Explain how natural selection acts on heritable variation so that advantageous traits become more common in a population over generations, and apply this to examples such as antibiotic resistance (MA STE HS-LS4-2, HS-LS4-3, cause and effect).

What this topic is asking

The Massachusetts STE framework (HS-LS4-2 and HS-LS4-3) asks you to construct an explanation for how natural selection leads to adaptation and to use data to support claims about which traits become more common. On the High School Biology MCAS, natural selection is usually tested with a scenario (camouflage, antibiotic resistance, beak shape) where you apply the mechanism, predict how a population changes, and identify the source of variation. The crosscutting concept is cause and effect, and the science practice is constructing explanations or arguing from evidence.

What natural selection is

The idea, first set out by Charles Darwin, is simple but powerful. In any population, individuals vary. Some variations make an individual better suited to its environment, so it is more likely to survive and reproduce. Because the trait is heritable, its offspring inherit it. Over many generations, the advantageous trait becomes more common, and the population becomes better adapted to its environment.

The four conditions for natural selection

The MCAS rewards knowing the ingredients, because applying them to a scenario is the standard question:

1. Variation. Individuals in a population differ in their traits (for example, color, size, or resistance).
2. Selection pressure. Something in the environment affects survival, such as a predator, a disease, a limited food supply, or an antibiotic.
3. Differential survival and reproduction. Individuals with the advantageous trait survive and reproduce more than those without it. This is sometimes called "survival of the fittest," where fittest means best suited, not strongest.
4. Inheritance. The advantageous trait is heritable, so it is passed to the next generation.

Repeat this over many generations and the proportion of the advantageous trait rises. The original variation comes from mutation (see mutations and biotechnology), and sexual reproduction shuffles it (see meiosis and sources of variation).

A key point: selection acts on populations, not individuals

This is the most rewarded subtlety. Natural selection does not change an individual during its life. A beetle does not turn brown because it would be safer; rather, brown beetles that already exist survive better and leave more offspring, so the population becomes browner over generations. An individual either has the advantageous trait or it does not; selection changes the proportions of traits in the population, not the traits of any one individual.

Antibiotic resistance: the textbook example

Antibiotic resistance is the example the MCAS uses most, because it shows natural selection happening quickly:

• A bacterial population varies: by chance, a few bacteria carry a mutation for resistance.
• An antibiotic is the selection pressure: it kills the non-resistant bacteria but not the resistant ones.
• The resistant bacteria survive and reproduce, passing on the resistance gene.
• Over time, most of the population is resistant.

This is why overusing antibiotics is dangerous: it selects for resistant bacteria. The same logic explains pesticide resistance in insects and is strong evidence that evolution is happening now, which connects to evidence for evolution.

Try this

Q1. State the four conditions needed for natural selection. [2]

• Cue. Variation, a selection pressure, differential survival and reproduction, and inheritance of the advantageous trait.

Q2. Explain why natural selection acts on populations rather than individuals. [2]

• Cue. An individual cannot change its heritable traits during life; selection changes the proportions of traits in the population over generations as the better-suited reproduce more.