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.
Understanding Natural Selection
A population contains a genetic toolkit, and each generation receives a shuffled sample of that toolkit. Mutations create new DNA versions, while sexual reproduction mixes existing versions into new combinations. Most new mutations have little effect or are harmful in a particular setting.
A few affect a visible feature, body function, or behavior in a way that changes the number of offspring produced. Selection does not plan ahead.
A trait that is useful today can become neutral or costly when conditions change. Thick fur helps in cold places but can cause overheating in a warmer climate.
The important unit is the population, not a single organism. One fast rabbit does not make the whole rabbit population faster. If its speed has a genetic basis and it produces more young that survive to reproduce, genes contributing to speed may appear more often in later generations.
Scientists can track this change by comparing DNA samples or counting trait forms across many individuals. Chance can complicate the pattern, especially in small populations.
A storm, fire, or random failure to find a mate may remove gene versions regardless of whether they were useful. This random process is called genetic drift, and it is different from selection.
Selection can take several patterns. Directional selection favors one end of a range, such as increasingly pesticide resistant insects after repeated pesticide use. Stabilizing selection favors intermediate forms.
For example, very small and very large bird eggs may be less likely to hatch than eggs near a middle size. Disruptive selection favors both extremes over the middle when different resources are available. These patterns are not fixed labels for a species.
They describe what happens under particular environmental conditions. A single trait can involve many genes, food quality, disease exposure, and development, so real data often show messy results rather than a simple winner and loser.
Humans create strong selection pressures through farming, medicine, fishing, hunting, and climate change. Antibiotics do not make bacteria resistant during treatment. Resistant bacteria may already be present because of earlier mutations.
When susceptible bacteria are killed, resistant ones can multiply rapidly because competition is reduced. This is why incomplete or unnecessary antibiotic use can worsen resistance. Similar reasoning applies to weeds exposed to herbicides and fish harvested before they grow large.
When studying examples, pay attention to the trait, whether it can be inherited, the environmental condition, and the number of offspring produced. Avoid saying organisms evolve because they need to. Evidence must show that inherited differences were linked to different reproductive outcomes over generations.
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.