DAT Review · Biology
Evolution and Natural Selection
On this page 7 sections
In 30 seconds
- Darwin's four postulates of natural selection are the foundation: variation, heritability, overproduction of offspring, and differential survival/reproduction.
- Fitness = reproductive success, NOT physical strength or speed. An organism that leaves more viable offspring has higher fitness, period.
- Individuals do NOT evolve — populations evolve across generations. This is the single most common trap on the DAT.
- Know the three modes of selection (directional, stabilizing, disruptive) and be able to identify each from a graph or scenario.
- Genetic drift, gene flow, and mutation are mechanisms of evolution beyond natural selection — the DAT frequently asks which mechanism is operating in a given scenario.
The college version
Core Review
Darwin's Theory and Natural Selection
Charles Darwin and Alfred Russel Wallace independently proposed the theory of evolution by natural selection. Darwin's On the Origin of Species (1859) articulated four key observations and inferences:
- Variation: Individuals within a population differ in their traits (morphology, physiology, behavior).
- Heritability: Some of these variations are heritable — passed from parents to offspring through genetic material.
- Overproduction: Populations produce more offspring than can survive — resources are limited, creating a struggle for existence.
- Differential reproduction: Individuals with traits better suited to their environment survive and reproduce more successfully, passing those advantageous alleles to the next generation.
Over many generations, this process shifts the allele frequencies in the population, leading to adaptation — the accumulation of traits that improve fitness in a given environment.
Fitness
In evolutionary biology, fitness is measured solely by reproductive success — the number of viable, fertile offspring an organism contributes to the next generation relative to others in the population. An organism that is physically strong but sterile has zero fitness. Fitness is always relative and environment-dependent; a trait that increases fitness in one environment may reduce it in another. Inclusive fitness extends this concept to include the reproductive success of relatives who share alleles by common descent (important for understanding kin selection and altruism).
Types of Natural Selection
Natural selection can shift the distribution of phenotypes in a population in three primary ways:
| Mode | Description | Effect on Distribution | Example |
|---|---|---|---|
| Directional | One extreme phenotype is favored | Mean shifts toward extreme; variance may decrease | Antibiotic resistance in bacteria: resistant individuals survive and reproduce |
| Stabilizing | Intermediate phenotype is favored; extremes are selected against | Mean unchanged; variance decreases | Human birth weight: very small or very large babies have lower survival |
| Disruptive | Both extremes are favored; intermediate is selected against | Mean may stay; variance increases; can lead to speciation | African seedcracker finches: large-billed birds eat hard seeds, small-billed birds eat soft seeds; intermediate fails at both |
Other Mechanisms of Evolution
Evolution is defined as a change in allele frequencies in a population over time. Beyond natural selection, three additional mechanisms drive this change:
Genetic drift is the random fluctuation of allele frequencies due to chance events. It is most powerful in small populations. Two special cases are tested heavily:
- Bottleneck effect: A catastrophic event (fire, flood, disease) randomly eliminates a large portion of the population. The surviving gene pool may not represent the original population's diversity. Example: northern elephant seals were hunted to ~20 individuals; today's population has severely reduced genetic diversity.
- Founder effect: A small group breaks off and colonizes a new area. The new population's gene pool reflects the founders, not the source population. Example: the Amish population's elevated frequency of Ellis–van Creveld syndrome (a rare form of dwarfism).
Gene flow (migration) is the movement of alleles between populations. When individuals move and breed, they transfer alleles, reducing genetic differences between populations. Gene flow can introduce new alleles or alter existing frequencies. A classic example: pollen from one plant population blown to another.
Mutation is the ultimate source of all new genetic variation. Mutations are random changes in nucleotide sequences. While most mutations are neutral or deleterious, rare beneficial mutations provide the raw material upon which natural selection can act. Mutation rates are typically low, so mutation alone rarely causes rapid evolution — but combined with selection, it powers long-term evolutionary change.
