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Natural Selection
How Environmental Pressures Shape Populations
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Natural selection is the process by which inherited traits that improve survival or reproduction become more common in a population over generations. It matters because it explains how populations adapt to changing environments, from insects resisting pesticides to animals changing color patterns in polluted habitats. Natural selection acts on variation that already exists among individuals. The environment does not choose what an organism needs, but it can make some traits more successful than others.
Environmental pressures include predators, climate, food availability, disease, competition, and human activities such as pollution or antibiotic use. Individuals with advantageous heritable traits tend to leave more offspring, so the alleles linked to those traits increase in frequency. Over many generations, this can shift the average traits of a population and sometimes lead to new species. Natural selection changes populations, not individual organisms during their lifetimes.
Key Facts
- Natural selection requires variation, heritability, overproduction of offspring, and differential survival or reproduction.
- Fitness means reproductive success, not physical strength.
- Allele frequency = number of copies of an allele / total number of allele copies in the population.
- A trait becomes more common when individuals with that trait leave more surviving offspring.
- Natural selection acts on phenotypes, but evolution is measured as changes in allele frequencies.
- Selection pressure can be biotic, such as predators or disease, or abiotic, such as temperature or drought.
Vocabulary
- Natural selection
- Natural selection is the process in which heritable traits that improve survival or reproduction become more common in a population over generations.
- Adaptation
- An adaptation is an inherited trait that increases an organism's fitness in a particular environment.
- Fitness
- Fitness is the ability of an organism to survive, reproduce, and pass its genes to the next generation.
- Selection pressure
- A selection pressure is an environmental factor that affects which traits improve survival or reproduction.
- Allele frequency
- Allele frequency is the proportion of a specific allele among all copies of that gene in a population.
Common Mistakes to Avoid
- Saying organisms evolve because they need to change. Individuals do not decide to evolve, since natural selection acts on existing inherited variation in a population.
- Confusing fitness with strength or speed. In biology, fitness means reproductive success, so a smaller or slower organism can be more fit if it leaves more offspring.
- Thinking natural selection creates perfect organisms. Selection only favors traits that work well enough in a specific environment, and tradeoffs or limits may remain.
- Forgetting that the environment can change which traits are advantageous. A trait that helps in one habitat may be harmful or neutral in another habitat.
Practice Questions
- 1 In a beetle population of 200 individuals, 80 are dark colored and 120 are light colored. After a new predator arrives, 70 dark beetles and 60 light beetles survive to reproduce. What percentage of each color survived?
- 2 A population has 60 copies of allele A and 140 copies of allele a. What are the allele frequencies of A and a?
- 3 A drought leaves mostly large, hard seeds in an island habitat. Explain how beak size in a finch population could change over several generations if beak size is heritable.