Unit 7 Natural Selection: How Evolution Acts on Populations

Population Genetics

Population genetics is the study of how the genetic makeup of a population changes over time. In AP Biology, the key idea is that evolution is defined as a change in allele frequencies in a population across generations. This is a subtle but crucial point: individual organisms do not evolve during their lifetimes. Instead, populations evolve because the proportions of different alleles in the gene pool shift.

Populations, gene pools, and allele frequency

A population is a group of individuals of the same species living in the same area that can interbreed. The combined genetic information in that population is called the gene pool. Population genetics tracks the allele frequencies in that gene pool.

• Allele frequency is the fraction of all copies of a gene that are a particular allele.
• If a gene has two alleles (say A and a), their allele frequencies are often written as p and q.
• For a two-allele system, the total must add to 1:

p + q = 1

Why this matters: once you can describe a population by allele frequencies, you have a quantitative way to detect evolution (frequency changes), compare populations, and predict genotype frequencies under specific conditions (Hardy-Weinberg, covered later).

From alleles to genotypes (and back)

Populations are made of individuals with genotypes (like AA, Aa, aa). You often need to convert between genotype counts and allele frequencies.

If you have genotype counts for a diploid population:

• Total individuals = N
• Total alleles at that gene locus = 2N

If n_{AA}, n_{Aa}, and n_{aa} are the genotype counts, then the number of A alleles is 2n_{AA} + n_{Aa} and the number of a alleles is 2n_{aa} + n_{Aa}.

So the allele frequencies are:

p = \frac{2n_{AA} + n_{Aa}}{2N}

q = \frac{2n_{aa} + n_{Aa}}{2N}

A common misconception is to think that the “most common genotype” must contain the “most common allele.” That can be true, but it is not guaranteed when dominance is involved or when genotype proportions differ from Hardy-Weinberg expectations. Allele frequency is the more fundamental population measure.

Microevolution: mechanisms that change allele frequencies

Microevolution refers to evolution within a population, typically described as changes in allele frequencies across generations. In AP Biology, you should be able to explain how each mechanism changes allele frequencies and, importantly, why it changes them.

Natural selection

Natural selection occurs when individuals with certain heritable traits leave more offspring than others. This difference in reproductive success changes allele frequencies over time.

Key logic chain:

1. Individuals vary in traits.
2. Some variation is heritable (genetic).
3. More offspring are produced than can survive.
4. Individuals with traits better suited to the environment tend to survive and reproduce more.
5. Alleles associated with those traits become more common.

Natural selection is the only major mechanism that consistently produces adaptation, meaning a match between organisms and their environment due to increased frequency of beneficial traits.

Genetic drift

Genetic drift is random change in allele frequencies due to chance events. It is strongest in small populations.

Why drift matters: drift can cause allele loss (reducing genetic variation) even if an allele is beneficial, simply because the individuals carrying it fail to reproduce by chance.

Two classic drift scenarios:

• Bottleneck effect: a population is drastically reduced (disaster, habitat loss). Survivors may not represent original allele frequencies.
• Founder effect: a small group starts a new population; their allele frequencies may differ from the original population.

A frequent exam trap is confusing drift with selection. If the prompt emphasizes randomness, accidents, storms, or “by chance,” that points to drift. If it emphasizes consistent advantage tied to a trait, that points to selection.

Gene flow

Gene flow is the movement of alleles between populations through migration and interbreeding.

Effects you should be able to articulate:

• Gene flow can introduce new alleles into a population.
• It tends to reduce differences between populations (it homogenizes allele frequencies).
• It can increase variation within a population (if migrants bring in alleles not previously present).

Mutation

Mutation is a change in DNA sequence that can create new alleles.

Mutation is the ultimate source of new genetic variation, but mutation alone usually changes allele frequencies slowly. Selection, drift, and gene flow often produce faster changes in allele frequencies at the population level.

Nonrandom mating and sexual selection

Nonrandom mating occurs when individuals choose mates based on phenotype (or are more likely to mate with nearby individuals). Nonrandom mating often changes genotype frequencies more directly than allele frequencies.

A key example is sexual selection, where traits that improve mating success increase in frequency (even if they reduce survival). This helps explain features like elaborate displays or combat structures in some species.

Fitness and selection vocabulary (used carefully)

In population genetics, fitness means relative reproductive success in a specific environment.

