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Migration
• Evolutionarily, this means the
movement of alleles, or gene
flow between populations
• This will always be less than
the movement of individuals
Migration
• When populations differ
in allele frequencies,
migration can be a
powerful force
Migration
• Genetic patterns due to
migration tend to be
temporary
• Over the longer term,
migration is a homogenizing
force—it makes populations
more genetically similar
D.R. Robertson
Migration
• Populations in Central and
Eastern Pacific (divided by
5000 km of deep ocean) are not
genetically different in these two
reef fish species*
*(and 18 out of 20 species studied:
Lessios and Robertson 2006)
D.R. Robertson
Geographic variation in
natural populations
Often, morphology varies geographically
from Futuyma (1998), p. 259
With low migration, allele
frequencies vary greatly over
small geographic distances
As in pocket gophers...
1999 R.M. Timm
from Futuyma (1998), p. 319
Geographic barriers to
dispersal often separate
genetically different
populations
Historical separations between Gulf of
Mexico and SE Atlantic drainages
from Avise (1994)
from Avise (1994), p. 244
The Florida peninsula is
a marine biogeographic
barrier
It forms a long-term
historical barrier to
migration and gene
exchange (gene flow)
within many species
Natural selection creates
patterns of geographic
variation
Clinal variation
from Volpe and Rosenbaum
(2000), p. 110
from Futuyma (2005)
In D. melanogaster, the alcohol dehydrogenase (Adh) locus shows
a cline—a regular change in frequency of a trait across a
geographic transect
from Futuyma (2005)
The frequency of AdhF decreases towards the equator on 3
continents: these are parallel clines
Clinal variation at
lactate
dehydrogenase-B in
the mummichog
Fundulus
heteroclitus: Ldh-Bb
increases towards
the north
This allozyme has a
higher catalytic
efficiency at lower
temperatures
Races and subspecies
Subspecies of Northern flicker (Colaptes)
Races and subspecies
• A race is a geographic
population with well
defined, discrete
differences in one or
more traits
Red-shafted C. auratus cafer
• Taxonomically, races
may be recognized and
named as subspecies
Yellow-shafted C. auratus auratus
Color races of
Heliconius
butterflies
geographic color races of
H. erato
geographic color races
of H. melpomene
these races display
warning (aposematic)
coloration to deter bird
predation
Allopatric color races
Most animal color races are
allopatric (they occupy nonoverlapping ranges)
Each color race of dart-poison frogs (Dendrobaetes) lives
on a different island in Bocas del Toro in western Panamá
Parapatric races or subspecies share
common borders
from Futuyma (2005)
Parapatric subspecies sometimes interbreed
where their borders meet
from Futuyma (1998), p. 258
Hamlets: sympatric races?
• 11-12 forms of Hypoplectrus in
Caribbean, Florida, Bahamas
• Morphologically identical, but
strikingly different color patterns
• 6 “races” live together on the
same reefs, and mate like-with-like
Racial variation
• morphological races
• host races in insects
– races feed and oviposit on different host
plants
– e.g. apple and hawthorn races of
Rhagoletis
• physiological races
• sex races
Physiological races
Vermont: embryos survive 5 28
New Jersey: 5 - 28
Northern leopard frog Rana pipiens
North Florida : 9 33
South Florida : 11 -
Sex races
from Futuyma (1998)