Population Genetics

Overview of Main Concepts

  • Population genetics explores the distribution and change of allele frequencies in populations.

Cladogram Basics

  • Cladogram: A diagram that shows the evolutionary relationships between different species.

Plant Development Timeline

  • Timeline Components:
      - Cherry Tree
      - Pine Tree
      - Fern
      - Moss
Key Features of Various Plants:
  1. Woody Stems
       - Pine Tree: Lives in wet areas, produces seeds on short shoots.
       - Cherry Tree: Has a woody stem and photosynthesis occurs primarily through the stem.
  2. Vascular Tissue
       - Present in woody plants (e.g., Cherry and Pine Trees).
       - Absent in non-woody plants (e.g., Moss).
  3. Sporophyte Dependency
       - Independent sporophyte: e.g., Ferns.
       - Dependent sporophyte: e.g., Moss.

Chapter 18: Genes and Variation

Introduction to Evolution

  • Darwin's contributions to evolutionary theory were limited by his lack of knowledge about genetics and DNA.
  • Key challenges faced by Darwin:
      1. Source of variation within species.
      2. Process of inheritance of traits across generations.

Populations and Gene Pools

  • Population: Group of individuals of the same species in a defined area.
  • Gene Pool: The complete set of genetic information within a given population, encompassing all alleles for each gene
Allele Frequency
  • Definition: The proportion of a specific allele among all alleles for that gene in a population.

Sources of Genetic Variation

Mutations

  • Definition: A heritable change in genetic information.
      - Can lead to the formation of new alleles.
  • Natural Selection: Can act on the resulting phenotypic variations.
  • Mutation Rates: Generally lower in plants and animals, and higher in prokaryotes due to faster reproduction.
  • Impact of Mutations:
      - Can be harmful, neutral, or beneficial; most are neutral to harmful.

Recombination Processes

  • Genetic Recombination:
      1. Independent Assortment: Random segregation of homologous chromosome pairs during gamete formation.
      2. Crossing-Over: Exchange of DNA segments between homologous chromosomes during meiosis, increasing genetic diversity.

Natural Selection

Overview of Natural Selection

  • Focus on how natural selection impacts allele frequencies and population phenotypes.
  • Reproductive Success is quantified through relative fitness, which considers survival and reproduction capabilities relative to others.
Measuring Effects of Natural Selection
  • Changes in phenotypic averages can provide insights into selection effects.

Modes of Natural Selection

  1. Directional Selection: Favors one extreme phenotype, moving the mean in that direction.
  2. Stabilizing Selection: Favors intermediate phenotypes, counteracting extremes.
  3. Disruptive Selection: Favors extremes of phenotypes, potentially leading to speciation.

Examples of Natural Selection Effects

  • Directional Selection: Longer necks in giraffes.
  • Stabilizing Selection: Human birth weights.
  • Disruptive Selection: Color variation in rock pocket mice.

Genetic Drift and Population Changes

Genetic Drift

  • Definition: Random fluctuation of allele frequencies in small populations due to chance events.
  • Consequences: Can reduce genetic diversity, not driven by selection mechanisms.

Bottleneck Effect

  • Occurs when a significant reduction in population size leads to altered allele frequencies and loss of genetic diversity.
  • Examples of causes: Natural disasters, habitat destruction, and overhunting.
  • Cheetah Population: Example of severe bottleneck, resulting in low genetic diversity due to historical events.

Founder Effect

  • Happens when a new population is established by a small subgroup of the original population, potentially resulting in different allele frequencies.

Gene Flow

  • Definition: Movement of alleles between populations through immigration/emigration of individuals.
  • Example: Pollen dispersal from one flower population to another, allowing allele mixing.

Scenarios Review

  • Scenario analysis to determine if they exemplify bottleneck effect, founder effect, or gene flow:
      1. Black robin population decline due to habitat loss - Bottleneck effect.
      2. Amish population polydactyly - Founder effect.
      3. Genetic diversity loss in marble trout - Bottleneck effect.
      4. Pollen dispersal leading to trait mixing - Gene flow.

Speciation

Definition and Introduction

  • Species: A group capable of interbreeding to produce viable, fertile offspring.
  • Speciation: The evolutionary process by which new biological species arise, leading to life's diversity.

Reproductive Isolation

  • Occurs when populations are prevented from interbreeding through mechanisms that maintain gene flow barriers.
  • Types of barriers:
      1. Prezygotic: Prevent mating or hinder fertilization.
      2. Postzygotic: Prevent development of viable, fertile hybrids.
Prezygotic Isolation Mechanisms
  1. Geographic Isolation: Separation of species due to physical barriers (e.g., mountains).
       - Example: Western vs Eastern bluebirds.
  2. Temporal Isolation: Species breed at different times, preventing mating.
       - Example: Mating times of western and eastern spotted skunks.
  3. Behavioral Isolation: Unique behaviors prevent interbreeding (e.g., courtship rituals in blue-footed boobies).

Types of Isolation Analysis

  • Examples provided for different types of prezygotic barriers, including geographic, temporal, and behavioral isolation.

Microevolution vs. Macroevolution

Distinction between Concepts

  • Microevolution: Changes in allele frequencies within a single species/population.
  • Macroevolution: Large-scale evolutionary changes, such as the emergence and extinction of species.

Patterns of Evolution

  1. Convergent Evolution: Unrelated species evolve similar traits due to analogous environmental pressures.
  2. Divergent Evolution: Related species evolve different traits as populations adapt to different environments.
  3. Coevolution: Mutual influence of closely interacting species on each other's evolution.

Speciation Rates

  • Punctuated Equilibrium: Rapid evolutionary changes followed by long periods of stability.
  • Gradualism: Change occurs slowly over long segments of time.