ACT Science Evolution and Natural Selection: Understand How Populations Change Over Time

Natural selection: Organisms with traits better suited to environment tend to survive and reproduce more. Over time, advantageous traits become more common in the population. Key concept: Individuals don't evolve; populations do. Evolution is change in allele frequency over generations. Mechanism: (1) Variation exists in populations (differences in traits). (2) Some variations confer advantage in current environment. (3) Advantaged individuals reproduce more. (4) Advantageous alleles increase in frequency. Adaptation: Trait that increases survival or reproduction (result of natural selection). Questions test whether you understand that evolution is not purposeful, traits don't develop "because they're needed," and natural selection drives change through differential reproduction.

Example: In a population of moths, darker color helps avoid predators in polluted forests. Dark moths survive more often, reproduce more, and allele frequency for dark color increases over generations.

Mistake 1: Thinking organisms evolve to meet environmental needs. Evolution is not purposeful. Traits that help survive are selected for; traits that don't help don't increase. Mistake 2: Confusing adaptation with individual change. A moth doesn't become darker during its life. Populations with more dark moths become common because dark moths survive better. Mistake 3: Thinking evolution requires a "perfect" adaptation. Natural selection favors traits that help, even if they're not optimal. A slightly better trait spreads through the population. Remember: Natural selection acts on existing variation. It doesn't create new traits; it changes frequencies of existing ones.

During practice, trace how traits spread through populations over generations due to survival/reproduction advantage.

Scenario 1: Rabbits in a field have brown or white fur. Predators see white easier. Brown rabbits survive more and reproduce. Over generations, population becomes mostly brown. Natural selection (brown color is advantageous in this environment). Scenario 2: A new disease kills plants that lack a certain enzyme. Plants with the enzyme survive and reproduce. Allele frequency for the enzyme increases. Scenario 3: A population has variation in beak size. Drought reduces food, favoring larger beaks (can crack hard seeds). Birds with larger beaks survive better. Beak size increases in population over time. Scenario 4: Mutation creates a bright color in a species. In the current environment, bright color attracts predators. Bright individuals don't survive as long. Allele frequency decreases. Natural selection can work against a new trait if it doesn't help survival. For each scenario, predict how natural selection changes allele frequencies.

Find three ACT Science passages with evolution questions. Predict population changes based on natural selection logic. By the third passage, selection mechanism understanding will be reliable.

Evolution and natural selection questions appear on some ACT Science sections. They test understanding of population genetics and adaptation. Students who correctly apply natural selection logic pick up 1 point because the mechanism is consistent and predictable.

On your next practice test, trace how traits spread through populations based on survival and reproduction advantage. By test day, you should explain evolution and natural selection confidently.