MODES OF SPECIATION

MODES OF SPECIATION

1. SPECIATION

   • evolution of strong, genetically-based isolating barriers between populations that prevent them from exchanging genes

2. MODELS

   • Rates of Speciation
     • Gradual
       • gradual evolution of genetic differences that ultimately results in reproductive isolation; takes many generations
       • extension of micro-evolutionary process
       • most common type of speciation
     • Sudden - also termed saltational, abrupt, and quantum speciation
       • sudden origin of a new species due to a single genetic change (or a few related changes); occurs in a single generation (or a few)
       • mechanism
         • 1) change of breeding system, where a sexually reproducing ancestor gives rise to an asexual line
           • mutation
             • e.g. ostracod - mutation in egg production from meiosis - mitosis
           • interspecific hybridisation
             • e.g. Whiptail Lizards (Cnemidophorus) - double chromosomes - duplicate couple with each other & undergo meiosis - diploid eggs
         • (2) chromosomal rearrangements --> chromosomal incompatibilities and reproduction isolation
           • e.g. goats beard (tragopogon)
             • have diploid sexual sp. interspecific hybrids - infertile (only reproduce vegitatively)  polyploid derivatives - 
   • Geography
     • allopatric
       • subdivision
         • - Large continuous population - A physical barrier more or less equally sub-divides the population - Immigration ceases: large sub-populations on either side of barrier evolve independently & gradually diverge  - Secondary contact without interbreeding = speciation
         • def: ancestral population is subdivided into two large sub-populations, each of which forms a new species
         • e.g. broad range of closely related species pairs in the western Atlantic and eastern Pacific - originated via the subdivision of ancestral populations as the Isthmus of Panama emerged ca. 20 - 3 million years ago (marine sp.)
         • e.g. Alpheus (in this genus - 20 pairs closely related)
         • CONC : - subdivision can generate congruent patterns of diversification in many taxa - speciation by subdivision is likely to be relatively slow (million of years)
       • peripheral isolation
         • def: new species evolves from a small population isolated at the edge of the ancestral species range
           • - Ancestral population  - Small number of individuals colonise an unusual habitat -- "Isolate" evolves independently & diverges relatively rapidly  - Isolated from parental population  - Secondary contactwithout interbreeding =speciation
         • e.g. Paradise Kingfishers (Tanysiptera) in New Guinea
         • CONC: speciation by peripheral isolation can occur over very small spatial and temporal scales
       • def: new species arise as a result of geographic isolation from the ancestral species (involves the evolution of an extrinsic barrier to evolution)
     • sympatric
       • def: • speciation within a population • involves the evolution of intrinsic barriers to gene flow
         • - Group of individuals adopt a new niche within the range of the parental species - Individuals mate within, and adapt to, their niche  - Evolution of reproductive isolation without geographic isolation
       • Conditions Required for Sympatric Speciation
         • most models postulate:  - heritable polymorphism based on resource use (food) - disruptive selection between ecomorphs (temp for mating) - assortative mating within ecomorphs
       • Why is the Idea of Sympatric Speciation Controversial?
         • • because the incipient species are in contact and any interbreeding between them will breakdown linkage disequilibrium and prevent the formation of genetically distinct subgroups • this is a problem for gradual speciation (sudden and sympatric models are compatible) • very few convincing examples (although this is also true of allopatric models)
         • sp in contact- probability to breed - prevent genetically distinct subgroups
         • problem for gradual speciation
       • e.g.apple maggot fly (Rhagoletis pomonella) in North America
         • • are morphologically indistinguishable • are genetically differentiated (very limited gene exchange) • show strong (& heritable) preference for their own fruit type
         • Origin of Apple race
           • - recent ~400 intro to N.A. - evolved from the hawthorn race in sympatry - divergence due to host shift (x geographic isolation) - gene exchange is limited - cz they court, mate, develop near the preferred fruit • the isolation is reinforced by the fact that the hosts fruit at different times • hence different alleles are favoured in different races because the larvae develop under different temperature regimes • there is limited gene flow between the races, but the effects are overwhelmed by selection
       • How Common is Sympatric Speciation?
         • (a) occurs mainly in specialised cases, such as via a host shift in phytophagous insects
         • (b) common & widespread
       • CONC: - sympatric speciation may underpin many adaptive radiations - it is necessarily relatively rapid
     • parapatric
       • - Ancestral population  - Individuals expand into new habitat but retain limited contact - Divergent selection counteracts the effects of limited gene exchange - Eventually reproductive isolation may evolve
       • e.g. Antoxanthum odoratum at mine sites
     • Species Flocks
       • def: an assemblage of very LARGE NUMBER of closely related species occurring in a SMALL GEOGRAPHIC AREA
       • e.g. cichlid flocks in African Great Lakes
         • so many different sp of Cichlids in a small area of water stream
       • origin - sympatric?? (uncertain) - adapt to eating on different preys

3. MECHANISM

   • Ecological
     • def: process by which barriers to gene flow evolve as a result of ecologically-based, divergent selection
     • CONTEXT:
       • speciation has traditionally been studied from a geographic perspective (allopatric versus sympatric)
       • recently, more emphasis has been placed on the mechanisms for the evolution of reproductive isolation
     • e.g. parallel evolution in the threespined stickleback (Gasterosteus)
       • - The marine form has repeatedly invaded freshwater. - Freshwater forms have independently reduced the amount of armour and pelvic structures.
       • limnetic and benthic forms
         • - A few lakes contain both limnetic and benthic forms. - These forms are morphological differentiated. - Parallel changes appear to have occurred independently in separate lakes.
       • Some of the changes appear to be linked to diet and intensity of predation, i.e., appear to be adaptive
     • CONC: The repeated independent evolution of some of the same traits implies an adaptive response that is driven by divergent natural selection.

4. REINFORCEMENT

   • def: • process by which natural selection increases reproduction isolation • can be prezygotic or postzygotic
   • Postzygotic - Hybrid inviability Hybrid infertility Hybrid breakdown
     • inviability-  infertility - zebroid (zebra mixed w other type of horses - infertile) breakdown - fertile hybrid-backcross into the sp but they never do quite as well (the hybrid never last very long through generations)
   • Prezygotic - Behavioural Temporal Habitat Chemical Mechanical
     • behavioural - choose their mate temporal - flowering time- seasons- difft flower types chemical - feromone type mech - structure of the flower
   • What Role does Reinforcement Play in Speciation?
     • not essential for allopatric models (although it could still play a role)
     • essential for sympatric and parapatric models
       • in exp, sp from allopatric speciation tend to breed w each other when put together (no prezygotic).  for sympatric - there's prezygotic speciation (they tend to breed w chosen sp - selection is so strong )

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