• Relative fitness compares reproductive output among genotypes.
• Selection acts on phenotypes, but evolution is tracked as changes in allele frequencies.

A common misconception is “the strongest survive.” In biology, the fittest are those who leave more viable offspring, not necessarily the biggest or strongest.

Worked example: calculating allele frequencies from genotype data

Suppose a population has 100 individuals with the following genotypes:

• AA: 36
• Aa: 48
• aa: 16

Step 1: compute total alleles: 2N = 200.

Step 2: compute number of A alleles:

• AA contributes 2 \times 36 = 72
• Aa contributes 1 \times 48 = 48
  Total A = 120

Step 3: compute number of a alleles:

• aa contributes 2 \times 16 = 32
• Aa contributes 1 \times 48 = 48
  Total a = 80

Step 4: compute frequencies:

p = \frac{120}{200} = 0.6

q = \frac{80}{200} = 0.4

Check: 0.6 + 0.4 = 1.0.

Exam Focus

• Typical question patterns:
  • Given genotype counts (or phenotype counts for dominant/recessive traits), calculate allele frequencies and interpret what a change over time means.
  • Identify which mechanism (selection, drift, gene flow, mutation, nonrandom mating) best explains a described scenario.
  • Explain, in words, why a mechanism changes allele frequencies and predict the direction of change.
• Common mistakes:
  • Treating evolution as something that happens to individuals rather than populations.
  • Confusing genetic drift (random) with natural selection (nonrandom differential reproduction).
  • Forgetting that diploid populations have 2N total alleles for a gene.

Hardy-Weinberg Equilibrium

Hardy-Weinberg equilibrium is a null model: it describes what genotype frequencies should look like in a population if no evolution is occurring at a particular gene locus. In AP Biology, Hardy-Weinberg is used mainly as a tool to detect when evolution is happening and to connect allele frequencies to expected genotype frequencies.

What Hardy-Weinberg says (and what it assumes)

Hardy-Weinberg provides a mathematical relationship between allele frequencies and genotype frequencies for a gene with two alleles A and a.

If the allele frequencies are p for A and q for a, then:

p + q = 1

And the expected genotype frequencies are:

p^2 + 2pq + q^2 = 1

Where:

• p^2 is the expected frequency of AA
• 2pq is the expected frequency of Aa
• q^2 is the expected frequency of aa

Hardy-Weinberg equilibrium requires specific conditions (assumptions). If these assumptions hold, allele frequencies remain constant from generation to generation at that locus.

The standard assumptions are:

• No mutation introducing new alleles at the locus
• Random mating with respect to the gene
• No natural selection (all genotypes have equal reproductive success)
• Extremely large population size (no genetic drift)
• No gene flow (no migration introducing/removing alleles)

Why this matters: real populations almost never meet all assumptions perfectly, so Hardy-Weinberg is not a description of “normal life.” It is a baseline you compare against. If observed genotype frequencies differ from Hardy-Weinberg expectations, you have evidence that at least one evolutionary force is acting.

Interpreting deviations: what does it mean if the population is not in equilibrium?

If a population is not in Hardy-Weinberg equilibrium for a locus, that means one (or more) assumptions is violated. A key AP skill is making a plausible link from a pattern to a cause.

Examples of how violations show up:

• Selection can change allele frequencies in a consistent direction (beneficial alleles increase).
• Nonrandom mating often changes genotype frequencies first (for example, more homozygotes than expected), sometimes without immediately changing allele frequencies.
• Gene flow can shift allele frequencies toward those of the incoming migrants.
• Drift can cause unpredictable fluctuations, especially in small populations.

A common misconception: “If a trait is recessive, it will disappear.” Hardy-Weinberg shows why recessive alleles can persist in heterozygotes even if the recessive phenotype is selected against.

Using Hardy-Weinberg to solve problems

Hardy-Weinberg problems are usually algebra with biological interpretation. The most common setup is that a recessive phenotype frequency is given, allowing you to infer q^2.

Notation reference (two-allele system)

Worked example 1: from recessive phenotype to carrier frequency

In a population, a recessive genetic disorder appears in 1% of individuals. Assume Hardy-Weinberg conditions for this locus.

