Campbell Biology Chapter 23 Review: The Evolution of Populations

23.1 Where genetic variation comes from

pp. 469–473

On the AP exam? Yes

Topics 7.2, 7.4, 5.2 and 6.7 cover why variation matters and where it comes from. Average heterozygosity, nucleotide variability and clines aren't in the current course, so treat them as background.

In the course: Topic 7.2 Natural Selection, Topic 7.4 Population Genetics, Topic 5.2 Meiosis and Genetic Diversity, Topic 5.5 Environmental Effects on Phenotype, Topic 6.7 Mutations, Topic 7.11 Variations in Populations (notes, videos and more questions)

Key points

Individuals don't evolve; populations do. Each organism keeps the genes it was born with, but over generations the share of each allele in the population can change. That change in allele frequencies is microevolution.
Evolution needs variation that can be inherited. Differences caused by diet, exercise or surroundings aren't passed on, so only the genetic part of the variation in a trait matters for evolution.
Some traits come in clear-cut types, often set by one gene. Most vary along a smooth range, like height, because several genes add their effects (and the environment adds more).
Biologists measure variation two ways. Gene variability is the average share of loci where an individual has two different alleles. Nucleotide variability is the share of DNA bases that differ between two individuals. The first is usually bigger, because one changed base anywhere in a gene makes a new allele.
Separate populations of one species often differ genetically. When a trait or allele changes gradually across a region, following something like temperature, it's called a cline.
Brand-new alleles come only from mutation, which happens at random, not because an organism needs it. Most mutations that change a protein are neutral or harmful, and in animals only mutations in cells that make gametes get inherited. A duplicated gene gives evolution a spare copy that can take on a new job.
Bacteria and viruses reproduce so fast that mutations build up variation quickly. In sexually reproducing species, most of the variation in each generation comes from shuffling existing alleles: crossing over, independent assortment and random fertilization.

Key terms (11)

microevolution: Change in how common alleles are in a population over generations. It's evolution on the smallest scale.
genetic variation: Differences in DNA sequence from one member of a population to another. Without it, evolution has nothing to work with.
discrete character: A trait that comes in a few distinct types with nothing in between, such as having or lacking horns. One gene often controls it.
quantitative character: A trait that varies along a continuous range, like body length. It usually depends on several genes plus the environment.
average heterozygosity: The average share of an individual's gene loci that carry two different alleles. A measure of gene-level variation.
nucleotide variability: The average share of DNA bases that differ when you compare two individuals' genomes, base by base.
geographic variation: Genetic differences between separate populations of the same species living in different places.
cline: A gradual change in a trait or allele frequency across a region, often tracking an environmental gradient like temperature or altitude.
mutation: A change in an organism's DNA sequence. It's the only source of brand-new alleles, and it isn't aimed at what the organism needs.
gene duplication: An error that leaves an extra copy of a gene. The spare copy can gather mutations and sometimes ends up with a new function.
somatic cell: Any body cell that doesn't make gametes. In animals, its mutations die with the individual and aren't inherited.

Check yourself: 23.1 Where genetic variation comes from

4 questions on 23.1 Where genetic variation comes from. Pick an answer to see if you got it, and why.

Question 1 of 4

Researchers follow a population of leaf beetles for 12 generations. Which observation is the clearest evidence that the population has evolved?

Question 2 of 4

A gardener roots cuttings taken from one shrub, so all the new plants are genetically identical. Plants set out in full sun grow small, thick leaves, and plants in shade grow large, thin leaves. She takes new cuttings only from the sun-grown plants with the smallest leaves and plants all of them in shade. What will the new plants' leaves most likely be like?

Question 3 of 4

In a lizard species, two individuals chosen at random differ at about 0.4% of their DNA bases. Yet a typical lizard is heterozygous at about 15% of its gene loci. What best explains why the gene-level number is so much larger?

Question 4 of 4

Long ago in a group of leaf-eating beetles, a gene for a digestive enzyme was accidentally copied, so the beetles had two versions. Today one copy still makes the digestive enzyme, and the other makes a protein that breaks down a toxin in the beetles' food plants. Which explanation is most likely?

0 of 4 answered

23.2 Hardy–Weinberg: the no-evolution baseline

pp. 473–476

On the AP exam? Yes

Topic 7.5 covers this, and both equations are on your formula sheet. Expect to work out allele and genotype frequencies and to use the model as a null hypothesis.

