variation usually structured spatially.
Many single-locus traits locally 'chaotic'
Colour & pattern variants occur within plant & animal species:
HbS scattered uniformly within a community.
show large-scale geographic patterns:
HbS is common in Congo Basin, uncommon outside Africa.
Variation often occurs as cline:
'character gradient' in space
produced by differential selection, balanced by gene flow.
absence of selection (s = 0),
exchange of two individuals between populations per generation
prevents evolutionary divergence.
If gene flow high, cline smooth;
if gene flow low, cline 'stepped'
Shape depends on
Intensity of selection: fitness differential between AA and BB
Biogeography: environmental gradient, barriers to dispersal
Vagility: potential for movement
ABO blood groups
f(B) in west = 0.0 , f(B) in east = 0.30
Smooth cline across Europe: q = 2.5 x 10-5 / km
Disease resistance? Historical invasions?
Lowest f(B) coincides with Basque population
Heavy-metal tolerance in wild
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
discrete (multiple 'types')
Within-species variation recognized as subspecies (Fig 9-21) with trinomials:
Ex.: Rat Snakes (Elaphe) occur as geographically disjunct subspecies
Ex.: Newfoundland pine marten (Martes americana atrata) larger & darker (endangered)
Patterns of subspecific variation well-documented:
Can they be interpreted biologically / adaptively?
Lynx (Lynx canadensis canadensis) monotypic on mainland:
L. canadensis subsolanus larger in Newfoundland
(Typical of highly vagile species)
Allopatric distributions: physically separate
Holarctic distributions: (=
occurring in Palearctic &
Rangifer tarandus is 'Reindeer' in OW, 'Caribou' in NW
Cervus elaphus is Red Deer in OW, Wapiti in NW
Nine of 20 subspecies of least weasel (Mustela) are insular
[This can be a circular argument: isolation => distinctness?]
[a lake is an island, to a fish]:
Mexican shrew (Sorex) lives only at high elevation.
Six subspecies are found on six different mountaintops.
physically contiguous (adjacent)
Distributions may overlap, producing clinal variation
vs. yellow-shafted ()
Narrow step-cline in US midwest
Sexual selection for mating preferences?
Colour / blotching patterns in California
"Rassenkreis" = 'race circle' of clinal change
'Tails' of circle overlap w/ reproductive isolation:
Pocket gopher (Thomomys)
lives in burrows (fossorial)
Only narrow ranges of soil types suitable
268 subspecies (!) separated by a few meters
Less common, because of competitive exclusion:
Forms with similar niche requirements cannot co-exist
But how does species see environment?
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
Comparative Method: Similar patterns imply analogous processes.
Rules summarize similar patterns of variation,
within species across multiple taxa.
Rule: size increases with latitude
Surface/volume ratio decreases in cold climate
Short-tailed weasels (Mustela
erminea) larger in north
However, related species don't follow same pattern.
White-tailed deer (Odocoileus) smaller in Neotropics than in Nearctic
Other neotropical cervids are also "miniaturized"
House Sparrows (Passer) larger in northern, cooler latitudes
Introduced to North America 150 years ago
Rapid spread & adaptation
Downy woodpeckers (Picoides) larger in north
Rule: 'stubbiness' increases with latitude
Extremities are reduced in cooler latitudes
Herring gulls (Larus)
have shorter wingspan in north
Hares (Lepus) 'chunky' in arctic, 'lean' in southern deserts
Dixon's Rabbucks show similar pattern, 50MY "After Man"
Rule: 'darkness' increases with humidity
or, reflectance increases with aridity
Human popujlations (Homo)
darker in more humid regions
Tree shrews (Tupaia) darker in southern (tropical) Asia
Long-tailed weasels (Mustela frenata) darker
in tropical & boreal regions (mesic: wetter)
than in desert regions (xeric: drier)
All of these relate to
reduction of heat loss in (cold) north,
avoidance of thermal stress in (hot) south
Increased survival presumably correlated with reproductive success => "adaptive"
Th. Dobzhansky (1900-1975) "Genetics of Natural Populations", 43 items
Dipteran chromosomes are polytene in
Ten doublings without karyokinesis 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:
"Standard" (ST) common on West Coast
"Arrowhead" (AR) common in Southwest
"Chiricahua" (CH) common on Mexican plateau
ST common at sea level
AR common at high altitude (> 5,000')
CH replaces ST in Spring
Cycle repeated annually for 40+ years
CH does better than ST at higher temperature
AR does better than ST at lower humidity
maintains multiple inversions in population
Inversions are co-adapted gene complexes: "supergenes"
Crossing-over suppressed within inversions:
inversions 'lock up' sets of alleles in cis -linked groups
Chromosome variation is adaptive to local conditions