MODES OF SPECIATION

1. Gradual
   1. gradual evolution of genetic differences that ultimately results in reproductive isolation; takes many generations
   2. extension of micro-evolutionary process
   3. most common type of speciation
2. Sudden - also termed saltational, abrupt, and quantum speciation
   1. 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)
   2. mechanism
      1. 1) change of breeding system, where a sexually reproducing ancestor gives rise to an asexual line
         1. mutation
            1. e.g. ostracod - mutation in egg production from meiosis - mitosis
         2. interspecific hybridisation
            1. e.g. Whiptail Lizards (Cnemidophorus) - double chromosomes - duplicate couple with each other & undergo meiosis - diploid eggs
      2. (2) chromosomal rearrangements --> chromosomal incompatibilities and reproduction isolation
         1. e.g. goats beard (tragopogon)

            Nota:

            • have diploid sexual sp. interspecific hybrids - infertile (only reproduce vegitatively)  polyploid derivatives - 

1. allopatric
   1. subdivision

      Nota:

      • - 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
      1. def: ancestral population is subdivided into two large sub-populations, each of which forms a new species
      2. 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.)
      3. e.g. Alpheus (in this genus - 20 pairs closely related)
      4. CONC : - subdivision can generate congruent patterns of diversification in many taxa - speciation by subdivision is likely to be relatively slow (million of years)
   2. peripheral isolation
      1. def: new species evolves from a small population isolated at the edge of the ancestral species range

         Nota:

         • - Ancestral population  - Small number of individuals colonise an unusual habitat -- "Isolate" evolves independently & diverges relatively rapidly  - Isolated from parental population  - Secondary contactwithout interbreeding =speciation
      2. e.g. Paradise Kingfishers (Tanysiptera) in New Guinea
      3. CONC: speciation by peripheral isolation can occur over very small spatial and temporal scales
   3. def: new species arise as a result of geographic isolation from the ancestral species (involves the evolution of an extrinsic barrier to evolution)
2. sympatric
   1. def: • speciation within a population • involves the evolution of intrinsic barriers to gene flow

      Nota:

      • - 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
   2. Conditions Required for Sympatric Speciation

      Nota:

      • most models postulate:  - heritable polymorphism based on resource use (food) - disruptive selection between ecomorphs (temp for mating) - assortative mating within ecomorphs
   3. Why is the Idea of Sympatric Speciation Controversial?

      Nota:

      • • 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)
      1. sp in contact- probability to breed - prevent genetically distinct subgroups
      2. problem for gradual speciation
   4. e.g.apple maggot fly (Rhagoletis pomonella) in North America

      Nota:

      • • are morphologically indistinguishable • are genetically differentiated (very limited gene exchange) • show strong (& heritable) preference for their own fruit type
      1. Origin of Apple race

         Nota:

         • - 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
   5. How Common is Sympatric Speciation?
      1. (a) occurs mainly in specialised cases, such as via a host shift in phytophagous insects
      2. (b) common & widespread
   6. CONC: - sympatric speciation may underpin many adaptive radiations - it is necessarily relatively rapid
3. parapatric

   Nota:

   • - 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
   1. e.g. Antoxanthum odoratum at mine sites
4. Species Flocks
   1. def: an assemblage of very LARGE NUMBER of closely related species occurring in a SMALL GEOGRAPHIC AREA
   2. e.g. cichlid flocks in African Great Lakes
      1. so many different sp of Cichlids in a small area of water stream
   3. origin - sympatric?? (uncertain) - adapt to eating on different preys

1. def: process by which barriers to gene flow evolve as a result of ecologically-based, divergent selection
2. CONTEXT:
   1. speciation has traditionally been studied from a geographic perspective (allopatric versus sympatric)
   2. recently, more emphasis has been placed on the mechanisms for the evolution of reproductive isolation
3. e.g. parallel evolution in the threespined stickleback (Gasterosteus)

   Nota:

   • - The marine form has repeatedly invaded freshwater. - Freshwater forms have independently reduced the amount of armour and pelvic structures.
   1. limnetic and benthic forms

      Nota:

      • - A few lakes contain both limnetic and benthic forms. - These forms are morphological differentiated. - Parallel changes appear to have occurred independently in separate lakes.
   2. Some of the changes appear to be linked to diet and intensity of predation, i.e., appear to be adaptive
4. CONC: The repeated independent evolution of some of the same traits implies an adaptive response that is driven by divergent natural selection.

Nota:

• 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)

Nota:

• behavioural - choose their mate temporal - flowering time- seasons- difft flower types chemical - feromone type mech - structure of the flower

1. not essential for allopatric models (although it could still play a role)
2. essential for sympatric and parapatric models

   Nota:

   • 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 )