Lecture 9: Speciation and Reproductive Isolation

Speciation: Accumulation of microevolutionary changes leading to new species; a form of macroevolution.

Morphological Species Concept: Defines species based on visible anatomical (morphological) differences.
- Used by Linnaeus.
Biological Species Concept (BSC): Groups of actually or potentially interbreeding natural populations reproductively isolated from other such groups.
- Members can mate and produce fertile offspring with each other.
- Different species cannot mate to produce fertile offspring.
- Limitations: Does not apply well to plants (due to hybridization) or asexually reproducing species.

Prevent genetic exchange between different species.
Prezygotic Isolating Mechanisms: Prevent mating or fertilization.
- Habitat Isolation (Geographic/Ecological):
  - Geographic Isolation: Physical barriers separate populations.
    - Allopatric: Geographically separated populations.
    - Sympatric: Populations in the same geographic area.
  - Ecological Isolation: Species utilize different parts of the same environment; rarely meet (e.g., lions vs. tigers).
- Temporal Isolation: Species reproduce at different times (seasons, day/night).
- Behavioral Isolation: Differences in courtship rituals or mate recognition signals prevent interbreeding (e.g., bird mating dances).
- Mechanical Isolation: Physical incompatibility of reproductive structures (e.g., insect genitalia, flower shapes).
- Gametic Isolation: Incompatibility of gametes; chemical signals from the egg may fail to attract or allow penetration by sperm from other species (common in broadcast spawners like abalone).
Postzygotic Isolating Mechanisms: Occur after fertilization.
- Hybrid Inviability: Hybrid embryo does not develop or offspring do not survive to adulthood.
- Hybrid Infertility: Hybrid offspring survive but are sterile (e.g., mules).
- Hybrid Breakdown: First-generation hybrids are fertile, but subsequent generations are infertile (e.g., offspring of ligers).

Occurs when populations are geographically separated (in allopatry).
Process:
1. Physical barrier separates an ancestral population into two.
2. No gene flow between separated populations.
3. Evolutionary forces (natural selection, genetic drift, mutation) act independently, causing differentiation.
4. Populations develop reproductive isolation.
Mechanisms of Population Separation:
- Dispersal Isolation: A small group moves to a new location (founder effect; e.g., Galapagos species).
- Vicariance Isolation: A new geographic barrier arises, splitting an existing population (e.g., new river, continental drift).
- Missing Link Isolation: Intermediate populations in a chain go extinct, creating a large, uncrossable gap between remaining populations.
Forces Driving Differentiation in Allopatry:
- Natural Selection: Different environmental pressures lead to adaptation and divergence (e.g., Anolis lizard dewlaps).
- Genetic Drift: Random allele frequency changes, especially in small, isolated populations.
- Mutation: Differential accumulation of new mutations.
Counteracting Force: Gene flow strongly opposes speciation.

Occurs when populations diverge into new species while inhabiting the same geographic area (in sympatry).
Mechanisms:
- Polyploidy: Whole-genome duplication (2n
  ightarrow 4n) due to meiotic error. This can create a new species that is reproductively isolated from the parent species in a single generation. Common in plants, rare in animals.
- Disruptive Selection + Assortative Mating:
  - Disruptive Selection: Favors extreme phenotypes over intermediate ones.
  - Assortative Mating: Individuals prefer to mate with others of similar phenotype.
  - Together, these can reduce gene flow and promote divergence within a sympatric population (e.g., Lake Barumbo cichlids, pea aphids).

Adaptive Radiation: Rapid formation of many new species from a single ancestral group, often when entering a new, resource-rich environment with few competitors (e.g., Hawaiian Islands, post-mass extinctions, Galapagos finches/tarweed).
Ring Species: A chain of populations encircling an uninhabitable area. Adjacent populations can interbreed, but populations at the ends of the chain, where they meet, are reproductively isolated, illustrating speciation in progress (e.g., Californian salamanders).