Speciation: The Splitting of Species

Introduction

Occurs when a species is divided by a physical barrier.
- Examples:
  - Glaciers during an ice age.
  - River creating a canyon.
  - Lava flow from a volcanic eruption.
  - Construction of a highway.
Isolated populations evolve separately due to:
- Adaptation to local environments.
- Random genetic drift.
Results in distinct species that cannot interbreed.
Examples:
- Squirrels on north and south rims of the Grand Canyon.
- Finches on different Galapagos islands.

Occurs without physical barriers, within the same habitat.
Requires:
- Disruptive selection: Natural selection drives a population in two directions.
- Assortative mating: Similar individuals choose each other as mates.
Results in groups with different ecological niches.
Examples:
- Apple Maggot Fly:
  - Originally laid eggs on hawthorn trees.
  - Adapted to domestic apples introduced in the 1600s.
  - By the mid-1800s, flies were specialized for either hawthorn or apple.
  - Genetically distinct populations within the same habitat.
- Cichlid Fish in Lake Apoyo, Nicaragua:
  - Single ancestral species split into two species.
  - Different body shapes and diets (insects vs. plant material).

Sympatric speciation is controversial due to:
- Specific conditions required for occurrence.
- Rarity of clear examples in nature.
Cases like apple maggot fly and Lake Apoyo cichlids support its possibility.
Uncertainty remains on how common sympatric speciation is.
Difficulty in determining past geographical conditions complicates understanding.