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Chapter 17: Sympatric Collapse and Niche Partitioning = Speciation Without Barriers

Against intuition, species can diverge while sharing the same geographic space. This chapter explores how ψ = ψ(ψ) achieves bifurcation through ecological differentiation rather than spatial separation.

17.1 The Sympatric Challenge​

Definition 17.1 (Sympatric Speciation): Divergence despite gene flow potential: One population→disruptive selectionTwo species in same location\text{One population} \xrightarrow{\text{disruptive selection}} \text{Two species in same location}

Requirements:

  • Disruptive selection
  • Assortative mating evolution
  • Linkage disequilibrium maintenance
  • Reproductive isolation emergence

17.2 Theoretical Foundations​

Theorem 17.1 (Sympatric Conditions): Speciation possible when: s>r2⋅1hs > \frac{r}{2} \cdot \frac{1}{h}

where ss is selection strength, rr is recombination rate, hh is dominance.

Proof: Strong disruptive selection must overcome recombination's homogenizing effect. ∎

Key insight: Selection must create correlation between ecological traits and mating preferences.

17.3 Disruptive Selection​

Fitness minima drive divergence:

W(ψ)=Wmax⁡−(ψ−θ1)2⋅(ψ−θ2)2W(\psi) = W_{\max} - (\psi - \theta_1)^2 \cdot (\psi - \theta_2)^2

Creating:

  • Two fitness peaks
  • Intermediate disadvantage
  • Divergent selection
  • Polymorphism maintenance

17.4 Assortative Mating Evolution​

Definition 17.2 (Mating Preference): Correlation between trait and preference: Pmating=exp⁡(−α∣ψm−ψf∣)P_{\text{mating}} = \exp(-\alpha|\psi_m - \psi_f|)

Mechanisms:

  • Pleiotropy (trait affects preference)
  • Linkage (preference alleles near trait alleles)
  • One-allele mechanism (universal preference)
  • Two-allele mechanism (matching preference)

17.5 African Cichlid Radiation​

Lake cichlids exemplify sympatric speciation:

>500 species in Lake Malawi\text{>500 species in Lake Malawi} >500 species in Lake Victoria\text{>500 species in Lake Victoria} >250 species in Lake Tanganyika\text{>250 species in Lake Tanganyika}

Mechanisms:

  • Visual system tuning
  • Jaw morphology specialization
  • Male coloration diversity
  • Behavioral differences

Creating species flocks in single lakes.

17.6 Polyploid Speciation​

Theorem 17.2 (Instant Isolation): Chromosome doubling creates barriers: 2n×2n→4n2n \times 2n \rightarrow 4n 2n×4n→3n (sterile)2n \times 4n \rightarrow 3n \text{ (sterile)}

Common in plants because:

  • Vegetative reproduction possible
  • Polyploidy often viable
  • Immediate isolation
  • Novel gene expression

17.7 Host Race Formation​

Specialization on different resources:

Fitness on host A×Preference for A>Fitness on B×Preference for B\text{Fitness on host A} \times \text{Preference for A} > \text{Fitness on B} \times \text{Preference for B}

Apple maggot fly example:

  • Native hawthorn host
  • Introduced apple trees
  • Divergent emergence times
  • Host preference evolution
  • Incipient speciation

17.8 Sexual Selection Drive​

Definition 17.3 (Fisherian Sympatric): Runaway process in place: dtdT=GTP⋅βP\frac{dt}{dT} = G_{TP} \cdot \beta_P dpdT=GTP⋅βT\frac{dp}{dT} = G_{TP} \cdot \beta_T

where trait-preference coevolution creates divergence.

Requirements:

  • Initial variation
  • Female preference polymorphism
  • Male trait response
  • Positive feedback loops

17.9 Magic Traits​

Traits under divergent selection that also affect mating:

ψmagic→affects{Ecology,Mating}\psi_{\text{magic}} \xrightarrow{\text{affects}} \{\text{Ecology}, \text{Mating}\}

Examples:

  • Body size (resource use + mate choice)
  • Coloration (crypsis + sexual display)
  • Timing (resource availability + breeding synchrony)
  • Habitat preference (ecology + encounter rate)

17.10 Chromosomal Mechanisms​

Theorem 17.3 (Recombination Suppression): Inversions link co-adapted alleles: rinverted≈0r_{\text{inverted}} \approx 0

Creating:

  • Supergenes
  • Linked ecological traits
  • Protected polymorphisms
  • Divergence despite gene flow

17.11 Microgeographic Variation​

Small-scale environmental heterogeneity:

Selection mosaic=∑isi(x)⋅ψi\text{Selection mosaic} = \sum_i s_i(\mathbf{x}) \cdot \psi_i

Examples:

  • Heavy metal tolerance on mine tailings
  • Thermal races around hot springs
  • Salinity adaptation in estuaries
  • Altitude races on mountains

Gene flow overwhelmed by selection.

17.12 The Sympatric Paradox​

Theory suggests sympatric speciation is difficult, yet spectacular radiations exist:

Difficulty: Gene flow homogenizes Reality: Explosive diversification in lakes

Resolution: Sympatric speciation requires special conditions—strong disruptive selection, sexual selection, or polyploidy—but when these align, diversification can be rapid. The key is establishing positive feedback between ecological divergence and reproductive isolation. Once initiated, this feedback accelerates: ecological differences reduce encounters, reduced encounters allow further divergence, further divergence strengthens mating preferences. Through this recursive process, ψ discovers how to bifurcate even while sharing space, proving that geographic isolation, while helpful, is not mandatory for speciation.

The Seventeenth Echo​

Sympatric speciation reveals ψ's capacity for differentiation through pure ecological and behavioral means. Without mountains or oceans to separate them, populations diverge through the subtle geography of resource space and the powerful dynamics of mate choice. Each sympatric species pair demonstrates that isolation is ultimately not about space but about gene flow—and gene flow can be interrupted by ecology as effectively as by geography. In these same-place speciations, we see evolution's creativity in its purest form: ψ multiplying itself through niche discovery rather than geographic accident.

Next: Chapter 18 explores ψ-Instability in Hybrid Zones, examining the dynamics where divergent populations meet.