Show & Tell 36.4 — PopulationManager: Speciation & Island Model Deep-Dive

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PopulationManager — Algorithm Deep-Dive

Speciation (Stanley & Miikkulainen 2002)

SPECIATE(population, species_list, threshold):
  for each individual in population:
    placed ← False
    for each species in species_list:
      rep ← species.representative
      if compatibility_distance(individual, rep) < threshold:
        species.add(individual)
        placed ← True
        break
    if not placed:
      new_species ← Species(representative=individual)
      species_list.append(new_species)
  // Update representatives
  for each species in species_list:
    species.representative ← random_choice(species.members)
  // Remove empty species
  species_list ← [s for s in species_list if len(s.members) > 0]

Dynamic Threshold Adjustment

ADJUST_THRESHOLD(threshold, num_species, target):
  if num_species > target:
    threshold += 0.3  // be more permissive
  elif num_species < target:
    threshold -= 0.3  // be more strict
  return max(0.1, threshold)

Island Model Migration

ISLAND_MIGRATE(islands, config):
  if generation % config.interval != 0:
    return islands
  for each island_i:
    neighbors ← get_neighbors(island_i, config.topology)
    for each neighbor in neighbors:
      migrants ← select_migrants(neighbor, config.migration_rate)
      replacees ← select_worst(island_i, len(migrants))
      island_i.replace(replacees, migrants)
  return islands

Topologies:

• Ring: each island connects to 2 neighbors — slow spread, preserves diversity
• Star: central island connects to all — fast spread, hub-and-spoke
• Fully Connected: all-to-all — fastest convergence, least diversity
• Random: stochastic connections each generation — balanced tradeoff

Diversity Metrics

1. Genotypic diversity: mean pairwise Hamming/Euclidean distance
2. Phenotypic diversity: behavioral characterization variance
3. Species count: number of active species (proxy for niche coverage)

Related: #742