Speciation

Biological Species Concept (Ernst Mayr): Species are groups of actually or potentially interbreeding populations that produce fertile offspring and are reproductively isolated from other such groups.

Strengths: • Emphasizes reproductive isolation (key to maintaining species identity) • Focuses on gene flow (or lack thereof) • Works well for many sexually reproducing organisms • Testable through breeding experiments

Limitations:

1. Cannot apply to: • Asexual organisms (bacteria, some plants) • Extinct organisms (fossils) • Organisms separated by geography (can't test interbreeding)

2. Ring species problem: • Adjacent populations can interbreed • But distant populations cannot • Where do you draw species line?

3. Microorganisms: • Extensive horizontal gene transfer • Species boundaries unclear

4. Plants: • Many species can hybridize and produce fertile offspring • Yet maintain distinct identities

Alternative concepts: • Morphological species (based on structure) • Ecological species (based on niche) • Phylogenetic species (based on evolutionary history) • Genetic species (based on DNA similarity)

Key insight: No single species concept works perfectly for all organisms!

ALLOPATRIC SPECIATION ("other homeland"): Mechanism: • Geographic separation of populations • Physical barrier prevents gene flow • Independent evolution in isolation • Reproductive isolation evolves as byproduct

Steps:

1. Geographic isolation (barrier forms)
2. Genetic divergence (mutation, drift, selection)
3. Reproductive isolation evolves
4. Populations can no longer interbreed (even if reunited)

Example: Darwin's finches on Galápagos Islands • Ancestral finch colonized islands • Island populations isolated • Evolved different beak shapes for different foods • Now 13+ distinct species

Other examples: Grand Canyon squirrels, Hawaiian silverswords

SYMPATRIC SPECIATION ("same homeland"): Mechanism: • Speciation WITHOUT geographic separation • Occurs within same area • Reproductive isolation evolves first • Gene flow reduced by behavioral or genetic factors

Mechanisms:

1. Polyploidy (especially in plants) • Chromosome doubling → instant reproductive isolation
2. Sexual selection • Mate preference divergence
3. Ecological specialization • Different niches → different selection pressures

Example: Polyploidy in wheat • Chromosome doubling creates new species instantly • Common in plants (30-70% of angiosperms) • Polyploid cannot breed with diploid parent

Other examples: Cichlid fish in African lakes, apple maggot flies

Key Difference: • Allopatric: Geographic isolation comes FIRST • Sympatric: Reproductive isolation comes FIRST (without geographic separation)

Reproductive barriers prevent gene flow between species.

PREZYGOTIC BARRIERS (before fertilization): Prevent hybrid zygote formation

1. Habitat Isolation • Species live in different habitats • Example: Water snakes vs. land snakes • Don't encounter each other

2. Temporal Isolation • Breed at different times • Example: Plants flowering in different seasons • Eastern and Western spotted skunks breed in different months

3. Behavioral Isolation • Different courtship behaviors • Example: Firefly flash patterns • Bird songs and dances

4. Mechanical Isolation • Incompatible reproductive structures • Example: Insect genitalia (lock-and-key fit) • Flower shapes matching specific pollinators

5. Gametic Isolation • Sperm cannot fertilize egg • Example: Sea urchin sperm-egg recognition proteins • Incompatible gamete surface proteins

POSTZYGOTIC BARRIERS (after fertilization): Hybrid zygote forms but has reduced fitness

1. Reduced Hybrid Viability • Hybrid embryos don't develop properly or die • Example: Sheep-goat hybrids die as embryos • Incompatible genes from different species

2. Reduced Hybrid Fertility • Hybrid adults are sterile • Example: Mule (horse × donkey) • Healthy but cannot produce offspring • Chromosome incompatibility during meiosis

3. Hybrid Breakdown • F1 hybrids viable and fertile • F2 or later generations have problems • Example: Some rice hybrids • Cotton hybrids

Key principle: • Prezygotic barriers save energy (no wasted gametes) • Postzygotic barriers are "last resort" • Natural selection favors prezygotic isolation

Polyploidy as Instant Speciation:

Mechanism:

1. Normal diploid plant (2n)
2. Error in meiosis or mitosis → chromosome doubling
3. Results in polyploid (3n, 4n, etc.)
4. Polyploid CANNOT breed with diploid parent • Different chromosome numbers • Meiosis produces unbalanced gametes • Instant reproductive isolation!

Types:

Autopolyploidy (within species): • Chromosome set duplicates within species • Example: 2n → 4n • 4n × 2n → 3n (sterile triploid) • But 4n × 4n → 4n (fertile!)

Allopolyploidy (between species): • Hybridization + chromosome doubling • Example: Species A (2n=14) × Species B (2n=16) • F1 hybrid (n=7+8=15) is sterile • Chromosome doubling → 30 chromosomes • Now can undergo normal meiosis (15 pairs) • Fertile new species!

Why more common in PLANTS:

1. Indeterminate growth • Plants can survive developmental abnormalities better • Animals have more rigid developmental programs

2. Flexible metabolism • Plants tolerate gene dosage imbalances • Animals' physiology more sensitive to gene dosage

3. Reproduction options • Plants can reproduce asexually • Can establish population even if initially sterile • Vegetative propagation (runners, bulbs, etc.)

4. Circulatory system • Plants lack closed circulatory system • Animals with extra chromosomes have blood cell problems • Cell size issues in closed systems

Examples of polyploid species: • Wheat (hexaploid: 6 sets of chromosomes) • Strawberries (octoploid: 8 sets) • Many ornamental flowers • 30-70% of flowering plants!

Evolutionary significance: • Major mechanism of plant speciation • Instant reproductive isolation • No geographic separation needed • Source of genetic variation

Reinforcement: The process by which natural selection strengthens prezygotic reproductive barriers between two populations, reducing the formation of inferior hybrids.

Key Concept: • If hybrids have low fitness, selection favors individuals who DON'T mate with other population • Leads to evolution of stronger prezygotic isolation • Completes the speciation process

Requirements for Reinforcement:

1. Two populations that diverged in allopatry
2. Come back into contact (secondary contact)
3. Can still interbreed but hybrids have reduced fitness
4. Selection against hybridization

Scenario Example:

Step 1: Geographic Isolation • Population of frogs separated by mountain range • Evolve different mating calls in isolation • Mountain range erodes → populations come back into contact

Step 2: Secondary Contact • Populations can still interbreed • Produce hybrid offspring • Hybrids have 50% survival rate (vs. 90% for pure individuals)

Step 3: Selection Against Hybrids • Individuals who mate with own population: more offspring survive • Individuals who mate with other population: fewer offspring survive • Females who prefer their own population's call: higher fitness

Step 4: Reinforcement • Selection favors stronger mating call preferences • Over generations, call preferences become more discriminating • Eventually: populations won't mate with each other at all • Complete reproductive isolation achieved!

Evidence for Reinforcement: • "Reproductive character displacement" • Sympatric populations show stronger prezygotic isolation than allopatric populations of same species • Example: Drosophila in overlapping vs. non-overlapping regions

Alternative Outcome - Fusion: • If hybrids have high fitness, populations merge back together • Reinforcement doesn't occur • Speciation reversed

Key Insight: Reinforcement is natural selection AGAINST hybridization, completing the speciation process!