Study Notes for Chapter 14: Nonadaptive Evolution and Speciation

Chapter 14: Nonadaptive Evolution and Speciation

Urban Evolution: How Cities Are Altering the Fate of Species

Definition: Gene Pool

• Gene pool: The total collection of alleles in a population.
• Importance: Understanding gene pools aids in grasping how species evolve and how human activities affect these changes.

Input of Urbanization into Evolution

• Urbanization has fragmented habitats leading to distinct populations, such as the white-footed mice in New York City.
• Population geneticists study these gene pools.

Key Concepts and Vocabulary

• Nonadaptive evolution: Any change in allele frequency not leading to a population becoming more adapted to its environment.
• Population genetics: The study of genetic makeup of populations and the changes in genetic composition through time.
• Allele frequency: The relative proportion of an allele in a population.
• Influential factors for gene pools include:
  • Mutation: Occurs when there is a change in the gene pool over generations.
  • Genetic drift and gene flow: Mechanisms that contribute to changes in allele frequencies.

Relationship between Gene Pool and Evolution

• DQ1LO1: A gene pool is the sum of all genetic information (alleles) within a specific population.
• DQ1LO2: Evolution requires changes in allele frequencies within a population’s gene pool, indicating that not all frequency changes result in evolutionary change.

Mechanisms Influencing Gene Pool Composition

Genetic Drift

• Increases variation through bottlenecks that reduce genetic diversity.
• Populations that undergo genetic drift tend to lose alleles, especially in small populations where chance events can have larger impacts.
• Genetic diversity can counteract bottleneck effects.

Key Vocabulary Related to Genetic Drift

1. Genetic drift: Random changes in allele frequencies between generations.
2. Founder effect: A type of genetic drift where a small number of individuals establish a new population, resulting in low genetic diversity.
3. Bottleneck effect: A significant reduction in population size, leading to loss of alleles.
4. Gene flow: Movement of alleles among populations can mitigate genetic drift and increase diversity.
5. Inbreeding: Breeding between closely related individuals increases the likelihood of homozygous genotypes.
6. Inbreeding depression: Reduced biological fitness due to accumulation of harmful alleles through inbreeding.

Genetic Drift: Founder vs Bottleneck Effects

Similarities

• Both start with a small population that becomes less genetically diverse.
• Changes in allele frequencies are random and nonadaptive.

Differences

• Founder effect: Occurs when a few individuals leap to form a new population, leaving the original intact.
• Bottleneck effect: Results from a drastic reduction in population size, limiting genetic diversity in future generations.

Increasing Genetic Diversity

• Pathways to increase genetic diversity include:
  • Gene flow: Introducing alleles through migration and mating with other populations, leading to more varied phenotypes.
  • The presence of diverse alleles makes populations more resilient to diseases or environmental changes.

Inbreeding Consequences

• High inbreeding diminishes genetic diversity, increasing the likelihood of deleterious alleles which can result in decreased fitness and adaptability.

Comparison of Adaptive vs Nonadaptive Evolution Mechanisms

Evolutionary Mechanisms Scenarios

Examples

1. Polar bears isolated by ice drift: Mechanism: Genetic drift, as small, isolated populations lose genetic diversity.
2. Disease in wild corn: Mechanism: Natural selection favoring alleles that confer resistance.
3. Mating hermit crabs in lagoon: Mechanism: Gene flow through interbreeding.
4. Volcanic eruption affecting beetles: Mechanism: Bottleneck effect, reducing the population drastically.

Hardy-Weinberg Principle

Significance and Conditions

• Hardy-Weinberg equilibrium: Describes conditions under which allele and genotype frequencies remain stable in a nonevolving population.
• Five conditions for Hardy-Weinberg equilibrium:
  1. No mutations.
  2. No natural selection.
  3. Infinitely large population to negate genetic drift.
  4. Random mating without selective pressures.
  5. No gene flow between populations.

Hardy-Weinberg Application

• Relationship: Evolutionary changes are detectable when allele frequencies shift away from Hardy-Weinberg predictions, indicating that at least one condition is violated.

Practical Application: Calculating Genetic Frequencies using Hardy-Weinberg

Example: Trait of Human Earlobes

• Understanding the equations:
  • Allele frequency equations: $p + q = 1$
  • Genotype frequency equations: $p^2 + 2pq + q^2 = 1$

Earlobe Trait in Population Example Calculation

• Given 500 individuals, 300 have attached earlobes (recessive trait).
  1. Total alleles = 1000.
  2. Recessive allele frequency (e): $q^2 = 0.25$ → $q = 0.5$.
  3. Dominant allele frequency (E): $p = 1 - q$ → $p = 0.5$.
  4. Genotype frequencies:
  • Frequency of homozygous recessive ($ee$): $q^2 = 0.25$ or 25%.
  • Frequency of heterozygous ($Ee$): $2pq = 0.5$.
  • Frequency of homozygous dominant ($EE$): $p^2 = 0.25$.

Speciation

Definitions and Importance

• Speciation: The process through which new species arise, often through reproductive isolation due to geographic or climatic factors.
• Importance extends to conservation biology and legal frameworks like the Endangered Species Act, highlighting the need for understanding biodiversity.

Mechanisms of Reproductive Isolation

1. Hybrid inviability: Offspring don't survive to reproduction.
2. Mechanical isolation: Incompatible mating structures.
3. Hybrid infertility: Viable offspring that cannot reproduce.
4. Ecological isolation: Different habitats prevent interbreeding.
5. Behavioral isolation: Different mating rituals prevent reproduction.
6. Temporal isolation: Different breeding seasons.
7. Gametic isolation: Sperm and egg cannot unite.

Process of Speciation

• Populations become reproductively isolated, leading to genetic divergence, influenced by geographical barriers or environmental changes.

Review Questions

1. A population with a large gene pool is more genetically diverse than one with a small gene pool (True).
2. Speciation occurs when populations can no longer interbreed (True).
3. Genetic drift, gene flow, and mutations are all influenced by chance (True).
4. Speciation often requires geographic isolation and subsequent adaptation (True).

Note: The above notes should facilitate understanding of the key concepts presented in Chapter 14, emphasizing definitions, examples, and necessary vocabulary directly relevant to the discussions on evolution and speciation.