lecture11 (1)

BIOL 3306 - Evolutionary Biology

Natural Selection IV

Natural Selection

Change in Allele Frequency

• Change in allele frequency from one generation to the next:

  • p = p0 + p

  • p = 0: allele frequency remains constant (equilibrium)

  • p > 0: allele frequency increases

  • p < 0: allele frequency decreases

Viability Selection Model

• Factors involved:

  • Zygotes

  • Viability Selection

  • Adults

  • Gametes

  • Parents

Case Study: Industrial Melanism

Overview of Biston Betularia

• Species: Peppered Moth

• Variants: typica and carbonaria

  • Kettlewell Study (1973)

  • Influence of pollution from the Industrial Revolution

Mark-Recapture Experiment Results

• Birmingham (city/industrial):

  • carbonaria: 447 | 27.5% recaptured

  • typica: 137 | 13.1% recaptured

• Dorset (countryside/nonindustrial):

  • carbonaria: 473 | 6.3% recaptured

  • typica: 496 | 12.5% recaptured

Reversal of Melanism

• Impact of Clean Air Acts on moth populations

• Analysis by C. A. Clarke

  • Sampling from 1959 to 2002

Equilibria

Types of Natural Selection

1. No Selection:

• Homogeneous fitness: w11 = w12 = w22

2. Directional Selection:

• Beneficial Allele: A1 advantageous

3. Heterozygote Advantage:

• Balanced: p = q = 0.5

Heterozygote Advantage and Disadvantage

Examples of Heterozygote Advantage

• Sickle-Cell Anemia:

  • Genotypes:

    • AA: No anemia, susceptible to malaria

    • AS: Mild anemia, resistant to malaria

    • SS: Severe anemia, resistant to malaria

Heterozygote Disadvantage (Underdominance)

• Unstable equilibrium:

  • Fitness: A1A1, A1A2, A2A2

Frequency-Dependent Selection

Definitions

1. Positive Selection:

• Fitness increases with genotype frequency

2. Negative Selection:

• Fitness decreases with genotype frequency

Causes of Negative Frequency-Dependent Selection

• Predation:

  • Example: Scale-eating cichlid, Perissodus microlepis

• Resource Partitioning:

  • Competition among species for different resources

Mutation-Selection Balance

Context of Mutations

• Deleterious Alleles:

• The balance of mutation rates versus selection persistence

• Examples:

  • Spinal Muscular Atrophy: recessive mutations in telSMN

  • Cystic Fibrosis: resistance to typhoid fever

Summary of Key Concepts

Types of Selection

• Directional Selection:

• Balancing Selection: includes heterozygote advantage and negative frequency-dependent selection

• Imperfect Adaptation: occurs through varying fitness levels due to environmental changes

Next Lecture

• Topic: Genetic Drift and Molecular Evolution

• Source: BD Chapter 8

BIOL 3306 - Evolutionary Biology

Natural Selection IV

Natural selection is a fundamental concept in evolutionary biology that explains the change in allele frequency from one generation to the next. In this context, the allele frequency can be calculated using the formula p = p0 + p, where p = 0 indicates that the allele frequency remains constant (equilibrium). If p > 0, the allele frequency increases, and if p < 0, it decreases.

Viability Selection Model

Several factors are involved in the viability selection model, which includes zygotes, viability selection, adults, gametes, and parents. An important case study in natural selection is the phenomenon of industrial melanism, particularly illustrated by the species Biston betularia, commonly known as the peppered moth. This species exhibits two main variants: typica and carbonaria. The Kettlewell Study in 1973 explored the influence of pollution from the Industrial Revolution on these moth populations through a mark-recapture experiment. Results showed that in Birmingham (a city with industrial activity), the carbonaria variant had 447 individuals with a recapture rate of 27.5%, while the typica variant had 137 individuals with a 13.1% recapture rate. Conversely, in the countryside of Dorset, carbonaria had a recapture rate of 6.3% from 473 individuals, while typica had a rate of 12.5% from 496 individuals. The impact of the Clean Air Acts on moth populations was analyzed by C. A. Clarke, with sampling conducted from 1959 to 2002.

Equilibria and Types of Natural Selection

Natural selection operates through various equilibria. There are different types of natural selection, including no selection where there is homogeneous fitness (w11 = w12 = w22), directional selection where a beneficial allele (A1) is advantageous, and heterozygote advantage characterized by balanced frequencies (p = q = 0.5). Heterozygote advantage can be exemplified by sickle-cell anemia, where genotypes AA are free of anemia but susceptible to malaria, AS individuals have mild anemia yet are resistant to malaria, while SS individuals experience severe anemia but are also resistant to malaria. Additionally, underdominance in heterozygote disadvantage can lead to unstable equilibrium.

Frequency-Dependent Selection

Frequency-dependent selection can be categorized into positive selection, where fitness increases with genotype frequency, and negative selection, where fitness decreases as the genotype frequency rises. Causes of negative frequency-dependent selection include predation and resource partitioning; for example, the scale-eating cichlid, Perissodus microlepis, demonstrates predation patterns that influence species dynamics.

Mutation-Selection Balance

In the context of mutations, deleterious alleles create a balance between mutation rates and the persistence of selection. Examples of this include spinal muscular atrophy, which arises from recessive mutations in the telomeric SMN (telSMN), and cystic fibrosis, which has been found to provide resistance to typhoid fever.

Summary of Key Concepts

In summary, the types of selection encompass directional selection and balancing selection, which includes heterozygote advantage and negative frequency-dependent selection, leading to imperfect adaptation in response to varying fitness levels due to environmental changes.

Next Lecture

The next lecture will cover the topic of genetic drift and molecular evolution and will reference BD Chapter 8.