Chapter 21: The Evolution of Populations – Study Notes

Chapter 21: The Evolution of Populations

Learning Objectives

• Define key terms: gene pool, population, species.

• Explain the relationship between alleles, genotypes, phenotypes, and fitness.

• Describe how mutation is the ultimate source of genetic variation and its necessity for evolution.

• Identify conditions that cause allele and genotype frequencies to change in a population.

• Compare and contrast natural selection and sexual selection.

• Describe limitations of natural selection that restrict evolution.

Overview: The Smallest Unit of Evolution

Populations, Not Individuals, Evolve

Evolution occurs at the population level, not in individual organisms. Natural selection acts on individuals, but only populations can evolve over generations.

• Example: Medium ground finches on Daphne Major Island showed increased average beak depth after a drought, as birds with larger beaks survived and reproduced more successfully.

• Microevolution is defined as a change in allele frequencies in a population over generations.

• Three main mechanisms cause allele frequency change:

  • Natural selection

  • Genetic drift

  • Gene flow

• Only natural selection consistently leads to adaptive evolution.

21.1 Genetic Variation Makes Evolution Possible

Genetic Variation

Genetic variation refers to differences among individuals in the composition of their genes or other DNA segments. It is essential for evolution, as it provides the raw material for natural selection.

• Phenotypic variation is observable in traits such as flower color or horse coat color.

• Molecular variation includes differences in DNA sequences, such as base-pair substitutions, insertions, or deletions.

• Some variation is visible (e.g., color), while other variation is molecular and not directly observable.

Sources of Genetic Variation

Genetic variation arises from several sources:

• Mutation: Changes in the DNA sequence that create new alleles.

• Recombination: Shuffling of alleles during sexual reproduction, producing new genetic combinations.

• Gene flow: Movement of alleles between populations.

Without genetic variation, evolution cannot occur.

21.2 The Hardy-Weinberg Equation: Testing for Evolution

Gene Pools and Allele Frequencies

The gene pool is the total collection of genes in a population at any one time. Allele frequencies describe how common an allele is in the population.

• For diploid organisms, the total number of alleles at a locus is twice the number of individuals.

• Allele frequencies are represented by p (dominant allele) and q (recessive allele), with the sum always equal to 1: $p + q = 1$.

Calculating Allele Frequencies

To calculate allele frequencies, count the number of each allele and divide by the total number of alleles.

• Example: In a population of wildflowers:

  • 320 red flowers ($C^RC^R$), 160 pink flowers ($C^RC^W$), 20 white flowers ($C^WC^W$)

  • Number of $C^R$ alleles: $(320 \times 2) + 160 = 800$

  • Number of $C^W$ alleles: $(20 \times 2) + 160 = 200$

  • Total alleles: $800 + 200 = 1000$

  • Frequency of $C^R$: $p = \frac{800}{1000} = 0.8$

  • Frequency of $C^W$: $q = 1 - p = 0.2$

The Hardy-Weinberg Equation

The Hardy-Weinberg equation predicts genotype frequencies in a non-evolving population:

• $p^2$ = frequency of homozygous dominant genotype

• $2pq$ = frequency of heterozygous genotype

• $q^2$ = frequency of homozygous recessive genotype

Equation: $p^2 + 2pq + q^2 = 1$

Hardy-Weinberg Equilibrium

A population is in Hardy-Weinberg equilibrium if allele and genotype frequencies remain constant from generation to generation, provided certain conditions are met.

• Genotype frequencies can be calculated using the rule of multiplication:

  • $C^RC^R = p^2 = (0.8)^2 = 0.64$

  • $C^RC^W = 2pq = 2(0.8)(0.2) = 0.32$

  • $C^WC^W = q^2 = (0.2)^2 = 0.04$

Conditions for Hardy-Weinberg Equilibrium

Five conditions must be met for Hardy-Weinberg equilibrium:

Condition	Consequence if Condition Does Not Hold
No mutations	Gene pool is modified if mutations occur or if genes are deleted/duplicated.
Random mating	Non-random mating changes genotype frequencies.
No natural selection	Allele frequencies change if individuals with different genotypes have different reproductive success.
Extremely large population size	Small populations experience genetic drift, causing allele frequencies to fluctuate.
No gene flow	Movement of alleles into or out of populations alters allele frequencies.

21.3 Natural Selection, Genetic Drift, and Gene Flow

Genetic Drift

Genetic drift is a random change in allele frequencies, especially significant in small populations.

• Can cause allele frequencies to change at random.

• May lead to loss of genetic variation within populations.

• Can cause harmful alleles to become fixed.

The Founder Effect

Occurs when a few individuals start a new population, leading to reduced genetic variation and different allele frequencies compared to the original population.

The Bottleneck Effect

Occurs when a population is drastically reduced in size by a sudden event, resulting in a loss of genetic diversity.

Gene Flow

Gene flow is the movement of alleles between populations due to migration of individuals or gametes.

• Can increase genetic variation within a population.

• Can reduce differences between populations.

21.4 Only Natural Selection Consistently Causes Adaptive Evolution

Modes of Selection

• Directional selection: Favors one extreme phenotype, shifting the population mean.

• Disruptive selection: Favors both extreme phenotypes over intermediate forms.

• Stabilizing selection: Favors intermediate phenotypes, reducing variation.

Balancing Selection

Maintains genetic diversity in a population by favoring heterozygotes or multiple alleles.

• Example: Sickle-cell allele in regions with malaria; heterozygotes have a survival advantage.

Frequency-Dependent Selection

The fitness of a phenotype depends on its frequency in the population.

• Example: Left- and right-mouthed fish; the rare phenotype has a selective advantage.

Sexual Selection

Sexual selection is a form of natural selection where individuals with certain traits are more likely to obtain mates.

• Leads to sexual dimorphism (differences in appearance between sexes).

• Examples: Peacocks, birds of paradise.

Limitations of Natural Selection

Natural selection cannot produce perfect organisms due to:

• Historical constraints

• Compromises between traits

• Chance events

• Available genetic variation

Summary Table: Effects of Genetic Drift

Effect	Description
Significance in small populations	Genetic drift has a greater impact when population size is small.
Random changes	Allele frequencies can change unpredictably from one generation to the next.
Loss of variation	Genetic drift can reduce genetic diversity within populations.
Fixation of harmful alleles	Harmful alleles may become fixed by chance.

Key Terms

• Gene pool: All the alleles for all the genes in a population.

• Allele frequency: Proportion of a specific allele among all alleles at a genetic locus in a population.

• Genotype: Genetic makeup of an organism.

• Phenotype: Observable traits of an organism.

• Fitness: Ability of an organism to survive and reproduce.

• Mutation: Change in DNA sequence.

• Genetic drift: Random change in allele frequencies.

• Gene flow: Movement of alleles between populations.

• Natural selection: Differential survival and reproduction of individuals due to differences in phenotype.

• Sexual selection: Selection for traits that increase mating success.

• Hardy-Weinberg equilibrium: Condition in which allele and genotype frequencies remain constant.

Example Application: The sickle-cell allele is maintained in populations where malaria is common due to heterozygote advantage, illustrating balancing selection.

Additional info: These notes expand on the provided slides with definitions, examples, and equations for clarity and completeness.