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Variations in
Populations

Explore how genetic variation fuels natural selection and drives the evolution of populations over time.

AP Biology

What Is Genetic Variation?

Genetic variation refers to the differences in DNA sequences among individuals within a population. These differences arise in alleles: the alternative forms of a gene that occupy the same locus on homologous chromosomes.

Genotype
The set of alleles an organism carries at a given locus
Phenotype
The observable trait produced by genotype and environment together

Sources of Genetic Variation

1
Mutation: Random changes in DNA sequence. The ultimate source of all new alleles in a population.
2
Sexual reproduction: Independent assortment and crossing over during meiosis shuffle existing alleles into new combinations.
3
Gene flow: Migration of individuals between populations introduces new alleles to the gene pool.
4
Horizontal gene transfer: In prokaryotes, transfer of DNA via transformation, transduction, or conjugation.

Phenotypic Variation: Bell Curve

Many traits in a population follow a normal distribution. Most individuals cluster around the mean, with fewer at the extremes. This is the starting point for understanding how selection shifts trait distributions.

Phenotype (e.g., body size) Frequency Mean -1 SD +1 SD

The Hardy-Weinberg Principle

The Hardy-Weinberg principle describes a non-evolving population: one where allele and genotype frequencies remain constant across generations. It serves as the null hypothesis for detecting evolution.

\( p^2 \)
Homozygous dominant frequency
\( 2pq \)
Heterozygous frequency
\( q^2 \)
Homozygous recessive frequency

Five Conditions for Equilibrium

A population is in Hardy-Weinberg equilibrium only if all five conditions are met. Violating any condition means the population is evolving.

1
No mutations: No new alleles are introduced or existing alleles altered.
2
Random mating: Individuals pair by chance, not by phenotype or genotype.
3
No natural selection: All genotypes have equal fitness and survival rates.
4
Extremely large population: No genetic drift (random changes in allele frequency).
5
No gene flow: No migration of individuals into or out of the population.

Discrete vs. Continuous Variation

Discrete Continuous
Definition Distinct, non-overlapping categories Range of values along a spectrum
Gene control Usually one or few genes Polygenic (many genes)
Environment effect Minimal Significant
Distribution Bar graph (distinct bars) Bell curve (smooth)
Example Blood type (A, B, AB, O) Human height, skin color

Three Modes of Natural Selection

Directional
Favors one extreme phenotype. The distribution shifts left or right.
Stabilizing
Favors the average phenotype. The distribution becomes narrower.
Disruptive
Favors both extremes. The distribution splits into two peaks.
Directional Stabilizing Disruptive

How to Apply Hardy-Weinberg

1
Identify the known value. You are usually given the frequency of the homozygous recessive phenotype, which equals \( q^2 \).
2
Solve for \( q \). Take the square root: \( q = \sqrt{q^2} \).
3
Solve for \( p \). Since \( p + q = 1 \), then \( p = 1 - q \).
4
Calculate genotype frequencies. Use \( p^2 \), \( 2pq \), and \( q^2 \) to find each genotype's proportion.
5
Multiply by population size to find the expected number of individuals per genotype.

Example 1: Cystic Fibrosis

In a population of 10,000, 4% are homozygous recessive and express cystic fibrosis. Find the allele and genotype frequencies.

Example 2: Detecting Evolution

In 2000, the frequency of allele A in a beetle population was \( p = 0.6 \). In 2020, it was \( p = 0.8 \). Is this population evolving?

2000 Frequencies
\( p = 0.6 \), \( q = 0.4 \)
\( p^2 = 0.36 \), \( 2pq = 0.48 \), \( q^2 = 0.16 \)
2020 Frequencies
\( p = 0.8 \), \( q = 0.2 \)
\( p^2 = 0.64 \), \( 2pq = 0.32 \), \( q^2 = 0.04 \)

Example 3: Genetic Drift in Small Populations

A population of 20 lizards on an island has an allele frequency of \( p = 0.5 \). A hurricane kills 15 randomly. The 5 survivors happen to have \( p = 0.8 \).

Before (n = 20) p = 0.50 Bottleneck After (n = 5) p = 0.80 = Allele A (dominant) = Allele a (recessive) This is the bottleneck effect: a form of genetic drift

The Founder Effect

When a small group colonizes a new area, the allele frequencies in the new population may differ drastically from the original. This is another form of genetic drift.

Amish communities
Founded by a small group of settlers. High frequency of Ellis-van Creveld syndrome due to the founder effect.
Island colonization
A few birds blown to an island carry only a subset of the mainland alleles. The new population diverges.
Ten genotypes and founder sample
Ten genotypes and founder sample
A beetle habitat accompanies a hypothetical source-to-founder comparison. An authored companion lists ten genotypes and highlights founders 1 AA and 4 Aa.
A founder event can change allele frequencies through sampling without a fitness advantage. In this synthetic example, 10 source diploids carry 10 A and 10 a copies; founders 1 and 4 carry 3 A and 1 a. The habitat image is contextual, not a measured sample.

Sexual Selection and Variation

Sexual selection is a form of natural selection in which individuals with certain heritable traits are more likely to obtain mates. It maintains or increases variation in certain traits.

Intersexual
Mate choice: one sex (usually females) selects mates based on traits like bright plumage or elaborate displays.
Intrasexual
Competition: members of one sex (usually males) compete directly for access to mates, favoring larger size or weapons.

Common Mistakes

✕ Confusing \( q \) with \( q^2 \)
Remember: \( q^2 \) is the frequency of the homozygous recessive phenotype. You must take the square root to find \( q \), the allele frequency.
✕ Drift = Selection
Genetic drift is random and does not favor beneficial alleles. Natural selection is non-random and favors advantageous traits. Do not conflate them.
✕ Ignoring carriers
Heterozygous individuals (\( 2pq \)) carry the recessive allele but show the dominant phenotype. They are a hidden reservoir of genetic variation.

Summary: Forces of Evolution

Force Effect on Variation Random?
Mutation Increases Yes
Gene flow Increases (if new alleles) No
Genetic drift Decreases Yes
Natural selection Varies by mode No
Non-random mating Changes genotype frequencies No

Question 1 of 5

In a population in Hardy-Weinberg equilibrium, the frequency of the homozygous recessive genotype is 0.09. What is the frequency of the heterozygous genotype?

Question 2 of 5

Which of the following is the ultimate source of all new genetic variation in a population?

Question 3 of 5

A population of birds shows a bimodal distribution for beak size, with birds having either very large or very small beaks surviving best. Which type of selection is operating?

Question 4 of 5

A hurricane destroys 90% of a mouse population at random. The surviving 10% have different allele frequencies than the original population. This is an example of:

Question 5 of 5

Which of the following conditions, if violated, would cause a population to deviate from Hardy-Weinberg equilibrium?