Chapter 12: The Forces of Evolutionary Change

The Forces of Evolutionary Change

Introduction to Charles Darwin

• Charles Darwin published On the Origin of Species on November 24, 1859.
• He introduced the concept of Natural Selection, a principle echoed by Alfred Russel Wallace and other contemporaries.
  • Key Ideas:
  • Organisms change over generations.
  • Individuals possessing heritable traits that enhance survival leave more offspring.

Evolutionary Adaptation

• Over generations, the consistent selection of beneficial traits leads to evolutionary adaptations.
  • Examples:
  • Flower mantid (Malaysia)
  • Trinidad tree mantid (mimics dead leaves)
  • Leaf mantid (Costa Rica)

Darwin's Research Context

• Darwin’s view contrasted the prevailing belief from Aristotle and Judeo-Christian teachings, which asserted species were unchanging and the Earth was young (approximately 6,000 years old).
• Darwin posited that the Earth was ancient and species had evolutionary relationships.

The HMS Beagle Voyage

• In 1831, Darwin embarked on the HMS Beagle as the onboard naturalist, collecting numerous specimens.
• Noted unusual distribution of species in the Galápagos Islands, particularly their resemblance to South American species.

Influence of Geology

• Influenced by Charles Lyell, whose work Principles of Geology supported the idea of an ancient Earth shaped by gradual processes.
• Darwin concluded that both the Earth and its organisms have changed over long periods through gradual processes.

Key Concepts in Evolution

• Descent with Modification: All current organisms are descendants of past species that may have differed significantly.
• Natural Selection: The mechanism by which descent with modification occurred.

Evidence for Evolution

• Understanding evolution involves multiple forms of evidence such as:
  • Fossils: Remnants of ancient organisms that provide a historical sequence of life.
  • Biogeography: Studies the geographical distribution of species to deduce common ancestry.
  • Comparative Anatomy & Embryology: Examines structural similarities and developmental processes among species indicating shared ancestry.
  • Molecular Biology: Analyzes genetic material to establish evolutionary relationships.

Fossils and the Fossil Record

• Fossils can be mineralized remains or impressions and are primarily found in sedimentary rocks.
• The fossil record documents the chronological appearance of organisms, confirming evolutionary changes over time.

Biogeography

• Understanding where species are located helps reveal their evolutionary history and relationships.
• Common Ancestors: Many species share ancestors from similar geographic locations, explaining their current distributions.

Comparative Anatomy and Embryology

• Comparative Anatomy shows that many species possess homologous structures, indicating a common ancestor.
• Comparative Embryology highlights early similar features in vertebrates, supporting evolutionary connections.

Molecular Biology

• Examines DNA sequences to establish genetic relationships and calculate divergence times using molecular clocks.

Natural Selection and Adaptive Evolution

• Example: Darwin’s finches showcase adaptive evolution driven by natural selection.
• Conditions for Natural Selection:
  1. Overproduction: More offspring are produced than can survive, creating competition.
  2. Variation: Individuals possess heritable variations.
  3. Reproductive Success: Individuals with favorable traits reproduce more successfully, influencing allele frequencies in the population.

Populations and Gene Pools

• A population consists of individuals of the same species in a specific area, representing the smallest biological unit capable of evolving.
• Gene Pool: Refers to the total alleles in a population, influencing genetic diversity and evolution.
  • Frequencies of alleles can be represented mathematically.

Hardy-Weinberg Principle

• The Hardy-Weinberg formula helps determine if a population is in genetic equilibrium, indicating it is not evolving.
• Conditions for Equilibrium:
  1. Large population size
  2. No gene flow
  3. No mutations
  4. Random mating
  5. No selection pressures

Mechanisms of Microevolution

1. Genetic Drift: Random changes in small populations can drastically alter allele frequencies.
2. Gene Flow: Movement of alleles between populations reduces genetic differences.
3. Mutations: Random changes in genomes can introduce new alleles to a population.
4. Natural Selection: Differential survival and reproduction based on advantageous traits.

Types of Natural Selection

• Directional Selection: Favors one extreme phenotype.
• Disruptive Selection: Favors both extreme phenotypes, leading to a potential evolution of polymorphism.
• Stabilizing Selection: Favors average phenotypes, reducing variation.

Sexual Selection

• Recognizes differences between males and females (sexual dimorphism) resulting from competition (intrasexual) or mate choice (intersexual).

Limitations of Natural Selection

• Four main limitations exist that prevent the perfection of adaptations:
  1. Historical constraints on organisms
  2. Compromises in adaptations
  3. Interaction of chance with natural selection
  4. Selection acts only on existing variations

Current Observations of Natural Selection

• Examples include pesticide resistance in insects and antibiotic-resistant bacteria, showcasing real-time evolution.

Founder Effect and Genetic Drift

• Founder Effect: A new colony often has reduced genetic variation leading to unusual alleles becoming more common.

Conclusion

• Population Genetics: Investigating the genetic makeup over time helps assess evolutionary changes.
• Genetic Variation: Primarily arises from mutations and sexual recombination, vital for evolution.