Natural Selection

Introduction

students, imagine a population of bacteria exposed to an antibiotic 💊. At first, most bacteria die, but a few survive because they already have a trait that helps them resist the drug. Those survivors reproduce, and over time the population changes. This process is natural selection, one of the main ways populations evolve. It is central to the IB Biology HL topic Continuity and Change because it explains how living things remain connected to their ancestors while also changing over generations.

By the end of this lesson, you should be able to:

• Explain the main ideas and terminology behind natural selection.
• Use examples and evidence to show how natural selection works.
• Apply IB Biology HL reasoning to situations involving change in populations.
• Connect natural selection to inheritance, cell division, molecular genetics, and the environment.
• Summarize why natural selection is important in continuity and change.

Natural selection does not happen because organisms “try” to adapt. It happens because individuals with inherited traits that improve survival or reproduction are more likely to pass those traits on. Over many generations, this changes the frequency of alleles in a population.

Core ideas and key terms

Natural selection depends on several conditions. First, there must be variation within a population. For example, some beetles may be green and others brown. Second, some of that variation must be inherited, meaning it is passed from parents to offspring through genes. Third, individuals with certain traits must have a selective advantage in a particular environment. If brown beetles are harder for birds to spot on dark soil, they may survive longer and produce more offspring.

Important terms include:

• Population: a group of organisms of the same species living in the same area.
• Trait: a characteristic, such as fur color or beak shape.
• Allele: an alternative version of a gene.
• Selective pressure: any environmental factor that affects survival or reproduction, such as predators, disease, or temperature.
• Fitness: the ability of an organism to survive and reproduce in its environment.
• Adaptation: an inherited trait that improves fitness.
• Evolution: change in allele frequencies in a population over time.

A common misunderstanding is that natural selection gives organisms what they “need.” Instead, random mutation and recombination create variation first, and the environment then favors some inherited traits over others. That is why natural selection is not random, even though the genetic variation it acts on arises by random processes.

How natural selection works step by step

Natural selection can be explained in a simple sequence:

1. Variation exists in the population.
2. Environmental pressure affects individuals differently.
3. Some individuals have traits that help them survive and reproduce more successfully.
4. Those individuals pass the helpful alleles to their offspring.
5. Over generations, the advantageous alleles become more common.

This is easy to see in a classic example: the peppered moth. Before industrial pollution, light-colored moths were better camouflaged on pale tree bark. When soot darkened the trees, dark moths became less visible to predators and survived more often. The environment changed, so the trait that was favored changed too 🦋.

Notice that individuals do not evolve during their lifetime. A moth does not become dark because it “needs” to. Rather, the frequency of dark-color alleles increases in the population because dark moths leave more offspring.

Natural selection acts on the phenotype: the observable traits of an organism. However, the long-term evolutionary change occurs in the genotype and allele frequencies. This connection is important in IB Biology HL because it links molecular genetics with population change.

Variation, mutation, and inheritance

Where does the variation come from? In sexually reproducing organisms, variation is generated by:

• Mutation: random changes in DNA sequence.
• Meiosis: creates new allele combinations through crossing over and independent assortment.
• Fertilization: combines genetic material from two parents in a new way.

Mutations are especially important because they create new alleles. For example, a mutation in a bacterial gene may alter the target of an antibiotic, making the drug less effective. If the mutation is inherited, natural selection can increase the frequency of that allele in the population.

The idea of inheritance is essential. A helpful trait must be heritable; otherwise, it cannot spread through the population. For instance, if a lion develops stronger muscles because it exercises, that acquired trait is not passed on genetically. Natural selection works with inherited variation, not acquired characteristics.

This is why molecular genetics matters. Genes influence proteins, proteins influence traits, and traits influence survival and reproduction. A change in a DNA base sequence can change an amino acid sequence, which can change the shape or function of a protein. That small molecular change can sometimes have a large impact on fitness.

Evidence and real-world examples

Natural selection is supported by many real-world examples. One of the most important is antibiotic resistance in bacteria 🧫. Some bacteria have alleles that make them resistant to an antibiotic. When the antibiotic is used, susceptible bacteria die, while resistant bacteria survive and reproduce. Over time, the resistant strain becomes more common. This is a major issue in medicine and public health.