Step 1: identify what 1% represents. If the disorder is recessive, affected individuals are aa, so:

q^2 = 0.01

Step 2: solve for q:

q = \sqrt{0.01} = 0.1

Step 3: solve for p:

p = 1 - q = 0.9

Step 4: find carrier frequency (heterozygotes, Aa):

2pq = 2(0.9)(0.1) = 0.18

Interpretation: about 18% of the population are carriers (heterozygotes), even though only 1% show the disorder.

Common mistake to avoid: treating 1% as q instead of q^2. If the phenotype is recessive and you are told the fraction of affected individuals, that fraction corresponds to q^2 under Hardy-Weinberg.

Worked example 2: testing equilibrium with observed vs expected

A sample of 200 individuals has genotype counts:

• AA: 98
• Aa: 84
• aa: 18

Step 1: compute allele frequencies from genotype counts.
Total alleles 2N = 400.

Number of A alleles:

• 2 \times 98 = 196 from AA
• 84 from Aa
  Total A = 280

So:

p = \frac{280}{400} = 0.7

Then:

q = 1 - p = 0.3

Step 2: compute expected genotype frequencies under Hardy-Weinberg.

p^2 = (0.7)^2 = 0.49

2pq = 2(0.7)(0.3) = 0.42

q^2 = (0.3)^2 = 0.09

Step 3: convert expected frequencies to expected counts (multiply by 200).

• Expected AA: 0.49 \times 200 = 98
• Expected Aa: 0.42 \times 200 = 84
• Expected aa: 0.09 \times 200 = 18

Observed matches expected exactly, so this sample is consistent with Hardy-Weinberg equilibrium for this locus.

Important nuance: On real data, you often see small mismatches due to sampling. AP questions may ask whether data “support” equilibrium rather than demanding perfect equality.

Exam Focus

• Typical question patterns:
  • Given recessive phenotype frequency, calculate q, p, and carrier frequency 2pq.
  • Given genotype counts, calculate allele frequencies, then expected Hardy-Weinberg genotype frequencies, and compare to observed.
  • Identify which Hardy-Weinberg assumption is violated based on a described situation (migration, selection, drift, etc.).
• Common mistakes:
  • Using Hardy-Weinberg formulas without checking the situation is framed as a “null model” or without being asked for expected frequencies.
  • Mixing up allele frequencies (p, q) with genotype frequencies (p^2, 2pq, q^2).
  • Forgetting to multiply expected genotype frequencies by total population size when the question asks for expected numbers of individuals.

Variations in Populations

Evolution by natural selection requires heritable variation. Without genetic differences among individuals, there is nothing for selection to “choose” among, and allele frequencies cannot shift in a directional way. This section focuses on where variation comes from, how it is measured, and how it is maintained or lost.

What “variation” means in biology

Genetic variation refers to differences in DNA sequences among individuals. Those differences may or may not affect traits you can observe.

• Some variation changes protein function or gene regulation and can affect phenotype.
• Some variation is neutral (no current effect on fitness).
• The environment also influences phenotype, so not all phenotypic differences are genetic.

Why this matters: natural selection acts on phenotypes in an environment, but only the genetic component of phenotypic variation is passed on. A classic misconception is assuming “if it helps an organism, it will be passed on.” Traits are passed on only if they have a genetic basis and the organism reproduces successfully.

Sources of genetic variation

Mutation: the ultimate source of new alleles

Mutations create new alleles by altering DNA sequences. Mutations can occur in coding regions, regulatory regions, or noncoding DNA.

Key points for AP Biology:

• Mutations are random with respect to the needs of the organism (they do not happen because an organism “tries” to adapt).
• Mutation can be beneficial, harmful, or neutral depending on environment and context.
• Only mutations in cells that give rise to gametes (or in gametes themselves) are heritable.

Sexual reproduction: reshuffling existing alleles

Sexual reproduction creates new combinations of alleles, which can create new trait combinations for selection to act on.

Main mechanisms:

• Independent assortment during meiosis: homologous chromosome pairs line up independently.
• Crossing over: homologous chromosomes exchange segments, creating recombinant chromosomes.
• Random fertilization: any sperm can fertilize any egg, generating many genotype combinations.

These processes do not create new alleles, but they greatly increase variation in genotypes (and often phenotypes) among offspring.

Gene flow: importing alleles from other populations

When individuals migrate and reproduce, they can introduce alleles not previously present in the population. This can increase genetic variation within a population and may provide new raw material for selection.

