Speciation: The Origin of New Species
According to Charles Darwin (1859),

     Natural Selection explains change within species.
      Over time, descendants diverge from their ancestors.
      When the difference is arbitrarily large enough,
             they are recognized as new species (nominalistic concept).
      Variation is therefore continuous.

    => The "Origin of Species" is not really about the origin of species,
         Natural Selection accounts for change within species (anagenesis)
         How do we explain speciation (cladogenesis)?

Modes of Speciation (See Futuyma 1997)

Traditional classification is by degree of geographic separation.
      How does reproductive isolation develop in a polytypic species?
        RIMS = Reproductive Isolating Mechanisms
                   arise fortuitously, epistatically
        SRMS = Species Recognition Mechanisms
                   arise by natural selection

Modern classification emphasizes population genetic processes involved.
      What genetic changes occur during speciation?

Speciation by Allopatric Divergence

1. A widespread species is held together by gene flow

2. Gene flow is interrupted or reduced
      isolation by distance - limited vagility produces polytypic subspecies
      extrinsic barriers - physical barriers to gene flow
             mountains, canyons, rivers, deserts, etc.
      vicariance  - populations move, or are moved apart
             continental drift, land bridges, range expansion
             change of watercourse, movement of glacier

3. Separated populations adapt to local conditions
      Populations 'track' different environments => they diverge biologically
      Divergence results in reproductive isolation

      Pre-mating reproductive isolation: failure to mate
        Physical: no meeting, no mating
        Ecological: Odocoileus spp. differ wrt microclimate
        Seasonal: Angiosperm flowering times differ on N / S cline
        Temporal: Reproductive cycles triggered by day length
        Ethological: divergence of mating behaviours
                                   different flashing patterns among firefly species (Lampyridae)

      Post-mating reproductive isolation: failure to mate successfully
        Mechanical: 'lock and key' genitalia in Insecta
        Gamete incompatibility: no capacitation reaction between egg & sperm
        Hybrid inviability: failure of gastrulation during development
        F1 sterility: female horse x male donkey => mule (v.v. => 'jenny')
        F2 / B1 breakdown: white-crowned sparrow (Zonotrichia) songs

4. Physical contact is re-established
      Pre-mating isolation may be sufficient to prevent mating
      Character Displacement= reproductive isolation 'exaggerated' in contact
             'Indiscriminant' parents leave fewer offspring,
                   ability to discriminate reinforced by selection
        Phlox species pink in allopatry; one spp. white in sympatry
        Sitta nuthatches have different bill sizes in sympatry

      Post-mating isolation may lead to 'dead end' hybridization
      Hybrid Sinks = hybrids found only in narrow zones bx species
        Mus musculus & M. domesticus meet & mate in Jutland: 100s yrs old

Speciation by Founder Effect

1. A widespread species, with physical limits determined by 'adaptive limits'
      ["Goldilocks' Rule": not too hot, too cold, too wet, too dry, etc.]
      Occasional opportunities in suitable habitat patches

2. Dispersers found new 'colonies' beyond species' periphery
      Colonies are 'experiments', in new & different habitats
      Most colonies are unsuccessful: they fail or are resorbed

3. Founders may be successful because of a "Genetic Revolution"
      Bottleneck: founders have small Ne, low H, non-random sample of alleles
      Drift  > Selection during first few generations, while Ne small
      => New colony is genetically 'unique': may 'fix' new alleles at many loci

      Isolation arises quickly & simultaneously for entire colony:

      Pre-mating isolation:
       runaway sexual selection fixes new mating behaviour in colony
       Hawai'ian picture-wing Drosophila  show aberrant morphotypes

      Post-mating isolation:
        chromosome rearrangements become fixed in colony (inversions, fissions, fusions)
             Mutation may confer selective advantage
              AR & CH vs. ST inversions in Drosophila
              Position Effect = expression of locus is influenced
                          by change of physical location wrt other loci
             Inversions are deleterious in heterokaryon
                   [heterokaryon =a chromosomal 'heterozygote';
                     cf. homokaryon]
             Crossing-over in heterokayrons produces duplications / deletions
                   semi-sterile (W = 0.50) if these are unfit

        Prob. of fixation of homokaryons greatly increased if
              small Ne (Founder Effect):
                          drift fixes homokaryon by chance
               high F (inbreeding) if founders are related:
                          higher proportion of homokaryons produced.

4. Successful colony expands to fill habitat patch, or contacts parental population.

     A computer model of Founder Effect speciation
            (NatSel Exercise #5)
             New chromosome mutant arises as heterokaryon (AB).
        BB has 50% advantage with respect to AA homokaryon,
        AB heterokaryon is semi-sterile (disruptive selection).
             Under what conditions can BB be fixed in a colony?

