Changes in Allele Frequency and Speciation PREREADING

Changes in Allele Frequency in Populations

• Hardy-Weinberg Equation

  • Used to determine if allele frequency changes occur in a population over time.

• Causes of Changes in Allele Frequency

  • Changes can result from mutation and natural selection.

  • Reproductive isolation can lead to genetic divergence, potentially resulting in new species.

Causes of Variation

• Primary Source: Mutation

  • Mutations: Mistakes in DNA replication; impactful if in gametes.

• Other Sources:

  • Meiosis and random fertilization during sexual reproduction generate additional genetic variation.

Conditions Necessary for Natural Selection

• Competition, Predation, and Disease contribute to differential survival of phenotypes.

• Summarized as Natural Selection where some phenotypes prevail over others due to selective advantages.

Struggle for Survival

• Variation alone does not lead to speciation; struggle for survival among individuals is required.

  • This struggle includes three interactions:

    1. Competition (for resources)

    2. Predation

    3. Disease

Natural Selection Principles

• Organisms with advantageous phenotypes are more likely to survive and reproduce, contributing their alleles to the next generation.

• The gene pool refers to all genes/alleles in a population, influencing allele frequency over generations.

Speciation Overview

• Criteria for Classifying Species:

  • Morphological, biochemical, behavioral, and genetic similarities.

  • Interbreeding capability resulting in fertile offspring.

• Definition of Speciation: Formation of new species from existing ones, often requiring variation in phenotypes influenced by genetic and environmental factors.

Isolating Mechanisms in Speciation

• Pre-zygotic Isolating Mechanisms include:

  • Temporal: Different mating seasons.

  • Ecological: Different habitats.

  • Behavioral: Different courtship behaviors.

  • Mechanical: Physical incompatibilities in species. - 4 above linked to sympatric

  • Geographical linked to allopatric

Types of Speciation

• Allopatric Speciation:

  • Occurs due to geographic isolation over time.

• Sympatric Speciation:

  • Occurs while species inhabit the same area, often through reproductive isolation mechanisms.

Genetic Drift

• Alongside natural selection, mutation, and migration, genetic drift is a key evolutionary mechanism.

• Key Concepts:

  • Some individuals leave more descendants by chance, which can change allele frequencies.

  • More significant in small populations.

Examples of Genetic Drift

• Founder Effect:

  • Occurs when a population originates from a small group, leading to limited genetic diversity (e.g., Amish population).

• Bottleneck Effect:

  • A drastic reduction in population size, leading to decreased genetic diversity (e.g., Cheetahs).

The Founder Effect and the Amish

• Original Amish population descended from 200 individuals; limited intermarrying led to conditions like Ellis-van Creveld syndrome due to recessive alleles being more prevalent.

The Genetic Bottleneck in Cheetahs

• Cheetahs share minimal genetic variation (<1%) due to historical bottlenecks caused by climate events and human activities like hunting.

Types of Isolating Mechanisms in Speciation

Isolating mechanisms prevent interbreeding between different species, facilitating speciation. They are categorized into two main groups: pre-zygotic and post-zygotic mechanisms.

Pre-zygotic Isolating Mechanisms

These mechanisms occur before fertilization and prevent the formation of zygotes.

1. Temporal Isolation: Species mate at different times (e.g., seasons or times of day).

2. Ecological Isolation: Species occupy different habitats within the same area, reducing encounters.

3. Behavioral Isolation: Different species have distinct mating rituals or behaviors that prevent them from mating.

4. Mechanical Isolation: Physical differences in reproductive organs prevent successful mating.

Post-zygotic Isolating Mechanisms

These occur after fertilization and result in reduced viability or fertility of hybrid offspring.

1. Hybrid Inviability: Hybrids fail to develop properly and die before reaching maturity.

2. Hybrid Sterility: Hybrids are sterile and cannot reproduce (e.g., mules, the offspring of a horse and donkey).

3. Hybrid Breakdown: First-generation hybrids are fertile, but their offspring are inviable or sterile.

Understanding these mechanisms helps clarify the processes that lead to the formation of new species and the maintenance of biodiversity in ecosystems.