In the course: Topic 7.5 Hardy–Weinberg Equilibrium, Topic 7.4 Population Genetics (notes, videos and more questions)

Key points

A population means members of one species that share an area and can breed with one another. Its gene pool is the whole stock of alleles its members hold, gene by gene. When every copy of a gene in the population is the same allele, that allele is fixed.
To find an allele's frequency, count its copies and divide by all copies of the gene. Diploid organisms carry two copies each. With two alleles, p + q = 1.
The Hardy–Weinberg model says that in a population that isn't evolving, allele and genotype frequencies stay the same in every generation, with genotypes at p², 2pq and q². Meiosis and fertilization reshuffle alleles but don't change how common they are.
The model needs five conditions: a very large population, no migration, no new mutations, random mating and no natural selection. Real populations never meet all five perfectly, so the model works as a null hypothesis: big gaps between real data and its predictions tell you something is changing the population.
For a recessive trait you can't tell carriers from homozygous dominant individuals, so take the share showing the trait as q², find q with a square root, get p = 1 − q, and calculate carriers as 2pq. This only works if you assume equilibrium.
Genotype frequencies always add up to 1, whether or not a population is in equilibrium. It's in equilibrium only if its actual genotype shares match p², 2pq and q² worked out from its own allele frequencies.
One population may be changing at some genes while sitting near equilibrium at others. When a harmful recessive allele is rare, most of its copies hide in healthy carriers.

Key terms (10)

population: Members of one species in one place that can mate with each other, so they share a gene pool.
gene pool: The combined set of alleles for every gene in a population: each copy, from each member, at one point in time.
fixed allele: An allele that's the only one left at its locus in a population, so every individual is homozygous for it.
allele frequency: The share of all copies of a gene in a population that are one particular allele. With two alleles, p + q = 1.
genotype frequency: The share of individuals in a population with a particular genotype, such as Aa.
Hardy–Weinberg equilibrium: The state of a population whose allele and genotype frequencies hold steady generation after generation.
Hardy–Weinberg equations: p + q = 1 for allele frequencies and p² + 2pq + q² = 1 for genotype frequencies in a non-evolving population.
null hypothesis: The "nothing is happening" prediction you test data against. In population genetics, it's usually that the population is at Hardy–Weinberg equilibrium.
random mating: Mating in which any individual is equally likely to pair with any other, with no preference based on genotype or relatedness.
carrier: A heterozygous individual that has one copy of a recessive allele but doesn't show the recessive trait.

Check yourself: 23.2 Hardy–Weinberg: the no-evolution baseline

4 questions on 23.2 Hardy–Weinberg: the no-evolution baseline. Pick an answer to see if you got it, and why.

Question 1 of 4Calculator allowed

A population of 250 land snails is genotyped at a shell-banding locus: 90 are BB, 100 are Bb and 60 are bb. What is the frequency of allele b?

Question 2 of 4Calculator allowed

Albinism in a cave salamander is caused by a recessive allele. In a survey of 2,500 salamanders, 4 are albino. Assuming Hardy–Weinberg equilibrium, about what percentage of the salamanders are carriers (heterozygous)?

Question 3 of 4Calculator allowed

Researchers genotype 400 plants in a population of a meadow clover at one locus (invented data). Genotype | Number of plants AA | 160 Aa | 120 aa | 120 Which statement best describes these data?

Question 4 of 4

In a population of field crickets, an allele for dark wings (D) is dominant to an allele for pale wings (d), and D has a frequency of 0.2. A student predicts that because D is dominant, it will become more common every generation. If the population meets all five Hardy–Weinberg conditions, which response is correct?

0 of 4 answered

23.3 Selection, drift and gene flow

pp. 476–480

On the AP exam? Yes

Topic 7.4 names genetic drift, the bottleneck and founder effects, gene flow and mutation, and Topic 7.11 asks why losing variation puts small populations at risk. You won't need any of the book's case-study details.