Evidence for Evolution
Key lines of evidence include: fossil records showing transitional forms; comparative anatomy (homologous structures indicating common ancestry vs analogous structures indicating convergent evolution); molecular biology (DNA and protein sequence comparisons); biogeography (distribution of species reflecting continental drift); and direct observation (antibiotic resistance, beak size changes in Darwin's finches).
Common Traps
- Colloquial "fitness" vs evolutionary fitness. The DAT will describe a strong, fast organism that fails to reproduce — and ask about its fitness. Answer: low/zero.
- Confusing bottleneck (population crash) with founder effect (small group colonizes). Both reduce diversity, but the mechanism differs.
- Thinking natural selection is the only mechanism of evolution. Genetic drift, gene flow, and mutation all change allele frequencies too.
- Believing organisms evolve during their lifetime. Lamarckian inheritance (use and disuse) is wrong — natural selection acts on existing variation across generations.
Eli explains
The same idea, in plain words
Explain it like I’m 10
Imagine a population of beetles — some green, some brown. Birds can easily spot green beetles on brown tree bark, so they eat more green ones. Brown beetles survive, mate, and have lots of brown babies. After many generations, nearly every beetle is brown. That's natural selection! The brown beetles didn't "try" to become brown — some were just born that way, and nature "chose" them. Here's the most important rule: a single beetle never changes; the whole population changes over time. Also, sometimes change happens just by luck — a random foot squashes all but a few beetles (that's genetic drift). And fitness isn't about being the strongest or fastest; it's about who has the most babies that survive to have babies of their own.
Key takeaways
- "Individuals do NOT evolve" — this is the most common trap. The phrase "the bacteria evolved resistance" really means the population evolved because resistant individuals were selected for.
- Directional → mean shifts; stabilizing → variance narrows; disruptive → both extremes thrive — memorize with graphs.
- Genetic drift is chance-driven — not adaptive. "A flood randomly killed most of the beetles" = bottleneck effect.
- Gene flow = movement of alleles between populations — reduces genetic divergence.
- Fitness is about reproduction, not strength. A bird with dull plumage that produces 10 offspring has higher fitness than a brilliantly colored bird that produces 3.
- Homologous structures = common ancestor (human arm, whale flipper, bat wing). Analogous structures = convergent evolution (bird wing, insect wing).
Check yourself
3 review questions from the chapter. Try each one, then open the answer.
A population of rabbits lives in a region where snow cover has increased over the last century. White rabbits survive predation better than brown rabbits. Over time, the population shifts from mostly brown to mostly white. This is an example of which type of selection?
Show answer
Directional selection. One extreme phenotype (white fur) is favored, and the population mean shifts toward that extreme over successive generations. This is classic natural selection in response to an environmental change.
A hurricane randomly kills 90% of a lizard population on a small island. The survivors happen to have a different allele frequency for scale color than the original population. This event is best described as what?
Show answer
A bottleneck effect — a form of genetic drift. The random, catastrophic reduction in population size changed allele frequencies by chance, not by selective advantage. The surviving population has reduced genetic diversity.
A farmer uses the same pesticide every season. After several years, the pesticide becomes ineffective. Explain this observation in terms of natural selection.
Show answer
The pesticide acts as a selective agent. Within the original pest population, a few individuals carried alleles conferring resistance. These resistant individuals survived and reproduced, passing resistance alleles to their offspring. Over generations, the population evolved resistance — the proportion of resistant alleles increased. Note: individual pests did not "develop" resistance during their lifetime; resistant genotypes were already present at low frequency.
Study tools & related lessonsYou’ll learn to · Related
You’ll learn to
- State Darwin's four postulates of natural selection and explain how they lead to adaptation.
- Define fitness in evolutionary terms and distinguish it from colloquial usage.
- Describe the three modes of natural selection and predict their effects on trait distributions.
- Compare genetic drift (including bottleneck and founder effects), gene flow, and mutation as evolutionary mechanisms.
- Explain why populations — not individuals — evolve.
Sources & references
- OpenStax Biology 2e, Chapter 18: "Evolution and the Origin of Species"
- NCBI Bookshelf, "How Natural Selection Works"
- Darwin, C. (1859). *On the Origin of Species*. Available through the Darwin Online project
This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.
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