Understanding Speciation

• Speciation is the evolutionary process by which new biological species arise.

Process of Speciation

• Cessation or reduction of gene flow allows for divergence between populations.
• Divergence is influenced by drift, selection, and mutation.
• When diverging populations come back into contact fusion, reinforcement, or stability can occur.
• Isolation prevents gene flow and can be physical (allopatry) or biological (sympatry).
• Genetic divergence occurs through drift and selection.
• Reproductive isolation happens through secondary contact.

Geography of Speciation

• Allopatric speciation involves geographic separation.
• Sympatric speciation occurs without geographic separation.

Physical Isolation - Allopatry

• Allopatry arises from dispersal, where a subset of the population disperses to a new, isolated region, leading to the founder effect.
• Allopatry also arises from vicariance, where extrinsic factors subdivide a population.
• The older the island, the more ancient the lineage.

Vicariance

• Vicariance is allopatry that arises because extrinsic factors act to subdivide a population.
• Snapping shrimp were divided by the Isthmus of Panama, resulting in six sister pairs of species.
• Snapping shrimp clade was used as an outgroup.

Biological Isolation - Sympatry

• Biological isolation results in speciation without a physical or spatial barrier.
• Snail coiling, where right and left-coiled snails cannot mate, is determined by a single gene.
• Chromosomal speciation is common in plants and involves polyploidy.
• Chromosomal speciation means an individual fails to reduce chromosomes during meiosis resulting in a 4N chromosome number.
• 2N X 4N results in 3N, which cannot reproduce due to unequal chromosome separation and creates infertile offspring.
• Plants can self-fertilize until they can mate with another polyploid individual (4N X 4N).

Speciation - Host Shift

• Some insects, such as the "apple maggot" Rhagoletis pomonella, can shift host, driving speciation.
• Apples are not naturally found in North America.

Geographic Isolation and Importance

• Two different species of three-spined sticklebacks exist in each of five lakes (10 species total).
• Benthic sticklebacks feed on large food on the bottom of the lake, while limnetic sticklebacks feed on small plankton in open water.
• Gene flow homogenizes populations and prevents speciation.
• Speciation can occur with gene flow.
• The isolation needed for speciation is variably.
• Focus has shifted to the cause of divergence.

Ecological Speciation

• Ecological speciation is somewhat parapatric and sympatric speciation from text, but broader.
• Ecological Speciation focuses on niche/resources instead of geography.
• Strong divergent selection is necessary for ecological speciation.

Secondary Contact

• Secondary contact is the 'moment of truth' in speciation.
• 'Secondary' contact might have been continuous.
• Isolating divergent populations, possible outcomes, and patterns in nature are important considerations.

Reproductive Isolation

• Reproductive Isolation prevents interbreeding.
• Dobzhansky categorized the types of reproductive isolation as prezygotic and postzygotic.
• Pollinator isolation is where species are pollinated by different pollinators.
• Temporal isolation means reproductive is different due to the time of reproduction.
• Brown and rainbow trout live in the same streams but breed at different times of the year.

Behavioral Isolation

• Behavioral Isolation means species are different due to behaviors.
• Gametic Isolation means that marine invertebrates have egg that does not recognize sperm due to incompatible receptors.

Postzygotic Barriers

• Fertilization occurs, but there is hybrid inviability, hybrid sterility, or low hybrid fitness.

Genetics of Isolation

• Most populations have substantial genetic variation.
• Reproductive isolation is caused by structural changes and incompatibilities.

Importance of Recombination

• Recombination provides variation but also inhibits speciation.

Chromosomal Inversion

• Adaptive divergence in the monkey flower Mimulus guttatus is maintained by a chromosomal inversion.
• Systems can demonstrate these genetic qualities without obvious chromosome-scale changes.
• Negative epistasis between alleles causes severe fitness cost on hybrids.
• Some systems don't have obvious chromosome-scale changes.
• It is important to consider ancestral populations over time when examining loci.
• Some loci may be different as sex chromosomes.
• Sex chromosomes determine sex. XX is female and XY is male. ZZ is male and ZW is female.

Haldane’s Rule

• Among hybrid offspring, if one sex is absent, it's the heterogametic one.

Secondary Contact Zones

• Secondary contact zones are areas of overlap in range after speciation.
• Hybrids may form if isolation is incomplete.
• Selection may act to reduce hybridization if hybrids have lower fitness than parents.

Hybrid Zones Over Time

• Hybrids of closely related species meet in a hybrid zone, there are three possible outcomes: strengthening of reproductive barriers (Reinforcement), weakening of reproductive barriers (Fusion), and continued formation of hybrid individuals (Stability).

Interactions In Sympatry

• Character displacement is the process of competition for resources that leads to divergence.
• Adaptive evolution of traits minimizes deleterious reproductive interactions between species: reproductive character displacement, Reinforcement.