PPT 12

Evidence for Evolution

Convergent Evolution

• Different ancestors produce similar solutions to environmental problems, not true homology.

• Similar adaptations arise from similar environmental pressures.

• Examples include:

  • Birds and Bats: Though they are different species, they developed the ability to fly.

  • Porcupines: American and Old World porcupines evolved from a common ancestor resembling a large furry rat, approximately 70 million years ago.

  • Comparison of marsupials versus placentals further illustrates this concept.

Divergent Evolution

• Shares a common ancestor but leads to variations in morphology based on different habitats, thus demonstrating true homology.

• Influenced by changing environmental conditions in isolated locations.

• This is often referred to as adaptive radiation.

• Examples:

  • Pentadactyl limbs: Found in various vertebrates like snakes, birds, and lizards, showcasing adaptations to different environments.

Summary of Evolution Forms

• Convergent Evolution: Similar morphology or lifestyle observed in organisms from different ancestors due to similar habitat solutions.

• Divergent Evolution: Similar history but resulting in differing physical traits and lifestyle adaptations due to varied environments.

Examples of Convergent and Divergent Evolution

• Convergent Examples:

  • Sugar Glider (North America) vs Flying Squirrel (Australia): Both can glide but evolved independently.

  • Hummingbirds exhibiting convergent traits with moths in floral adaptation.

• Divergent Examples:

  • Various tortoises: Sea turtles, pet shop turtles, and desert tortoises representing diverging adaptations to their specific habitats.

Evidence through Fossil Record

• Fossils provide insights into past life forms indicating evolution and extinction cycles:

  • Only hard structures are typically fossilized; soft tissues decay readily.

  • Fossils are preserved best in environments that protect against decay, like sedimentary rock.

  • Carbon dating using isotopes can estimate the age of fossils, but is imprecise.

Understanding Radiometric Dating

• Half-life of isotopes aids in dating fossils:

  • Example: Carbon-14 has a half-life of 5730 years.

  • Oldest known fossils: Stromatolites estimated at 3.5 billion years old, indicating early life.

• Extension of knowledge gained from examining the geological time scale via sediment layers.

Biogeography: Organism Distribution and Adaptation

• The geographical distribution of organisms plays a critical role in evolution, influenced by continental drift and habitat availability.

• Island Biogeography: Numerous endemic species evolve distinctly when isolated from mainland species, as seen with Galapagos finches.

• The impact of competition leads to adaptive radiation between placental and marsupial mammals.

Comparative Anatomy and Embryology

• Analysis of common structures reveals evolutionary relationships:

  • All vertebrates share basic limb structure, indicative of a common ancestor. Example: Pentadactyl limb in humans, cats, whales, bats:

    • Humerus, radius, ulna, carpals, metacarpals, phalanges.

• Embryology offers insights into evolutionary paths, as early developmental stages show similar structures pre-differentiation.

Molecular Biology Evidence

• Comparison of DNA sequences leads to understanding evolutionary relationships:

  • Closely related species exhibit fewer differences in DNA sequences compared to those more distantly related.

  • Rate of mutations provides a timeline for evolutionary changes.

  • Peppered moth example illustrates adaptive responses to environmental changes, demonstrating natural selection in action.

Causes of Microevolution

1. Mutations: New alleles arise.

2. Genetic Drift: Random changes in allele frequency, significantly affecting small populations.

3. Founder Effect: New populations formed from a small number of individuals, resulting in limited genetic variation.

4. Bottleneck Effect: Occurs after a significant reduction in population size due to an event, leading to decreased genetic diversity.

5. Gene Flow: Changes in allele frequencies through migration of individuals.

6. Assortative Mating: Preference for similar phenotypes in mate selection.

7. Natural Selection: Adaptation creating increased fitness within the population, where favorable traits proliferate.