In-Depth Notes on Cladogenesis, Species Definition, and Speciation Types

Cladogenesis and Cladogenesis

• Cladogenesis: The process through which a common ancestor splits into multiple species, forming a group called a clade.
  • Diagram shows ancestry with branches representing the split into new species.
  • The split leads to reproductive isolation, a critical factor in defining new species.

Definition of Species

• Species are defined by reproductive isolation: when two organisms cannot reproduce with each other, they belong to different species.
• Example: Lions (Panthera leo) and Tigers (Panthera tigris) can mate and produce ligers, but these hybrids are sterile, indicating that lions and tigers are separate species.

Factors Leading to Reproductive Isolation

Physical Barriers

• Example: Two populations of species are adapted to different environments (terrestrial vs. aquatic) and never encounter one another, thus remaining separate.

Behavioral Isolation

• Species may live together but do not mate due to different behaviors or mating rituals.
  • Courtship rituals serve as a signal for potential mates (e.g., dances of different bird species).

Temporal Isolation

• Related species breed at different times (e.g., cicada species emerging in different years).
• Even if two species live in the same area, differences in breeding times can prevent mating.

Mechanical Isolation

• Physical incompatibility (e.g., size differences between two species) prevents successful mating.
• Example: A male Chihuahua and a female Great Dane cannot mate due to size differences.

Gametic Isolation

• Even if mating occurs, if sperm cannot fertilize an egg, reproductive isolation is maintained.

Viability Issues in Hybrids

• Reduced Hybrid Viability: Some hybrids may develop but may be weak or die early.
• Reduced Hybrid Fertility: Mules (a cross between a horse and a donkey) are strong but sterile, unable to produce offspring.

Hybrid Breakdown

• Offspring of hybrids may initially be fertile but lose viability over generations.

Speciation Types

Allopatric Speciation

• New species arise due to physical separation (e.g., formation of a river or a mountain range) that keeps two populations from interbreeding.
  • Examples include the separation of squirrels by the Grand Canyon.

Sympatric Speciation

• New species arise in the same geographic area without physical separation, often due to chromosomal changes or behavioral differences.
  • Example: Mutations in apple maggot flies that shift their food preferences lead to reproductive isolation from others.

Modes of Evolution

Gradualism

• The theory that changes occur slowly over long periods,
  • e.g., human evolution from earlier ancestors.

Punctuated Equilibrium

• A model where changes happen quickly within complements, followed by long periods of stability.

Exaptations

• Traits that evolve for one purpose may come to serve other functions over time (e.g., feathers, which may have first served for insulation and later evolved for display or flight).