AP Biology 7.3-7.4: Population Genetics

Population genetics

The study of how allele and genotype frequencies are distributed within populations and how evolutionary forces change those frequencies over time.

How does population genetics define evolution?

Evolution is a change in the frequency of alleles in a population over generations. Changes in allele frequencies can produce changes in the population’s genetic structure and phenotype distribution.

Allele frequency

The proportion of all copies of a gene in a population that are a particular allele: $\text{allele frequency} = \frac{\text{copies of the allele}}{\text{total copies of the gene}}$.

Gene pool

The complete collection of alleles present in a population at a particular time.

How can natural selection change a population’s allele frequencies?

If an inherited allele produces a phenotype that improves survival or reproductive success, individuals carrying it tend to leave more offspring. The allele can therefore become more common over generations.

Fixed allele

An allele is fixed when every individual in a population carries it, so its frequency is $1$ or $100\%$.

Genetic drift

A change in allele frequencies caused by random chance rather than by an allele’s effect on survival or reproduction. Drift is strongest in small populations.

Why does genetic drift have a larger effect in small populations?

Random loss or reproduction of even a few individuals represents a large fraction of a small population’s gene pool. In a large population, the same chance event generally changes allele frequencies less.

Founder effect

A form of genetic drift that occurs when a small group establishes a new population. Because the founders may not represent the original population’s allele frequencies, the new population can have unusual allele frequencies and reduced genetic diversity.

Bottleneck effect

A form of genetic drift caused by a sudden, usually random reduction in population size. The survivors’ alleles become the gene pool of the next population, often reducing genetic variability and changing allele frequencies.

How do the founder effect and bottleneck effect differ?

The founder effect begins when a small group leaves or becomes isolated to start a population. The bottleneck effect begins when a catastrophe or other event sharply reduces an existing population.

Polymorphism

The presence of two or more forms of a characteristic within a population. These forms usually reflect different alleles or phenotypes.

Genetic variability

The diversity of alleles and genotypes in a population. Greater genetic variability provides more heritable variation on which evolutionary forces can act.

Heritability

The fraction of variation in a population’s phenotypes that is attributable to genetic differences among individuals. A trait must have a heritable component for natural selection to change its frequency across generations.

Why can natural selection act on inherited muscle-building ability but not usually on muscles gained through exercise?

An inherited genetic basis for muscle development can be transmitted to offspring, whereas muscles acquired through exercise generally do not alter the offspring’s DNA. Natural selection acts on heritable variation, not on most acquired traits.

Inbreeding depression

The reduction in health or reproductive success that can result when closely related individuals mate. Inbreeding increases homozygosity, making harmful recessive alleles more likely to occur in homozygous form.

Gene flow

The movement of alleles into or out of a population through migration of individuals or movement of gametes, such as pollen. It can introduce new alleles and reduce genetic differences between populations.

Mutation as an evolutionary force

Mutation changes DNA and is the ultimate source of new alleles. Mutations may be harmful, beneficial, or neutral; their effects on population frequency depend partly on natural selection and drift.

Nonrandom mating

A mating pattern in which individuals do not choose partners randomly. Mate choice, physical proximity, or preference for particular phenotypes can alter genotype frequencies and influence evolutionary change.

Assortative mating

A form of nonrandom mating in which individuals preferentially mate with partners that have similar phenotypes.

What is the Hardy–Weinberg principle used for?

It provides a mathematical model of a non-evolving population. Comparing observed allele or genotype frequencies with Hardy–Weinberg predictions allows scientists to infer whether evolutionary forces may be acting.

What does it mean for a population to be in Hardy–Weinberg equilibrium?

Allele and genotype frequencies remain constant from generation to generation. The population is not undergoing evolutionary change at the locus being analyzed.

What conditions are required for Hardy–Weinberg equilibrium?

The model assumes a very large population, random mating, no mutation, no migration or emigration, and no natural selection favoring or opposing any genotype. Real populations rarely meet all conditions exactly.

For a gene with two alleles, what relationship must allele frequencies satisfy under Hardy–Weinberg analysis?

If $p$ and $q$ are the frequencies of the two alleles, then $p+q=1$. This means the two allele frequencies account for all copies of that gene in the population.

What are the Hardy–Weinberg genotype-frequency equations for two alleles?

The expected homozygous dominant, heterozygous, and homozygous recessive frequencies are $p^2$, $2pq$, and $q^2$, respectively. They satisfy $p^2+2pq+q^2=1$.

Why is the heterozygous genotype frequency represented by $2pq$?

A heterozygote can form when the first allele is chosen from one frequency category and the second from the other in either order: dominant then recessive or recessive then dominant. Thus the probability is $pq+qp=2pq$.

A population has $p=0.8$ for a dominant allele and $q=0.2$ for its recessive allele. What genotype frequencies are predicted by Hardy–Weinberg equilibrium?

$p^2=0.64$ homozygous dominant, $2pq=0.32$ heterozygous, and $q^2=0.04$ homozygous recessive.

In a Hardy–Weinberg population of 500 organisms with $p=0.8$ and $q=0.2$, how many individuals of each genotype are expected?

Expected counts are $500(0.64)=320$ homozygous dominant, $500(0.32)=160$ heterozygous, and $500(0.04)=20$ homozygous recessive.

How are phenotype frequencies inferred from Hardy–Weinberg genotype frequencies when the dominant allele determines the phenotype?

The dominant phenotype includes both homozygous dominant and heterozygous individuals, so its frequency is $p^2+2pq$. The recessive phenotype appears only in homozygous recessive individuals, with frequency $q^2$.

Why can observing a recessive phenotype reveal genotype but observing a dominant phenotype usually cannot?

A recessive phenotype requires two recessive alleles, so its genotype is known to be homozygous recessive. A dominant phenotype can result from either a homozygous dominant or heterozygous genotype.

How can a recessive phenotype frequency be used to calculate allele frequencies under Hardy–Weinberg equilibrium?

The recessive phenotype frequency equals $q^2$. Calculate $q$ by taking its square root, then calculate $p$ using $p=1-q$. The expected genotype frequencies are then $p^2$, $2pq$, and $q^2$.