Chapter 24

Concept 24.1: The Biological Species Concept

  • Species = a group of populations whose members can potentially interbreed in nature and produce viable, fertile offspring
  • Gene flow between populations holds a species together genetically
  • Emphasizes reproductive isolation between species

Reproductive Isolation (overview)

  • Biological barriers that prevent interbreeding or reduce fitness of hybrids
  • Can act before (prezygotic) or after (postzygotic) fertilization

Prezygotic Barriers

  • Block fertilization from occurring or prevent mating
  • Examples: habitat, temporal, behavioral, mechanical, and gametic barriers

Postzygotic Barriers

  • Hybrids form but have reduced viability or fertility, or hybrid lineages are weak (hybrid breakdown)

Reproductive Isolation: Barriers in Detail (8 of 8 essential types)

  • Habitat Isolation: species in different habitats rarely meet (e.g., apple vs. blueberry maggot flies)
  • Temporal Isolation: breed at different times (season or time of day)
  • Behavioral Isolation: distinct courtship -> effective barriers (e.g., blue-footed boobies)
  • Mechanical Isolation: morphological differences prevent mating (e.g., Bradybaena snails)
  • Gametic Isolation: sperm and eggs incompatible or sperm cannot fertilize eggs
  • Postzygotic: Reduced Hybrid Viability, Reduced Hybrid Fertility, Hybrid Breakdown

Reproductive Isolation: Key Limitation

  • May not apply to fossils or asexual organisms; examples of occasional interbreeding (grolar bears)

Other Definitions of Species

  • Morphological Species Concept: based on structural features; applicable to sexual/asexual; subjective
  • Ecological Species Concept: defined by ecological niche; emphasizes disruptive selection

Concept 24.2: Speciation with or without Geographic Separation

  • Allopatric Speciation: populations geographically isolated
  • Sympatric Speciation: populations not geographically isolated

Allopatric Speciation

  • Gene flow interrupted by geographic isolation; new species form in separated subpopulations
  • Examples: lakes, colonization of remote areas (Galápagos cormorant)

Process of Allopatric Speciation

  • Isolated gene pools diverge via mutation, natural selection, genetic drift
  • Reproductive isolation may arise as a by-product of divergence
  • Example: mosquitofish divergence under different predation levels

Evidence of Allopatric Speciation

  • Lab populations diverged due to different environments (e.g., fruit flies with different diets)
  • Nature: snapping shrimp lineages diverged after Isthmus of Panama formation
  • Isolated regions tend to have more species; geographic distance correlates with reproductive isolation

Allopatric vs. Geographic Isolation Note

  • Physical separation prevents interbreeding but is not a biological barrier on its own

Sympatric Speciation

  • Occurs in the same geographic area; less common; barriers are intrinsic to the organism
  • Mechanisms: Polyploidy, Sexual Selection, Habitat Differentiation

Polyploidy (Sympatric Speciation Mechanism)

  • Definition: presence of extra chromosome sets; n, 2n, 4n as key levels
  • Autopolyploids: more than two chromosome sets from a single species
    • Tetraploid 4n can arise from a diploid 2n; fertile offspring via self-fertilization or mating with other 4n
  • Allopolyploids: chromosome sets from different species; hybrids typically sterile unless chromosome doubling occurs
    • After doubling, can form fertile tetraploids that can interbreed with each other
    • Do not interbreed with either parent species
    • Example: Tragopogon ( Tragopogon species example of allopolyploid speciation)

Polyploidy in Practice

  • Polyploidy common in plants; underlies many agricultural crops (e.g., oats, cotton, potatoes, tobacco, wheat)
  • New polyploid species can arise rapidly within a generation

Sexual Selection and Habitat Differentiation (Sympatric Speciation)

  • Sexual selection: mate-choice can drive isolation (e.g., cichlids in Lake Victoria)
  • Habitat differentiation: exploiting new habitats/resources can reduce gene flow (e.g., hawthorn vs. apple flies)
  • Consequences: host or habitat switches can create premating or postzygotic barriers

Allopatric and Sympatric Speciation: Quick Review

  • Allopatric: geographic isolation + divergent evolution -> reproductive barriers
  • Sympatric: reproductive barriers arise without geographic separation (polyploidy, sexual selection, habitat shift)

Concept 24.3: Hybrid Zones and Reproductive Isolation

  • Hybrid zone: region where members of different species mate and produce hybrids
  • Arises when barriers are incomplete

Patterns within Hybrid Zones

  • Some zones are narrow bands where related species meet and interbreed
  • Hybrids often have reduced viability or fertility
  • Outside zones, gene flow may be limited by selection in parental habitats

Hybrid Zones and Environmental Change

  • Climate or habitat changes shift hybrid zones or create new ones
  • Alleles can flow between species via hybrids, aiding adaptation to new conditions

Hybrid Zones Over Time: Possible Outcomes

  • Reinforcement: strengthen barriers if hybrids have lower fitness
  • Fusion: barriers weaken; species fuse into one
  • Stability: ongoing hybridization with maintained barriers

Reinforcement Details

  • Stronger prezygotic barriers when hybrids are less fit
  • Often stronger in sympatric populations

Fusion Details

  • Substantial gene flow if hybrids are as fit as parents
  • Pollution can blur species distinctions (e.g., cichlids in Lake Victoria)

Stability Details

  • Continued gene flow into the hybrid zone can maintain hybrids and prevent full isolation

Concept 24.4: Speciation can be Rapid or Slow; Few or Many Genes Involved

  • Timeframe of speciation is debated; can be rapid or gradual
  • Evidence from fossil record and genetics

Patterns in the Fossil Record

  • Punctuated equilibria: rapid changes interspersed with long periods of stasis
  • Some lineages show gradual change

Speciation Rates

  • Rates vary widely across taxa; evidence from lab studies and fossils
  • Example: Helianthus anomalus arose via hybridization followed by rapid speciation
  • Time between speciation events ranges widely among groups

Genetics of Speciation

  • Speciation may involve changes in a single gene or many genes
  • Examples: Japanese Euhadra snails (single gene affects mating orientation of shells), Mimulus (flower color involves at least two loci)

From Speciation to Macroevolution

  • Accumulation of differences across many speciation events leads to major evolutionary changes
  • Extinction reduces lineage size; macroevolution is the cumulative effect of speciation and extinction