Patterns of Descent with Modification

• Biological Evolution is a process that gradually selects the organisms that are better adapted to their environment to continuously change life and make all living organisms in our world the way they are today.
• Evolution

  • is not a finished event wherein humans are the final product.
  • Continuous process which has been changing and forming life on Earth for billions of years and continues to do so.
• Species

  • groups of interbreeding natural population that are reproductively isolated from other such group.
  • is a closely related organism that are very similar and capable of producing fertile offspring.
• Pre-zygotic isolation Mechanisms

  • prevent fertilization and zygote formation.
  • happens before fertilization occurs between gametes.
  • occurs after members of two different species have mated and produced a zygote.
• Geographic or Ecological or Habitat Isolation

  • occurs when two species that could not interbeed due to them living in different areas.
  • The two species will not encounter one another.
• Temporal or Seasonal Isolation

  • Different groups may not be reproductively mature.

  e.g.

  • population of plants may reproduce flowers in different seasons; making mating between the populations impossible.
• Behavioral Isolation

  • When two populations are capable of interbreeding but does not occur since they have differences in reproductive strategies that involve behavior.
• Mechanical Isolation

  • Differences in reproductive organs prevent successful interbreeding
  • occurs when mating is physically impossible.
• Gametic Isolation

  • Incompatibilities between egg and sperm to prevent fertilization.
  • animals do come in contact, but the gametes are not compatible.
• Post-zygotic Isolation mechanism

  • Hybrid Inviability
  • Hybrid Sterility
  • Hybrid Breakdown
• Hybrid Inviability

  • Fertilized egg fails to develop past the early embryonic stages.
• Hybrid Sterility

  • Hybrids are sterile because gonads develop abnormally or there is abnormal segregation of chromosomes during meiosis.
  • Mismatched chromosomes make it hard to make viable sperm and eggs.
• Hybrid Breakdown

  • Some first-generation hybrids are fertile, but when they mate with another species or with either parent species, offspring of the next generation are feeble or sterile.
• Speciation

  • is the evolutionary process by which populations evolve to become distinct species.
  • process by which new species develop from existing species.
• Types of Speciation

  1. Allopatric
  2. Sympatric
  3. Parapatric
• Allopatric Speciation

  • occurs when some members of a population become geographically separated from other members; therefore, preventing gene flow.
• Sympatric Speciation

  • occurs when members of a population that initially occupy the same habitat within the same range diverge into two or more different species.
  • involves abruptic genetic changes that quickly lead to reproductive isolation.
• Parapatric Speciation

  • occurs when the groups that evolved to be separate species are geographic neighbors.
  • Abrupt changes in the environment over a geographic border and strong disruptive selection is also present in parapatric speciation.