In the course: Topic 7.4 Population Genetics, Topic 7.2 Natural Selection, Topic 7.5 Hardy–Weinberg Equilibrium, Topic 7.11 Variations in Populations (notes, videos and more questions)

Key points

Breaking any Hardy–Weinberg condition can cause evolution, but not equally. New mutations shift allele frequencies only slightly in each generation. Nonrandom mating, like inbreeding, changes genotype frequencies (more homozygotes) without changing allele frequencies. Selection, drift and gene flow change allele frequencies directly.
In natural selection, alleles that help individuals survive and reproduce in their current environment end up in more of the next generation, so they become more common. As long as the environment stays the same, the push goes the same way.
Genetic drift is random change in allele frequencies caused by chance: which individuals happen to survive and breed, and which gametes happen to meet. Its direction can't be predicted, so a frequency may climb for a while and then slide back, and its effects are biggest in small populations.
In the founder effect, a few individuals start a new population, so its allele frequencies reflect whatever the founders carried. In the bottleneck effect, a disaster leaves a few random survivors. A population that bounces back in numbers can still be short on variation for many generations, because lost alleles don't come back on their own.
Drift usually lowers genetic variation, because alleles can be lost or fixed purely by chance. When numbers are tiny, chance can even carry a mildly harmful allele all the way to fixation. Low variation leaves a population less able to cope with new diseases or a changing environment.
Gene flow is the movement of alleles between populations when individuals migrate or gametes travel, like pollen on the wind. It makes populations more alike and can keep them from splitting apart. It can bring in useful alleles, or flood a well-adapted population with poorly suited ones.
Conservation managers sometimes move individuals between populations on purpose, using gene flow to bring lost alleles back into a small, inbred population.

Key terms (10)

natural selection: Individuals with inherited traits that suit their environment survive and reproduce more, so the alleles behind those traits become more common.
genetic drift: Random, unpredictable shifts in allele frequencies over generations. It's strongest in small populations.
founder effect: Drift that happens when a few individuals start a new population, so its gene pool reflects only what those founders carried.
bottleneck effect: Drift that happens when a population is suddenly cut to a few survivors. Which alleles survive is mostly luck.
gene flow: Alleles carried from one population to another when individuals or their gametes move. It makes populations more alike.
migration: Movement of individuals from one population to another. When migrants breed, it causes gene flow.
nonrandom mating: Choosing mates based on genotype, traits or relatedness. It changes genotype frequencies but not, by itself, allele frequencies.
inbreeding: Mating between close relatives, including plants fertilizing themselves. It raises the share of homozygotes in a population.
loss of genetic variation: A fall in how many different alleles a population carries, often after drift in a small population. It makes adapting to change harder.
genetic rescue: Bringing individuals into a small, inbred population on purpose so their alleles restore lost variation.

Check yourself: 23.3 Selection, drift and gene flow

4 questions on 23.3 Selection, drift and gene flow. Pick an answer to see if you got it, and why.

Question 1 of 4

To protect a rare frog, a zoo starts a breeding colony with six frogs collected from a large wild population. After ten generations of breeding only within the zoo, a skin-spot allele with a frequency of 0.02 in the wild has a frequency of 0.35 in the colony. The spot pattern has no effect on survival or mating in the zoo. What best explains the difference?

Question 2 of 4

Researchers set up 40 lab populations of flour beetles. In every one, a color-marker allele that doesn't affect survival or reproduction starts at a frequency of 0.5. Half the populations are kept at 10 breeding adults each generation, and half at 500. After 30 generations they record the result (invented data). Breeding adults | Populations | Allele lost | Allele fixed | Both alleles still present 10 | 20 | 7 | 6 | 7 500 | 20 | 0 | 0 | 20 Which conclusion is best supported?

Question 3 of 4

A wind-pollinated grass grows on soil polluted by an old copper mine and in a clean meadow right next to it. An allele for copper tolerance has a frequency of 0.80 on the mine and 0.05 in the meadow, and tolerant plants grow more slowly than others on clean soil. Strong winds often carry meadow pollen onto the mine site. What is the most likely effect of this pollen?

Question 4 of 4

A population of a flowering herb loses its insect pollinators and switches to mostly self-pollination. Assume there is no selection, mutation, gene flow or drift at a flower-color locus. Over several generations, what will most likely happen at that locus?

0 of 4 answered

23.4 How selection builds adaptations

pp. 480–485

On the AP exam? Yes

Topics 7.1 and 7.2 cover fitness and selection, and Topic 8.7 names heterozygote advantage. Directional, stabilizing and disruptive selection, sexual selection and frequency-dependent selection aren't named in the current course, so treat them as background that helps you read graphs of trait changes.

