(A4.1) - Evolution & Speciation - IB Biology (SL/HL)

Overview of Evolution and Speciation

  • Exploration of concepts like evolution, speciation, and related evidence

Definition of Evolution

  • Evolution: Cumulative change in heritable characteristics of a population over generations.

    • Keywords:

      • Generations: Changes occur slowly over many generations, not across just one lifetime.

      • Population: Evolution affects entire populations, not just individual organisms.

      • Heritable: Genetic traits must be passed down through generations.

      • Cumulative: Incremental changes accumulate over time.

Example of Giraffes

  • Historical context: Giraffes originally had short necks due to lack of resource competition.

  • As resources became scarce, giraffes with longer necks survived, reproducing over generations leading to the long-necked giraffes we see today.

Misconceptions About Evolution

  • A change in an individual's appearance over a lifetime (e.g., weight change) does not constitute evolution since genetics remain unchanged.

  • Evolution entails genetic changes across generations, not within an individual's lifetime.

Darwinism vs. Lamarckism

  • Darwinism: Evolution occurs through natural selection over long periods, based on survival of favorable traits.

  • Lamarckism: An outdated theory claiming organisms acquire traits in their lifetime (e.g., a giraffe stretching its neck) and pass them on to offspring. This is incorrect and not supported by evidence.

Evidence for Evolution

  • Three primary evidence points:

    1. Homologous Structures:

      • Similar anatomical structures in different species, suggesting a common ancestor (e.g., human arm, bird wing).

      • These structures serve different functions but share a similar basic design, a result of divergent evolution.

      • Definition: Anatomical features showing structural similarities but functional differences, indicating common ancestry.

    2. Analogous Structures:

      • Structures that serve similar functions but do not have a common ancestor (e.g., wings of birds vs. wings of insects).

      • Definition: Anatomical features showing differences in structure but similarities in function.

    3. Genetics:

      • Evolution can be traced by analyzing DNA sequences from different organisms, revealing similarities and differences

      • Phylogenetic trees help illustrate genetic relationships and evolutionary history.

      • Important: Notably observe variations among many genes, not just a single gene sequence.

Selective Breeding

  • Process whereby humans intentionally breed organisms for specific traits (e.g., breeding dogs for certain characteristics).

  • Example: Teosinte (ancient corn) transformed into modern maize through generations of selective breeding by early farmers.

  • Supports evolution as it demonstrates how traits can change over time through selective pressures.

Speciation

  • Definition: Formation of a new species often due to reproductive isolation.

    • Occurs when populations cannot interbreed due to geographic or other barriers.

  • Example using iguanas:

    • Storm causes iguanas to be separated by water, leading to the evolution of distinct species (mainland vs. marine iguanas) over generations.

  • Geographical barriers, behavioral barriers, and temporal barriers as means to prevent interbreeding.

Importance of Speciation

  • Reproductive isolation leads to new species as populations develop different adaptations.

  • Contrasts with extinction, which is the loss of species.

Key Definitions and Concepts

  • Adaptive Radiation: Rapid evolution of different species from a common ancestor, an intense form of divergent evolution.

  • Homologous Structures: Similar structures from a common ancestor; show differences in function.

  • Analogous Structures: Structures with similar functions but different origins; show differences in structure.

  • Convergent Evolution: Similar traits arise in unrelated species due to similar environmental pressures.

  • Divergent Evolution: Many species evolve from a common ancestor, leading to increased differences.

Review Questions

  1. What is the biological definition of evolution? Answer: Cumulative change in heritable characteristics of a population.

  2. What are homologous structures?

    • Answer: Similar structures indicating common ancestry but differing in function.

  3. What evidences support evolution?

    • Homologous structures and selective breeding are significant evidences.