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.
Understanding Evolution & Natural Selection
Natural selection is not a force that gives organisms what they need. Variation appears first through mutation and the reshuffling of genes during sexual reproduction. The environment then affects which variants leave more descendants.
A thick coat can help an animal survive cold winters, yet the same coat may cause overheating in a warm habitat. Many traits involve tradeoffs like this. Selection depends on the current conditions, including predators, food, disease, climate, and competition.
A trait is not permanently better or worse. It is only more useful in a particular setting.
Students often confuse dominance with usefulness. A dominant allele can be rare, harmful, or disappear from a population. A recessive allele can remain hidden in heterozygous individuals, even when the recessive trait is selected against.
Hardy Weinberg calculations give a useful snapshot for comparing expected patterns with real data. For example, if one out of every one hundred individuals shows a recessive condition, the recessive allele frequency is one tenth because one tenth times one tenth equals one hundredth. The other allele frequency is nine tenths.
From there, expected genotype proportions can be calculated. A difference between expected and observed numbers suggests that one or more evolutionary influences may be operating.
Small populations can change in surprising ways through chance alone. After a flood, fire, or disease outbreak, the survivors may not represent the original population well. Their alleles become the starting point for later generations.
This bottleneck effect can reduce genetic diversity. A founder effect occurs when a few individuals establish a new population elsewhere. Gene flow can have the opposite effect by bringing alleles into a population when organisms migrate and reproduce.
Mutation usually changes one allele at a time, but it supplies the raw material that later processes can spread or remove. In exam questions, pay close attention to population size, movement, and random events because they point to the mechanism involved.
New species form gradually when groups stop exchanging genes for long enough. A physical barrier such as a mountain range can separate populations, though reproductive isolation can develop without a barrier. Different breeding seasons, courtship signals, chromosome changes, or incompatible gametes can prevent successful reproduction.
Phylogenetic trees help organize evidence about these relationships. The tips represent groups being compared, while each branching point represents a shared ancestor. Groups that sit next to each other on the page are not necessarily closest relatives.
The key evidence is the most recent shared branching point. DNA comparisons, fossils, body structures, and geographic patterns can all support a tree, but each source has limits. Scientists compare many lines of evidence rather than relying on one feature alone.