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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
No Selection:
Homogeneous fitness: w11 = w12 = w22
Directional Selection:
Beneficial Allele: A1 advantageous
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
Positive Selection:
Fitness increases with genotype frequency
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.