Explain how the role of variation and mutations drives natural selection, producing adaptation and changing the heritable traits of a population over generations (Virginia 2018 Biology SOL BIO.7.b).

What this topic is asking

Virginia Biology SOL standard BIO.7 is about how populations change through time, and substandard BIO.7.b focuses on the role of variation and mutations in the process of natural selection. For the Biology EOC that means knowing the ingredients of natural selection (variation, overproduction, competition, and differential survival and reproduction), being able to apply them to a scenario (camouflage, antibiotic resistance, beak shape), and holding on to the central idea that populations evolve, individuals do not. Items almost always give a scenario, a data table, or a graph and ask you to predict or explain the change.

The ingredients of natural selection

If a scenario contains all four, it is natural selection. The trait must be heritable (genetic), and the change happens across generations in the population, not within a single individual's lifetime.

Where the variation comes from (the BIO.7.b focus)

Natural selection can only act on variation that already exists. The standard names two sources of that variation:

• Mutations. A mutation is a change in the DNA base sequence, and it is the original source of new alleles. Most mutations are neutral or harmful, but the rare beneficial one supplies new raw material for selection.
• Sexual reproduction. Meiosis and fertilization shuffle existing alleles into new combinations, so siblings differ. This is why sexually reproducing populations carry so much variation to begin with.

Selection does not create variation; it filters it, favoring the variants that work best in the current environment. This is the heart of BIO.7.b: variation and mutation provide the differences, and natural selection sorts them.

Fitness and adaptation

A trait that raises fitness in a given environment is favored by selection. Over generations this produces adaptation, the visible result of natural selection acting on variation. Camouflage, a thick coat in a cold climate, a beak shape suited to the available food, and antibiotic resistance in bacteria are all adaptations. The same trait can be an advantage in one environment and a disadvantage in another, so fitness is always relative to the environment.

Populations evolve, not individuals

Evolution is a change in the heritable traits (allele frequencies) of a population over generations. An individual organism does not evolve during its life; it is the population that changes as the proportions of different alleles shift from one generation to the next. This is one of the most common EOC misconceptions to avoid. A single mouse does not turn dark; rather, dark mice become a larger fraction of the population over generations because they survive and reproduce more. Always describe the change at the population level.

Try this

Q1. State the four conditions required for natural selection to occur. [2]

• Cue. Variation in the population; overproduction and competition for resources; differential survival and reproduction (fitness); and inheritance of the advantageous heritable traits.

Q2. Explain the role of mutation in natural selection. [2]

• Cue. Mutation is the original source of new alleles, so it supplies the heritable variation that selection acts on; without variation there would be nothing for selection to favor.