              #AB=1, #AA= 9 or 99, F= 0.0 or 0.50, W0 = 1.0, W1=0.5, W2=1.5

Biological & geological patterns of speciation

Speciation by Allopatric Divergence is characteristic of 'typical' vertebrates
      Mammals (especially Carnivora) & birds
             Look for ecogeographic, clinal subspecies
             highly vagile, large Ne, K-selected, ecological generalists
        Martes shows western Palearctic -> eastern Nearctic size cline
      Freshwater fish:
             waterfalls & shallows, watershed recaptures create barriers
      Epicontinental marine fishes:
             N/S temperature gradients, river outflow
             (Ex.: Gulf of St. Lawrence)

      Biogeography of Nearctic heavily influenced by Pleistocene Glaciations
        Refugia in BC islands, American SW & SE during Wisconsin Ice (7-12,000 BP)
             Newfoundland glaciated < 7,000 BP: Flemish Cap exposed
                   [E. C. Pielou, "After the Ice"]

Founder Effect speciation is characteristic of two contrasting types
      Vagile, K-selected, generalists with potential for very small Ne
             Successful colonies founded by 'families'
                   2 parents + 2 offspring is basic social unit
                   single gravid female
             Primates, ungulates [Equidae, Cervidae], some herps
                   Did a "lemur on a log" colonize Madagascar?
                   Neotropical deer have extensive chromosome polymorphism

      Non-vagile, r-selected, specialists with very large R0
             Successful colonies 'bounce back' from initial low N
              Ex.: Drosophila on Hawai'ian kipukas (lava islands)
             Ex.: Guppies (Cyprinidontiformes) have internal fertilization
                    spawning is withheld until environment is right

      Look for peripheral & island isolates, chromosomal complexes

Time course of speciation:

      In classical diagrams (Divergence models), evolutionary change is
        "slow & gradual": 10,000s ~ 1,000,000s of years (Model "B")
                   Process is (in principal) observable in fossil record
                   Rates of change are constant before, during, and after speciation
                   Adaptation occurs continuously
             => Anagenesis produces all evolutionary change

      In 'revisionist' diagrams (Founder models), evolutionary change is
        "rapid & punctuated": 100s ~ 1000s of years (Model "A")
                   "geologically instantaneous": not observable in fossil record
                   Adaptive divergence occurs only during speciation
                   Established lineages undergo no change
             => Cladogenesis produces all evolutionary change

      Iconography influences interpretation:
        Mixed models ("A" vs. "B" & "C" vs. "D") are possible

Variants: Parapatric & Sympatric speciation

Parapatric speciation (AKA Stasipatricspeciation):
          New colonies are founded within home range of parent species
          Characteristic of species with extremely low vagility
          Post-zygotic isolation arises simultaneously with entry to new niche

Case study: Israeli blind mole rat (Nannospalax [Spalax] ehrenbergi)
        [Nevo (1991). Evolutionary Biology 25:1-125].

          1. Four chromosomal races: 2N = 54 52 58  60
                centric fusions

          2. Environmental gradients in temperature, humidity, soil type
                cold north  hot south
                humid coast  arid inland

          3. Very narrow hybrid zones
                Deficiency of heterokaryons in hybrid sinks

          4. Pre-zygotic ethological isolation
                Chromosomal races have different olfactory, vocal signals,
                Intraspecific aggression exaggerated between adjacent races
               e.g., 52 / 58 meet aggressively, 54 / 58 less so

          => Incipient species:
                 Original cytogenetic differences are being reinforced
                     by adaptation to habitat & divergence of pre-mating RIMS
                 [Did chromosomal mutations 'cause' adaptive shifts?]

       cf. Flightless Australian grasshoppers (Morabinae: Orthoptera) (M.J.D. White 1978)

Sympatric speciation

          Habitat patches are sympatric: within "cruising range" of each other
          Strong host / parasite interactions
                host recognition has simple genetic basis
                reproductive isolation precedes niche shift

          A two-locus model:
           Disruptive selection for host preference (AA > A'A < A'A')
           Assortative mating for mate preference (BB > B'B < B'B')
                => two races, AABB & A'A'B'B' "meet & mate" in isolation

 Case study: Apple maggot (Rhagoletis pomonella) (Tephritidae) (Bush 1969)
           1. A parasitoid of fruit (related species on blueberry)
           2. Host shifts from hawthorn   apple (1864)   cherry (1960s)
           3. Extremely limited vagility: "Meeting & Mating" on same piece of fruit
           Chemotaxis for host recognition
           Mating, oviposition, diapause are keyed to fruiting time
           4. Primary mutation for host recognition
                secondary mutations for host shifts

          How common? Unlikely in vertebrates (50,000 spp.) because of vagility
                                     But:  > 500,000 species of parasitoid insects

Text material © 2010 by Steven M. Carr