Develop and use mathematical models to support explanations of how undirected genetic changes, including genetic drift and gene flow, alongside natural selection, lead to changes in populations of organisms (GSE SB6.d).

What this topic is asking

Standard SB6.d asks you to use mathematical models to explain how undirected genetic changes, including genetic drift and gene flow, alongside natural selection, change populations. For the Georgia Milestones Biology EOC that means knowing the mechanisms that shift allele frequencies, telling the random ones (drift and mutation) from the non-random one (selection), and using a simple model such as $p + q = 1$. Items often describe a scenario (a random die-off, migration, a new mutation) and ask which mechanism it shows.

Evolution as a change in allele frequencies

For a gene with two alleles, the frequencies must add up to 1:

where $p$ is the frequency of one allele and $q$ the frequency of the other. If $p$ rises, $q$ must fall by the same amount. The mechanisms below are simply the different ways $p$ and $q$ can change.

Mutation: the source of new alleles

Mutation is a change in DNA, and it is the only mechanism that creates brand-new alleles. Most mutations are neutral or harmful, but the rare beneficial ones supply the raw material the other mechanisms act on. Without mutation there would be no new variation for selection or drift to work with, which is why mutation is the ultimate source of genetic variation. Mutation alone changes frequencies slowly; its importance is as the origin of variation.

Natural selection: the non-random mechanism

Natural selection changes allele frequencies non-randomly: alleles that build advantageous, heritable traits increase because their carriers survive and reproduce more. Selection is the mechanism that produces adaptation, because it consistently favors variants that suit the environment. It is the only mechanism that reliably makes a population better matched to its surroundings; drift and gene flow do not aim at anything.

Genetic drift: change by chance

The key signal of drift is randomness: the change is not based on which traits are advantageous. If an EOC scenario says survival was random, or a small group founded a new population, the mechanism is genetic drift.

Gene flow: alleles move between populations

Gene flow is the movement of alleles between populations, carried by migrating individuals or by gametes (such as wind-blown pollen). Individuals moving in add alleles to the gene pool; individuals leaving remove them. Gene flow tends to make separate populations more similar, and it can introduce a new allele to a population. Stopping gene flow (for example by a geographic barrier) is an important step toward speciation.

The non-evolving baseline (Hardy-Weinberg)

To recognize when a population is evolving, it helps to picture one that is not. The Hardy-Weinberg idea describes a population whose allele frequencies stay constant because none of the mechanisms is acting (no mutation, no selection, no drift, no gene flow, random mating). In that ideal case $p + q = 1$ holds with the same values each generation. Real populations almost always violate a condition, so they do evolve; the baseline is a comparison, not a real population.

Try this

Q1. State the difference between the bottleneck effect and the founder effect. [2 points]

• Cue. A bottleneck is a sudden random reduction of an existing population (survivors carry a chance sample of alleles); the founder effect is when a small group leaves to start a new population carrying only some of the original alleles. Both are genetic drift.

Q2. Explain why mutation is described as the ultimate source of genetic variation. [2 points]

• Cue. Mutation is the only mechanism that creates brand-new alleles; selection, drift, and gene flow only rearrange or change the frequencies of variation that already exists, so the new variation must originate from mutation.