In the course: Topic 7.1 Introduction to Natural Selection, Topic 7.2 Natural Selection, Topic 8.7 Disruptions in Ecosystems, Topic 7.11 Variations in Populations (notes, videos and more questions)

Key points

Natural selection has a random step and a non-random step. Mutation and recombination make variation at random, but selection keeps favoring whatever works best in the current environment, so its result isn't random. It's the only mechanism that steadily makes organisms better suited to where they live.
Relative fitness is how much an individual adds to the next generation's gene pool compared with others. Survival only counts because it leads to reproduction: an individual that lives a long time but leaves no offspring has zero fitness.
Selection acts on phenotypes, the traits you can see or measure. It affects genotypes only through the traits they produce.
For a trait that varies along a range, selection can take three shapes. Directional selection favors one extreme and shifts the average. Disruptive selection favors both extremes and can split the population. Stabilizing selection favors the middle and trims away the extremes.
Sexual selection favors traits that help in getting mates, either by winning contests within a sex (intrasexual) or by being chosen by the other sex (intersexual, or mate choice). It can produce big differences between males and females and showy traits that cost survival but raise mating success.
Variation survives selection in several ways. Some differences are neutral. Diploidy hides recessive alleles in heterozygotes. Balancing selection keeps more than one allele: in heterozygote advantage heterozygotes have the highest fitness (the classic human case is the sickle-cell allele where malaria is common), and in frequency-dependent selection whichever type is rarer does better.
Selection can't build perfect organisms. It can only work with variation that exists, it remodels old structures instead of starting over, every adaptation is a trade-off, and chance and changing environments get in the way. An allele that helps in one environment can hurt in another.

Key terms (13)

relative fitness: How many offspring an individual adds to the next generation compared with others in its population.
adaptive evolution: Evolution that makes a population better suited to its environment. Only natural selection produces it consistently.
directional selection: Selection that favors one end of a trait's range, so the population's average shifts that way.
disruptive selection: Selection that favors both ends of a trait's range over the middle, which can split a population into two groups.
stabilizing selection: Selection that favors the middle of a trait's range and removes the extremes, narrowing variation.
sexual selection: Selection for traits that help an individual get mates, even if those traits don't help it survive.
sexual dimorphism: Clear differences between the males and females of a species in features like body size, coloring or courtship displays.
intrasexual selection: Competition among members of the same sex, often males, for access to mates.
intersexual selection: Mate choice: members of one sex, often females, choose which members of the other sex to mate with.
neutral variation: Differences in DNA that don't help or hurt survival or reproduction.
balancing selection: Selection that keeps two or more forms of a gene or trait in a population instead of letting one win out.
heterozygote advantage: When individuals with two different alleles at a locus out-survive or out-reproduce both kinds of homozygote, so selection keeps both alleles around.
frequency-dependent selection: Selection in which a type's fitness rises or falls with how common it is, often favoring whichever type is rarer.

Check yourself: 23.4 How selection builds adaptations

4 questions on 23.4 How selection builds adaptations. Pick an answer to see if you got it, and why.

Question 1 of 4

A field study follows four female lizards in one population for their whole lives (invented data). Lizard | Lifespan (years) | Body mass (g) | Offspring that survived to breed W | 9 | 42 | 3 X | 4 | 30 | 11 Y | 6 | 55 | 7 Z | 12 | 61 | 0 Which lizard has the highest relative fitness, and why?

Question 2 of 4

In a meadow plant, flowering date varies and is heritable. Early-flowering plants are pollinated by bees before a midsummer drought, and late-flowering plants are pollinated by moths after the rains return. Plants that flower during the drought get few pollinator visits and set little seed. If this pattern lasts for many generations, what is most likely?

Question 3 of 4

The larvae of a small insect develop inside swellings, called galls, that they cause on plant stems. Gall size is heritable. Researchers measure how many larvae survive in galls of different sizes (invented data). Gall diameter (mm) | Larvae surviving (%) 14 | 22 18 | 48 22 | 61 26 | 45 30 | 19 If these survival rates hold for many generations, which change in gall size is most likely?

Question 4 of 4

In a species of dung beetle, males with long horns win fights over the tunnels where females lay their eggs. In a species of anole lizard, females watch males flash colorful throat fans and mate more often with the males whose fans are brightest. Which statement describes these cases correctly?

0 of 4 answered