Evolution 2: Gene Flow, Speciation, & Macroevolution

Gene Flow

• Definition: the (random) movement of alleles among populations

• Modes: via movement of individuals (immigration & emigration) or movement of reproductive structures (pollen, seeds, broadcast spawning)

• Role: can introduce new alleles/traits → source of variation in a population

• Notation: movement of allele(s) between Population A and Population B

Terminology

• Gene Pool: aggregate of all alleles of all individuals within a population

• Example alleles: Bb, BB, bb (illustrative)

Mechanisms of Evolution

• Natural Selection: environment-based; produces adaptive, consistent evolution

• Sexual Selection: subset of natural selection; mate choice drives traits

• Artificial Selection: human-driven trait change

• Genetic Drift: random change; reduces variation; adaptive shifts may be due to luck

• Gene Flow: random event-driven; can be adaptive or introduce new variation

• Mutation: random change introducing new alleles

Speciation and Macroevolution

• Microevolution: small-scale evolution within a population

• Macroevolution: large-scale evolution across species (speciation, extinctions, etc.)

• Evolution on two scales: microevolution vs macroevolution (speciation)

What is a species? (concepts)

• Biological species concept: a group with the potential to interbreed in nature and produce viable, fertile offspring; reproductive compatibility/isolation matters

• Alternative species concepts:

  • Morphological species concept: based on structure

  • Ecological species concept: based on niche

  • Phylogenetic species concept: based on common ancestry

Reproductive Isolation and Barriers

• Reproductive isolation + time → reproductive incompatibility: inability to mate or produce viable, fertile offspring

• Prezygotic barriers: prevent mating or fertilization

  • Habitat isolation, Temporal isolation, Behavioral isolation, Mechanical isolation, Gametic isolation

• Postzygotic barriers: after fertilization; reduce viability or fertility of hybrids

  • Reduced hybrid viability, Reduced hybrid fertility, Hybrid breakdown

Species Barriers Illustrated

• Prezygotic barriers impede mating or fertilization if mating occurs

• Postzygotic barriers prevent viable, fertile offspring

Allopatric vs Sympatric Speciation

• Allopatric: geographic separation creates barriers to gene flow; divergence → speciation

  • Geographic barrier → reproductive isolation → divergence → speciation

• Sympatric: speciation within the same geographic area

  • Mechanisms: habitat differentiation, behavioral changes, sexual selection, polyploidy

Allopatric Speciation

• Geographic barrier leads to reproductive isolation and divergence

• Process: ORIGINAL POPULATION — geographic barrier — reproductive isolation — SPECIATION

Sympatric Speciation

• Mechanisms include habitat differentiation, behavioral differences, sexual selection, polyploidy

• Result: reproductive isolation → divergence → speciation

Sexual Selection

• Definition: preference for certain traits by a subset of individuals can lead to reproductive isolation

• Examples: mate preferences can drive speciation (e.g., color/morphology differences affecting mate choice)

Rate of Speciation

• Can be rapid (punctuated equilibrium) or gradual (gradualism)

• Key figures: Mayr; Eldredge & Gould (punctuation) vs Dennett; Dawkins, Trivers (gradualism)

• Time scales: intervals between speciation events can range from 4{,}000 years to 40{,}000{,}000 years, with an average of 6{,}500{,}000 years

Studying the Genetics of Speciation

• Number of genes involved in new species formation varies

• Examples:

  • Japanese Euhadra snails: shell spiral controlled by a single gene

  • Mimulus (monkey flowers): two loci affect flower color and pollinator preference

• Pollinator-driven isolation: shifts toward hummingbirds vs bees can drive reproductive isolation

• Other organisms may involve larger numbers of genes and gene interactions

Figures and Examples

• Illustrative genetic changes can involve few or many loci depending on the organism

• Specific examples highlight how genetic differences translate to reproductive isolation