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Population Ecology and Hardy-Weinberg Cheat Sheet
A printable reference covering population growth, carrying capacity, community interactions, Hardy-Weinberg equations, allele frequencies, and evolution checks for grades 11-12.
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Population ecology studies how populations change in size, density, and distribution over time. This cheat sheet helps students connect growth models, limiting factors, and species interactions to real biological patterns. It also supports genetics units by showing how allele frequencies can be used to test whether a population is evolving. These ideas are important for ecology, evolution, conservation biology, and data-based exam questions. The core population formulas include growth rate, exponential growth, logistic growth, and carrying capacity. Hardy-Weinberg equilibrium uses p + q = 1 and p^2 + 2pq + q^2 = 1 to predict genotype frequencies in a non-evolving population. If observed genotype frequencies do not match expected values, one or more evolutionary forces may be acting. Students should be able to calculate allele frequencies, interpret graphs, and explain how natural selection, mutation, migration, genetic drift, and nonrandom mating change populations.
Key Facts
- Population size is the total number of individuals in a population, while population density is calculated as density = number of individuals / area or volume.
- Population growth rate can be estimated with r = (births + immigration - deaths - emigration) / initial population size.
- Exponential growth occurs when resources are unlimited and follows the pattern dN/dt = rN, where N is population size and r is growth rate.
- Logistic growth slows as a population nears carrying capacity and is modeled by dN/dt = rN((K - N) / K), where K is carrying capacity.
- In Hardy-Weinberg equilibrium, allele frequencies add to 1, so p + q = 1 for two alleles.
- Hardy-Weinberg genotype frequencies are p^2 + 2pq + q^2 = 1, where p^2 is homozygous dominant, 2pq is heterozygous, and q^2 is homozygous recessive.
- A population is in Hardy-Weinberg equilibrium only if there is no mutation, no migration, no natural selection, random mating, and a very large population size.
- Evolution is defined as a change in allele frequencies in a population over generations.
Vocabulary
- Population
- A population is a group of individuals of the same species living in the same area at the same time.
- Carrying capacity
- Carrying capacity is the maximum population size an environment can support over time with its available resources.
- Limiting factor
- A limiting factor is any resource or condition that restricts population growth, such as food, space, predators, or disease.
- Allele frequency
- Allele frequency is the proportion of a specific allele among all copies of a gene in a population.
- Hardy-Weinberg equilibrium
- Hardy-Weinberg equilibrium is a condition in which allele and genotype frequencies remain constant from generation to generation.
- Genetic drift
- Genetic drift is a random change in allele frequencies, especially strong in small populations.
Common Mistakes to Avoid
- Confusing allele frequency with genotype frequency is wrong because p and q describe alleles, while p^2, 2pq, and q^2 describe genotypes.
- Using the recessive phenotype frequency as q instead of q^2 is wrong because the recessive phenotype represents the homozygous recessive genotype frequency.
- Assuming every population is in Hardy-Weinberg equilibrium is wrong because real populations often experience selection, migration, mutation, drift, or nonrandom mating.
- Treating carrying capacity as a fixed number is wrong because K can change when resources, climate, disease, or human impacts change.
- Ignoring immigration and emigration in population growth is wrong because movement into or out of a population can change population size and allele frequencies.
Practice Questions
- 1 A pond has 240 frogs in an area of 60 square meters. What is the population density in frogs per square meter?
- 2 In a population, the recessive phenotype frequency is 0.16. Assuming Hardy-Weinberg equilibrium, find q, p, p^2, 2pq, and q^2.
- 3 A population begins with 500 rabbits. In one year there are 90 births, 20 deaths, 15 immigrants, and 35 emigrants. What is the growth rate r for that year?
- 4 A small island population loses many individuals after a hurricane, and rare alleles become common in the next generation. Explain which Hardy-Weinberg condition was violated and why this can cause evolution.