Evidence for Evolution

21.1 The Beaks of Darwin’s Finches: Evidence of Natural Selection

• Darwin's Research on Finches

  • Collected 31 specimens from 3 islands in the Galápagos Islands.

  • Not an expert on birds; specimens were taken to England for identification.

  • Identified as a closely related group of distinct species.

  • Notable similarities except for beak characteristics; all 14 species recognized.

21.2 Beaks of Darwin’s Finches: Food Gathering and Natural Selection

• Variation in Beak Shapes

  • Darwin hypothesized beak variation relates to food gathering.

  • Different shapes corresponding to different food sources.

• Modern Verification of Darwin’s Hypothesis

  • Three conditions critical for natural selection:

  1. Phenotypic variation must exist among individuals.

  2. Variation leads to differences in lifetime reproductive success.

  3. Variation must be genetically transmissible to the next generation.

• Research by Peter and Rosemary Grant

  • Focused on medium ground finch on Daphne Major island.

  • Observed beak depth variations in population.

  • Changes in average beak depth linked to environmental conditions:

    • Drought years favored birds with deeper beaks (able to crack large seeds).

    • Normal rainy conditions resulted in decreased average beak depth (small seeds).

21.3 Peppered Moths and Industrial Melanism

• Pepper Moths Color Variation

  • Color controlled by a single gene; dark form rare pre-1850s.

  • Industrial pollution led to increased dark moths— phenomenon termed industrial melanism.

• Natural Selection's Role

  • Dark moths had better survival in polluted areas; explained by predation (Tutt’s hypothesis).

  • Pollution control measures reversed trends, favoring light moths once again.

21.4 Artificial Selection: Human-Initiated Change

• Definition of Artificial Selection

  • Change instigated by humans favoring certain phenotypic traits.

  • Favors reproduction of individuals with desirable traits.

• Directional Selection Implications

  • Results in evolutionary changes where one extreme of a trait is favored, shifting population averages.

• Examples of Agricultural Selection

  • Differences from generations of human selection include livestock with greater milk production and larger corn ear sizes.

• Domestication Examples

  • Produced various breeds of cats, dogs, pigeons with specific traits (e.g., dachshunds for badger hunting).

  • Significant differences between dog breeds surpass those observed between wild Canid species.

21.5 Fossil Evidence of Evolution

• Fossils: Creation Process

  • Fossils are preserved remains of once-living organisms formed through:

  1. Burial in sediment to prevent decay.

  2. Mineralization of calcium in bone or hard tissue replacing organic material.

  3. Hardening of surrounding sediment into rock.

• Challenges in Fossilization

  • Fossilization is a rare event, leading to gaps in the fossil record.

• Evolutionary Transitions Demonstrated by Fossils

  • Intermediates exist, showing major transitions in evolution.

  • Example: Archaeopteryx—transition fossil demonstrating features of both birds and dinosaurs.

21.6 Anatomical Evidence for Evolution

• Homologous Structures

  • Features shared by different species derived from a common ancestor; e.g., mammalian forelimbs.

• Early Embryonic Development Evidence

  • Similarities seen in early vertebrate embryos, such as pharyngeal pouches:

  • In humans: develop into glands and ducts.

  • In fish: develop into gill slits.

• Imperfection in Adaptations

  • Some organisms are not perfectly adapted; Natural selection works with available traits.

  • Example: Giraffes have long necks but only possess 7 cervical vertebrae, typical for mammals.

21.7 Vestigial Structures

• Vestigial Structures Explanation

  • Structures that have lost their original function; examples:

  • Human ear muscles with no function.

  • Hip bones in pythons and boa constrictors.

  • Flippers in manatees resembling ancestral leg structures.

• Pseudogenes

  • Traces of genes that functioned in ancestors but are no longer active.

21.8 Homoplasy and Convergent Evolution

• Definition of Homoplasy

  • Similar traits in different species not derived from a common ancestor, often arising from convergent evolution.

• Convergent Evolution Explanation

  • Independent evolution of similar traits due to similar environmental pressures.

  • Example: Similar adaptations seen in marsupial and placental mammals.

21.9 Biogeography and Evolution

• Biogeographic Studies

  • Observed by Darwin during voyages; noted island species often diverged from continental relatives.

  • Concluded island species evolve through adaptive radiation.

21.10 Darwin’s Critics

• Evolutionary Acceptance and Controversy

  • While natural selection is accepted by biologists, it faces objections in public discussions:

  1. Claims of evolution not being solidly demonstrated; misunderstood theory.

  2. Assertions that there are no fossil intermediates despite contrary evidence.

  3. Intelligent design arguments questioning complexity arising from randomness.

  4. Misunderstanding of the Second Law of Thermodynamics concerning energy systems.

  5. Claims about improbability of protein formation based on random events.

  6. Misunderstanding the role of natural selection in explaining major changes.

  7. Belief in irreducible complexity undermining evolution explanations.