Evolution & Natural Selection Cheat Sheet

A printable reference covering natural selection, allele frequencies, Hardy-Weinberg equations, speciation, evidence for evolution, and phylogenetic trees for grades 10-12.

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Evolution explains how populations change over generations as inherited traits become more or less common. This cheat sheet helps students connect natural selection, genetic variation, adaptation, and speciation into one clear framework. It is useful for reviewing vocabulary, interpreting diagrams, and solving allele frequency problems. Students also need these ideas to understand biodiversity, antibiotic resistance, and the history of life on Earth. The core idea is that evolution acts on populations, not individuals, because allele frequencies change across generations. Natural selection favors traits that improve fitness in a specific environment, but mutation, genetic drift, gene flow, and nonrandom mating can also change populations. Hardy-Weinberg equations describe a population that is not evolving under ideal conditions. Evidence for evolution includes fossils, homologous structures, DNA similarities, embryology, biogeography, and observed evolutionary change.

Key Facts

Evolution is a change in allele frequencies in a population over time.
Natural selection requires genetic variation, heritability, overproduction of offspring, and differential survival or reproduction.
Fitness means reproductive success, so the most fit organisms are those that leave the most viable offspring.
Hardy-Weinberg allele frequencies are p + q = 1, where p is the dominant allele frequency and q is the recessive allele frequency.
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, random mating, no natural selection, very large population size, and no gene flow.
Speciation can occur when populations become reproductively isolated and accumulate enough genetic differences to form separate species.
A phylogenetic tree shows evolutionary relationships, and the most recent common ancestor is found at the node where branches meet.

Vocabulary

Adaptation: An inherited trait that increases an organism's chance of survival or reproduction in a particular environment.
Allele frequency: The proportion of a specific allele among all alleles for a gene in a population.
Genetic drift: A random change in allele frequencies, especially strong in small populations.
Gene flow: The movement of alleles between populations when individuals migrate and reproduce.
Speciation: The formation of a new species when populations become reproductively isolated and genetically different.
Common ancestor: An ancestral species from which two or more later species evolved.

Common Mistakes to Avoid

Saying individuals evolve is wrong because individual organisms do not change their inherited allele frequencies during life. Populations evolve across generations.
Assuming natural selection always creates perfect organisms is wrong because selection only acts on existing variation and is limited by tradeoffs, history, and environment.
Confusing fitness with strength is wrong because biological fitness means reproductive success, not size, speed, or physical power alone.
Using p^2 as the dominant allele frequency is wrong because p is the dominant allele frequency, while p^2 is the homozygous dominant genotype frequency.
Reading a phylogenetic tree as a ladder of progress is wrong because branches show relationships and common ancestry, not higher or lower organisms.

Practice Questions

1 In a population, the recessive phenotype frequency is 0.16. Assuming Hardy-Weinberg equilibrium, find q, p, and the heterozygote frequency 2pq.
2 A population has 200 individuals: 80 are AA, 100 are Aa, and 20 are aa. Calculate the frequency of allele A and allele a.
3 A beetle population has both green and brown beetles. Birds more easily see green beetles on dark soil, and after many generations brown beetles become more common. Identify the evolutionary mechanism and explain the change.
4 Two squirrel populations are separated by a new river and no longer interbreed. Over many generations, their mating calls and allele frequencies become different. Explain how this could lead to speciation.