Patterns of Geographic Variation within Species


      Are observed patterns adaptive?
      Does within-species variation contribute to among-species variation?
      Do species differences have their origin within species?

Polymorphic variation is usually structured spatially.
      Many single-locus traits are locally 'chaotic'
       Colour & pattern variants occur within plant & animal species:
       HbS is scattered uniformly within a community.

      Many show large-scale geographic patterns:
       HbS is common in Congo Basin, uncommon outside Africa.

      Variation often occurs as a cline: a 'character gradient' in space
             produced by differential selection, balanced by gene flow.

          In the absence of selection (s = 0),
             exchange of two individuals between populations per generation
                 prevents evolutionary divergence.
            If gene flow is high, cline is smooth;
                 if gene flow is low, cline is 'stepped'
          Shape depends on
       Intensity of selection: fitness differential between AA and BB
       Biogeography: environmental gradient, barriers to dispersal
       Vagility of organisms (= potential for movement)

       ABO blood groups in Homo
            f(B) in west = 0.0 , f(B) in east = 0.30
             Smooth cline across Europe: q = 2.5 x 10-8 / meter
             Disease resistance? Historical invasions?
                Lowest f(B) coincides with Basque population

      Heavy-metal tolerance  in wild grass (Agrostis)
              Mine tailing are contaminated with heavy metals (Cu++)
             15%  55% over 40m at mine,  20% downwind over 160 m
                [seed tolerance > adult tolerance, downwind]
                rapid change: q = ~ 1% / m

Polytypic variation is discontinuous, discrete (multiple 'types')
      Within-species variation is recognized as subspecies (Fig 9-21) with trinomials:
           Ex.: Rat Snakes (Elaphe obsoleta) occur as geographically disjunct subspecies
           Ex.:  Newfoundland pine marten (Martes americana atrata) is larger & darker (endangered)

      Patterns of subspecific variation are well-documented:
      Can they be interpreted biologically / adaptively?

Monotypic species: no geographic variation
       Lynx (Lynx canadensis canadensis) is monotypic on mainland:
          L. canadensis subsolanus is larger in Newfoundland
            (Typical of highly vagile species)

      Allopatric distributions: physically separate

       Holarctic distributions: (= occurring in Palearctic & Nearctic zones)
           Rangifer tarandus is 'Reindeer' in OW, 'Caribou' in NW
           Cervus elaphus is Red Deer in OW, Wapiti in NW

             Oceanic islands:
                   Nine of 20 subspecies of least weasel (Mustela erminea) are insular
                   [This can be a circular argument: isolation => distinctness?]

             Ecological 'islands' [a lake is an island, to a fish]:
              Mexican shrew (Sorex) lives only at high elevation.
                   Six subspecies are found on six different mountaintops.
 

      Parapatric distributions: physically contiguous (adjacent)
             Distributions may overlap, producing clinal variation

             Red-shafted (W) vs. yellow-shafted (E) flickers (Colaptes)
            Narrow step-cline in US midwest
                   Sexual selection for mating preferences?

         Colour / blotching patterns in California Salamander (Ensatina)
                 "Rassenkreis" = 'race circle' of clinal change
                   'Tails' of circle overlap w/ reproductive isolation:
                          separate species?

         Pocket gopher (Thomomys umbrinus) lives in burrows (fossorial)
                   Only narrow ranges of soil types are suitable
                  268 subspecies (!) separated by a few meters

      Sympatric distributions: physically overlapping
             Less common, because of competitive exclusion:
                   Forms with similar niche requirements cannot co-exist
                   [But how does the species see the environment?]

             Host races of Rhagoletis fruit flies
               parasitoids of apple, cherry, hawthorn, blueberry fruits
                   life cycle complete on single tree

       Sibling species of Drosophila may subdivide single cactus
            Anolis lizards subdivide crown & trunk of trees

       Species flocks among fish (Haplochromis, Cichlidae) in African lakes
                 100's of 'species' in same lake with no apparent barriers
            Feeding niche, depth, substrate differences



Is geographic variation adaptive?
      Intuitively, Yes: organisms 'fit' environment
      But it's difficult to identify & measure fitness components
             So, we tell "Just So Stories"
                   "How the Elephant Got Its Trunk" (Kipling)
                   "How the Bear Got It's Mouth" (Darwin)

      Comparative Method: Similar patterns imply analogous processes.

Ecogeographic Rules summarize similar patterns of variation,
      within species across multiple taxa.

Bergmann's Rule: size increases with latitude
             Surface/volume ratio decreases in cold climate

      Short-tailed weasels (Mustela erminea) are larger in north [Lab #2]
             However, related species don't follow same pattern.
      White-tailed deer (Odocoileus) are smaller in neotropics than in nearctic
             Other neotropical cervids are also "miniaturized"
     House Sparrows (Passer) are larger in northern, cooler latitudes
            Introduced to North America 150 years ago
            Rapid spread & adaptation
      Downy woodpeckers (Picoides) are larger in north
 

Allen's Rule: 'stubbiness' increases with latitude
                    Extremities are reduced in cooler latitudes

      Herring gulls (Larus) have shorter wingspan in north
      Hares (Lepus) are 'chunky' in arctic, 'lean' in southern deserts
      Dixon's Rabbucks show similar pattern, 50MY from now

Gloger's Rule: 'darkness' increases with humidity
                           or, reflectance increases with aridity

      Human races (Homo) are darker in more humid regions
      Tree shrews (Tupaia) are darker in southern (tropical) Asia
      Long-tailed weasels (Mustela frenata) are darker
           in  tropical & boreal regions (mesic: wetter)
           than in desert regions (xeric: drier)
 

All of these relate to thermoregulation:
      reduction of heat loss in (cold) north,
             avoidance of thermal stress in (hot) south
      Increased survival is presumably correlated with reproductive success => "adaptive"



Adaptive chromosome variation within Drosophila

Th. Dobzhansky (1900-1975) "Genetics of Natural Populations", 43 items

Dipteran chromosomes are polytene in salivary glands
      Ten doublings yield 210 = 1,024 chromatids side-by-side
      D. pseudoobscura shows extensive paracentric inversion polymorphism
       Evolutionary relationships can be inferred
             Geographic variants occur: "Standard" (ST) is standard.

Chromosome III  varies:

      (1) Geographically
       "Standard" (ST) common on West Coast
             "Arrowhead" (AR) common in Southwest
             "Chiricahua" (CH) common on Mexican plateau

      (2) Altitudinally
           ST common at sea level
           AR common at high altitude (> 5,000')

    (3) Seasonally
           CH replaces ST in Spring
             Cycle repeated annually for 40+ years

             Population cage experiments
                CH does better than ST at higher temperature
                AR does better than ST at lower humidity

Balancing selection maintains multiple inversions in population
         Inversions are co-adapted gene complexes: "supergenes"
         Crossing-over is suppressed within inversions:
             inversions 'lock up' sets of alleles in cis -linked groups
         Chromosome variation is adaptive to local conditions


Text material © 2010 by Steven M. Carr