Topic 9

Anagenesis: The gradual evolution of a species without branching.
Cladogenesis: Branching evolution, where one ancestral species evolves into two or more species.

Speciation (Cladogenesis): One ancestral species evolves into two reproductively isolated descendant species.

Definition of a Species: A group of organisms that commonly interbreed in the wild and do not interbreed with other similar species.

Importance of Reproductive Isolation: Critical for maintaining species integrity and allowing independent evolution.
Types of Reproductive Isolation:
- Pre-zygotic (before fertilization occurs):
- Examples include:
  - Different mating displays (e.g., jumping spiders).
  - Different habitats (e.g., aquatic vs terrestrial garter snakes).
- Post-zygotic (after fertilization occurs):
- Hybrid inviability (e.g., North American frogs).
- Hybrid sterility (e.g., mules).

Gene Flow: The amount of divergence between populations relies on the balance between selection and gene flow.
In the absence of gene flow, populations may diverge more dramatically.

Only Applicable to Sexual Organisms: The concept primarily applies to those that reproduce sexually.
Challenges in Demonstration: Requires substantial work to prove reproductive isolation.
Not Always Effective:
- Example of the Herring Gull and Lesser Black-backed Gull: No interbreeding in the UK, but occasional interbreeding on the continent.

Physiological, Morphological, and Ecological Species Concepts: These concepts provide different perspectives on what constitutes a species.
Phylogenetic Species Concept: Focuses on the evolutionary history and relationships among species.

Genetic studies on various elephants reveal differences:
- Asian Elephants and African Forest Elephants.
- African Savannah Elephants.
Genetic divergence indicated (e.g., 0.001 changes noted).

No Single Species Concept is Perfect: Each concept has limitations and applicability issues.

Reproductive isolation evolves mainly through geographic isolation in the Allopatric Model of Speciation. (a theory that proposes new species arise when a single species is split into two or more geographically isolated populations)
Steps in Allopatric Speciation:
- Step 1: Initial population in a single habitat.
- Step 2: Geographic barrier splits the population.
- Step 3: Divergence occurs in isolation due to lack of gene flow.
- Step 4: Populations may meet but are now reproductively isolated.

Various finch species demonstrate adaptations and morphological differences within geographic isolation facilitated by environmental pressures.
- Small insectivorous finch, large insectivorous finch, vegetarian tree finch, woodpecker finch, and more, each adapted to different ecological niches.

Speciation can also occur without geographic isolation, known as sympatric speciation.
Typically more difficult due to ongoing gene flow.
Example: Rhagoletis fruit flies, which adapted by invading new habitats (hawthorn to apple) leading to temporal isolation in mating.

Periods of geographic isolation can facilitate the evolution of reproductive isolation.
Allopatric speciation is likely the main mechanism of speciation in animals, while sympatric speciation, though common in plants (e.g., polyploidy), is rare in animals.
Sympatric speciation may occur under unusual ecological conditions, but proving it remains challenging.