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 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 ()
vs. yellow-shafted ()
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
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"
Th Dobzhansky (1900-1975) "Genetics
of Natural Populations",
48
items (1938 - 1976)
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