Evolution and Natural Selection
Evolution is the change in living things over long spans of time, and its main driving force is natural selection. This is one of the biggest ideas in all of biology, and the test expects you to understand how it works. The good news is that natural selection follows a clear, logical chain you can learn in one sitting.
The Logic of Natural Selection
Natural selection works through a few connected steps. First, individuals in a species vary — they are not all identical. Second, some variations help an individual survive and reproduce better in its environment. Third, those individuals pass their helpful traits to more offspring. Over many generations, the helpful traits become more common, and the species gradually changes. This is often summarized as “survival of the fittest,” where fittest means best suited to the environment — not necessarily the strongest.
A classic example: in a population of beetles, the ones whose color hides them from birds survive longer and have more offspring. Over time, that camouflage color spreads through the population.
Where Variation Comes From
Natural selection needs variation to work on, and that variation comes mainly from mutations (random changes in DNA) and from the mixing of genes during reproduction. Selection does not create traits on purpose; it simply favors whichever existing traits happen to help. The environment does the “selecting” by determining who survives to reproduce.
Adaptation and Common Misunderstandings
An adaptation is a helpful trait that becomes common because it improved survival. A key point the test checks: individuals do not choose to change or evolve during their lifetime. A giraffe does not stretch its neck longer and pass that on. Instead, giraffes with naturally longer necks reached more food, survived better, and had more offspring. Evolution acts on populations over generations, not on individuals during their lives.
Watch: A Short Video Lesson
Natural History Museum walks through this skill clearly in a few minutes. It is a helpful companion to the reading above:
A Routine for Evolution Questions
- Start with variation: individuals differ.
- Helpful traits improve survival and reproduction.
- Those traits are passed on and become more common over generations.
- Remember: the environment selects; individuals do not choose to evolve.
Practice
- What is evolution?
- What does “fittest” mean in natural selection?
- Where does the variation that selection acts on come from?
- What is an adaptation?
- Does an individual evolve during its lifetime?
- What “selects” which traits become common?
Answers
- The change in living things over long periods of time.
- Best suited to the environment.
- Mutations and the mixing of genes during reproduction.
- A helpful trait that becomes common because it aided survival.
- No — evolution acts on populations over generations.
- The environment.
Where This Fits in Your Science Prep
Natural selection builds on heredity and leads into genetic variation, common ancestry, and cladograms. See all topics on the Science Topics Hub.
Recommended Prep Books
These study guides and practice books help you keep building momentum as you prepare:
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