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Chapter 20: The Evidence for Evolution

Introduction

• Understanding how evolution is supported through various forms of evidence.

• Emphasis on Darwin's observations and modern research supporting evolutionary theories.

Darwin’s Observations on Finches

• Darwin’s Finches:

  • Collected 31 specimens from 3 Galápagos Islands.

  • Not a bird expert; relied on ornithologist for identification.

  • Identified as closely related species differing primarily in beak characteristics.

  • Total of 14 species recognized today.

Beak Characteristics

• Different beak shapes linked to feeding habits:

  • Observed that each species adapted to exploit different food sources.

  • Hypothesis posited by Darwin: each species modified for specific ends based on available resources.

Natural Selection Principles

• Conditions for Natural Selection:

  • Variation must exist in the population.

  • Variation must influence reproductive success.

  • Variation must be heritable, allowing it to pass to future generations.

Case Study: Medium Ground Finch

• Peter and Rosemary Grant's research:

  • Studied beak depth variations on Daphne Major island.

  • Average beak depth fluctuated according to available seed types caused by environmental changes.

  • Droughts: increased average beak depth due to availability of tough seeds.

  • Normal rain: reduced depth when small seeds predominated.

Evolutionary Tree and Adaptive Radiation

• DNA Comparisons of Darwin’s Finches:

  • Tree predicts evolutionary relationships and adaptations to vacant ecological niches.

  • Demonstrates the process of adaptive radiation among finch species.

Peppered Moths and Industrial Melanism

• Environment’s influence on trait selection in species:

  • Light and dark variations of Biston betularia moth and the rise of dark forms due to predation on light forms.

  • Resulted from habitat darkening, primarily during the industrial era.

  • Clean Air Acts in the mid-20th century led to light-colored forms regaining dominance.

Artificial Selection

• Human-driven selection influences species development:

  • Selection favors individuals with desired traits for reproduction.

  • Example using Drosophila melanogaster to illustrate rapid changes over generations (bristle number).

Domestication and Selection Effects

• Selection of traits leads to distinguishable breeds:

  • Example with dogs and specific traits for functions (e.g., Dachshunds for chasing badgers).

  • Silver fox experiment: consistent selection for docility led to notable changes in traits similar to dogs.

Fossil Evidence and Evolution

• Types of fossils and dating methods:

  • Fossils provide a historical record of life on Earth.

  • Relative Dating:

    • Age determined by the position of rock layers.

  • Absolute Dating:

    • Uses radioactive decay to establish age.

Intermediate Forms and Evolutionary Transitions

• Archaeopteryx:

  • Significant fossil representing a transition between reptiles and birds, showcasing evolutionary traits from both groups.

• Discoveries of aquatic mammals illustrate the evolution from land-dwelling ancestors.

Anatomical and Developmental Evidence

• Homologous structures across species:

  • Similar structures serve different functions, indicative of common ancestry (e.g., forelimbs of mammals).

• Comparative embryology:

  • Early similarities in embryonic development among vertebrates provide evidence for common descent.

Imperfections in Adaptation

• Many organisms are adaptations that reflect a history of evolution, arising from natural selection, not perfect design.

• Examples include vertebrate eyes with structural imperfections compared to mollusk eyes.

Vestigial Structures

• Traits that no longer serve significant functions:

  • Include structures like the human ear-wiggling muscles and hip bones in snakes, evidencing evolutionary history.

Genetic Evidence of Evolution

• Evolutionary changes in DNA:

  • Mutations in alleles accumulate over generations, reflecting evolutionary paths.

  • Molecular clocks reveal how genetically distinct species are over time.

Biogeography and Evolution

• Study of species distribution influences understanding of evolution:

  • Example of convergent evolution in marsupial and placental mammals with similar adaptations in similar environments.

• Darwin’s conclusions on island species:

  • Species on islands derive from nearby continental relatives, leading to adaptive evolution in response to available niches.

Addressing Objections to Evolution

• Common misunderstandings about evolutionary mechanisms:

  • Misconceptions surrounding the concept of ‘theory’ in science and evidenced-through fossil records.

  • Discussion of intelligent design and inherent complexity versus random processes in evolution.

Conclusion

• Natural selection plays a significant role in evolutionary changes alongside artificial selection.

• Continuous advancements in scientific understanding refine the mechanisms of evolution.