Another example is pesticide resistance in insects. If a pesticide kills most insects but a few carry resistant alleles, those survivors reproduce. Farmers may then need different control methods. This shows how human activity can create strong selective pressures.

A third example is Darwin’s finches. On islands, different food sources favor different beak shapes. Birds with beaks better suited to available seeds, insects, or flowers survive and reproduce more successfully. Over time, populations can diverge. This is one way natural selection can contribute to speciation, the formation of new species.

In IB Biology HL, you may need to use evidence such as changes in allele frequency, survival rates, or phenotype distributions to explain natural selection. Graphs often show that one trait becomes more common over time while another decreases. This is strong evidence of selection acting on a population.

Natural selection and Continuity and Change

Natural selection fits the theme of Continuity and Change because life shows both stability and transformation. Continuity comes from the fact that DNA is passed from one generation to the next. Change comes from mutation, recombination, and selection. Organisms stay connected to their ancestors through inheritance, but populations are not fixed. They respond to changing environments.

This topic also connects to homeostasis and climate change. As environments change, selective pressures change too. For example, warmer temperatures may favor organisms with traits that help them avoid heat stress. In changing ecosystems, species with greater genetic variation may have a better chance of surviving because some individuals are more likely to possess helpful alleles.

Natural selection is also linked to sustainability. Human actions such as habitat destruction, overuse of antibiotics, and intensive agriculture can alter selective pressures quickly. Understanding natural selection helps explain why biodiversity matters. A population with low genetic variation may struggle to adapt to new diseases or climate stress, while a more diverse population may have a greater chance of long-term survival.

It is important to remember that natural selection acts on existing variation. It does not produce perfect organisms. An adaptation may be helpful in one environment but harmful in another. For example, a trait that improves survival in cold conditions may reduce performance in hot conditions. Evolution has no goal; it simply reflects which traits are favored in a given environment.

Applying IB Biology HL reasoning

When answering IB-style questions about natural selection, focus on cause and effect. A strong answer usually includes:

• A clear statement of the environmental pressure.
• Identification of variation in the population.
• Explanation of which phenotype has higher fitness.
• Description of inheritance of the beneficial allele.
• A conclusion about how allele frequency changes over time.

For example, if a question asks why a population of moths changed color over several generations, you should not say the moths “chose” to change. Instead, explain that moths with a color that improved camouflage were less likely to be eaten, more likely to reproduce, and passed their alleles to offspring. That is the biological logic of natural selection.

If data are given, interpret them carefully. A rise in the number of resistant individuals, a shift in phenotype frequency, or a survival advantage in one group can all indicate selection. Always use the evidence provided and connect it to inheritance and reproduction.

Conclusion

Natural selection is a powerful explanation for how populations change while remaining connected to previous generations. It relies on inherited variation, environmental pressures, differential survival and reproduction, and the passing on of advantageous alleles. It connects molecular genetics to large-scale change in populations and helps explain adaptation, antibiotic resistance, and biodiversity. Within Continuity and Change, natural selection shows how life is both stable through inheritance and dynamic through evolution 🌍.

Study Notes

• Natural selection is the process in which individuals with inherited traits that improve fitness leave more offspring.
• Variation must exist in a population for natural selection to act.
• Variation comes from mutation, meiosis, and fertilization.
• Natural selection acts on phenotypes, but evolution is measured as change in allele frequencies.
• Individuals do not evolve during their lifetime; populations evolve over generations.
• A trait must be heritable to spread through a population.
• Selective pressures include predators, disease, competition, climate, and human activities.
• Examples include antibiotic resistance, pesticide resistance, peppered moths, and Darwin’s finches.
• Natural selection is closely linked to molecular genetics because DNA changes can alter proteins and traits.
• In Continuity and Change, natural selection explains how inheritance maintains continuity while environments drive change.
• Populations with greater genetic diversity may be better able to adapt to environmental change.
• IB Biology HL answers should explain variation, selection pressure, survival, reproduction, inheritance, and change over time.