Click image to open full size
Natural selection is the process by which helpful inherited traits become more common in a population over generations. It matters because it explains how living things become better suited to their environments without any individual organism trying to change. In a population of beetles, finches, or rabbits, individuals are not identical, and some differences affect survival or reproduction. When the environment favors one variation, that trait can spread through the population.
Key Facts
- Natural selection requires variation, inheritance, differential survival or reproduction, and time.
- Fitness means reproductive success, not physical strength alone.
- Allele frequency = number of copies of an allele / total number of allele copies in the population.
- If a helpful trait improves survival or reproduction, its allele frequency may increase across generations.
- Natural selection acts on phenotypes, but evolution is measured as changes in allele frequencies.
- A trait is helpful only in a specific environment, so a trait that helps in one habitat may not help in another.
Vocabulary
- Natural selection
- Natural selection is the process in which inherited traits that improve survival or reproduction become more common in a population over generations.
- Variation
- Variation is the presence of differences in traits among individuals of the same species.
- Fitness
- Fitness is an organism's ability to survive, reproduce, and pass its genes to the next generation.
- Adaptation
- An adaptation is an inherited trait that helps an organism survive or reproduce in a particular environment.
- Allele frequency
- Allele frequency is the proportion of a specific gene version in a population.
Common Mistakes to Avoid
- Saying individuals evolve, which is wrong because evolution describes changes in populations over generations, not changes within one organism's lifetime.
- Assuming organisms choose helpful traits, which is wrong because natural selection works on existing inherited variation and does not involve conscious choice.
- Calling every useful feature an adaptation, which is wrong unless the trait is inherited and became common because it improved survival or reproduction.
- Thinking natural selection always produces perfect organisms, which is wrong because selection is limited by available variation, tradeoffs, and changing environments.
Practice Questions
- 1 In Generation 1, 20 out of 100 beetles have a dark color trait. In Generation 4, 70 out of 100 beetles have the trait. What percent of the population has the dark trait in each generation, and by how many percentage points did it increase?
- 2 A rabbit population has 40 individuals. Each rabbit has 2 copies of a fur color gene, so there are 80 total allele copies. If 50 allele copies code for thicker fur, what is the allele frequency of the thicker fur allele?
- 3 A population of finches lives through a drought that leaves mostly large, hard seeds. Explain why finches with larger beaks may become more common over several generations.