Lecture 11

Understanding how populations evolve via:
- Mutation.
- Gene flow.
- Genetic drift.
- Natural selection.
- Sexual selection as a specific form of non-random mating.
Defining species primarily using the biological species concept, which states:
- Species are groups of actually or potentially interbreeding populations that are reproductively isolated from other groups.

Focus Questions: How do changes in genetic structure lead to new species?
- 1) Basic genetics of speciation.
- 2) Geographic context of speciation: allopatry and sympatry.
- 3) Hybridization.
- 4) Rates of speciation.

Overview of reproductive barriers:
- Prezygotic barriers: Such as habitat isolation, temporal isolation, and mechanical isolation.
- Postzygotic barriers: Such as hybrid inviability and hybrid sterility.
Concerns regarding reproductive barriers:
- A mutation that causes reproductive isolation cannot increase in frequency if the individual carrying it cannot mate with others (unless self-fertilization occurs).
Dobzhansky-Muller Model:
1. A single ancestral population divides into two separate populations evolving independently (no gene flow).
2. New alleles can get fixed at different loci in each population.
3. New alleles from these populations may not be compatible with each other, leading to hybrids that are inviable or less fit.

Case Study: The monkeyflower, Mimulus guttatus
- Specific population developed a new allele for copper tolerance due to environmental influences at a contaminated site.
- Other populations developed independently with different alleles, leading to hybrid inviability (death of early embryos due to incompatible alleles).

Incompatibilities accumulate as populations diverge due to:
- Independent evolutionary paths and lack of gene flow.
- Initial cessation of gene flow is critical for independent evolution.

Allopatric Speciation:
- Defined as speciation occurring when populations are separated by geographic barriers (e.g., mountains, sea level changes).
- Examples include:
- Rising sea levels isolating populations of flowers and causing distinct evolutionary paths.
- Experimental studies with fruit flies showing reproductive isolation through preferred food sources over generations.
- Adaptations in Gambusia hubbsi (mosquitofish) in different environments leading to physical and mating preference differences.

Marine species separation due to the rise of the Isthmus of Panama around 3 million years ago, leading to distinct species pairs.
Founding events on isolated islands (like Hawaiian Islands) resulting in numerous species arising from a single introduction.
Galapagos finches illustrating allopatric speciation wherein different island environments led to divergence from a common ancestral species.

Sympatric Speciation:
- Defined as the evolution of reproductive isolation without geographic separation.
- Thought to occur less frequently than allopatric speciation.
- Mechanisms may include:
- Disruptive selection on habitat preference (e.g., apple maggot flies adapting to new hosts).
- Sexual selection leading to reproductive isolation based purely on female choice.
- Polyploidy leading to reproductive isolation through chromosomal duplications, either autopolyploidy or allopolyploidy.
Experimental studies such as those conducted with Drosophila melanogaster demonstrating habitat preference leading to reproductive isolation over generations.

Hybrid zones can form where previously isolated populations reunite.
- Example: hybrid zones of toads in Europe showing distinct breeding patterns.
Outcomes of Hybridization:
1. Fusion: High gene flow may lead to species fusing when not sufficiently differentiated.
2. Stability: Hybrid zones may maintain for generations, despite low fitness in hybrids.
3. Reinforcement: Natural selection favors prezygotic barriers to reduce hybridization, enhancing reproductive isolation, observable in some frog species.