechanism of evolution explains this?
Show the solution
The founder effect, a kind of genetic drift. The few individuals that found the new population happen, by chance, to have allele frequencies unlike the main population. No selection is needed; chance explains the change.