Variation, phenotype distributions, and polygenic traits

Many traits (height, skin pigmentation in humans, yield in crops) are polygenic, meaning they are influenced by multiple genes, often producing continuous variation.

When many genes contribute small effects, phenotypes often form a bell-shaped distribution. This matters because different selection patterns can act on different parts of that distribution.

How selection acts on variation: three common patterns

Selection is often described by how it changes the distribution of phenotypes.

Directional selection

Directional selection favors one extreme phenotype, shifting the population’s average.

Example idea: if larger beak size allows birds to crack harder seeds during drought, alleles contributing to larger beaks may increase.

Stabilizing selection

Stabilizing selection favors intermediate phenotypes and acts against extremes, reducing variation.

Example idea: very low and very high birth weights may have higher mortality; intermediate weights have higher survival.

Disruptive selection

Disruptive selection favors both extremes over intermediates, potentially increasing variation and sometimes contributing to divergence.

Example idea: if small seeds and large seeds are common but medium seeds are rare, birds with very small or very large beaks may do best.

A common misunderstanding is thinking stabilizing selection “stops evolution.” It can reduce phenotypic variation, but allele frequencies can still change (for example, if intermediate genotypes have higher fitness).

Maintaining genetic variation in populations

If selection and drift can reduce variation, why does variation persist?

Several mechanisms can maintain it:

Heterozygote advantage (overdominance)

Heterozygote advantage occurs when heterozygotes have higher fitness than either homozygote, maintaining both alleles in the population.

The key concept is balancing selection: selection can actively preserve multiple alleles rather than eliminating all but one.

Frequency-dependent selection

In frequency-dependent selection, the fitness of a phenotype depends on how common it is.

• Negative frequency-dependent selection: rare phenotypes have an advantage, which can maintain diversity.

Spatial and temporal variation in the environment

If environments differ across locations or change over time, different alleles may be favored at different times or places. Gene flow among subpopulations can then help maintain overall variation.

Losing variation: drift, bottlenecks, and inbreeding

Genetic drift reduces variation

Drift can eliminate alleles, especially rare ones, which reduces genetic diversity. This can make populations less able to respond to environmental change.

Bottlenecks and founder events reduce diversity

After a bottleneck, the surviving population may have:

• fewer alleles overall
• different allele frequencies than the original population

Founder events can produce similar outcomes in a newly established population.

Inbreeding increases homozygosity

Inbreeding (mating among relatives) does not necessarily change allele frequencies immediately, but it increases homozygosity and can reveal harmful recessive alleles as homozygous genotypes become more common.

This is a common point of confusion: inbreeding is not the same as genetic drift, though both are more likely in small populations and both can reduce effective genetic diversity.

Worked example: why small populations drift more (conceptual)

Imagine a population where p = 0.5 and q = 0.5.

• In a very large population, random sampling of gametes each generation tends to average out, so allele frequencies stay close to 0.5.
• In a very small population, just a few random reproductive outcomes (who mates, who survives, how many offspring) can push p to 0.6 or 0.4 in a single generation.

Over many generations, drift in small populations can lead to fixation (an allele reaches frequency 1) or loss (frequency 0), even without selection.

Real-world application: why variation is central to conservation and medicine

• In conservation biology, maintaining genetic variation helps populations adapt to new diseases, climate change, and habitat shifts.
• In pathogens, high genetic variation (via mutation and rapid reproduction) can allow quick evolution of drug resistance. Selection then increases resistant alleles.

A helpful memory aid: Mutation makes new alleles; meiosis mixes them; selection sorts them; drift shuffles them randomly; gene flow shares them between populations.

Exam Focus

• Typical question patterns:
  • Explain the source of variation in a scenario (mutation vs recombination vs gene flow) and connect it to potential evolutionary outcomes.
  • Identify which selection pattern (directional, stabilizing, disruptive) matches a described shift in phenotype distribution.
  • Predict how bottlenecks, founder effects, or inbreeding will affect genetic diversity and allele frequencies.
• Common mistakes:
  • Claiming mutations occur because organisms “need” them (mutations are random with respect to fitness).
  • Confusing gene flow (movement between populations) with genetic drift (random sampling within a population).
  • Treating phenotypic variation as automatically heritable, without considering environmental effects and genetic basis.