Georgraphic Variation occurs in birds across the country and has been documented in several cases. Clinal factors for instance, have been found in even a single species. The Fox sparrow from California has a deeper bill than that from Idaho and Oregon. It is believed that this geographic variation and divergence is caused by local selective pressures such as by mate choice or rivalry.
When thinking about natural selection and adaptation it is important to remember that it works in two ways: 1) within a species (called intraspecific selection) and 2) on a newly formed species.
It is important to ask the question then: What generates this speciation and how does it persist (what maintains it)?
But, before we go too far into the discussion lets clarify some terms:
- Hybrid zone - a zone of intergradation that can be stable.
- Equilbrium model - hybrids are dead ends (sinks) that can not reproduce thus creating a sharp boundry between two pure populations
- Bounded Superiority Model - hybrids are of equal or better adaption for habitat in an intergradation zone so as to out-compete the further encroachment onto the other's range.
- Clinal - gradients of character states, such as body size or feather color. The expression of the opposing actions of divergent selection and blending gene flow in contiguous populations can be dynamic or static.
Case 1: Hybridization and speciation
A study on blue-winged and golden-winged warblers represents a case of hybridization and the ecological aspects of speciation. The blue-winged warblers can live in a wider range of habitats and thus hybridizes with the golden-winged warblers. Thus ecological aspects will eventually decide the fate fo the golden-winged warblers.
Case 2: Yellow and black and Black morph Bananaquit
The Y&B morph dispersed to Grenada from other islands. Now it is replacing the B morph in a clinal fashion. There is a gradual increase in Yellow-black morphs. Is it a better competitor? Is this sexual selection? As it turns out females of the black morph actually prefer the yellow and black morph males.
The rest is straight from my notes
EX: Mascarine white-eye: Grey, Brown, Brn/Gry – very steep gradient
Generation of Geographical Variation
1. if heritable characteristics
2. non-heritable envt. Variation (House finch – food determines
plumage color not genes)
3. Genetic drift on neutral or weakly selected characters
Genetic drift can overwhelm selection pressures if the latter is weak.
Heritability = ratio of variance attributed to sires and dams (G)/ envt.
Variation + G
Heritibility of bill length = .35 to .36..plenty of additional variation
in variance of bill length; raw material for nat sel. To act upon.
If no bill length changes there is stabilizing selection
RRBB eggs moved from the everglades to talahasee looked more like Talahasee
birds and Oklahome birds to Minnessota looked more like Minessota birds
but in both cases not totally showing that genetic component of heritability
-Amakihi nest morphology on big island
-Also young birds not re-captured as often as adults this shows substantial
natal dispersal. Effective pop.size decreases with the decrease in natal
dispersal distance. Small effective pop. sizes evolve faster and often
in directions dictated by chance.
-Honeycreeper male plumage cline along Hawaiian Island chain evidence
of clinal female sexual selection.
-latitudeinal gradient of clutch size N – greater S less,
- porosity of egg shells with altitude
- repertoire size .. W marsh wren and E marsh wren. The former has a larger
song repertoire and this seems to keep the species from hybridizing –
no intermediate forms have been detected however the W males may attract
more E females and be slowly moving eastward
- area of A and B different # of allozymes (neutral character –
no selection) but non-neutral selection may be occurring.
Same suite of changes throughout the pop. Dispersal and gene flow “enetic
Individuals on each end of the range on 2 different evolutionary trajectories……….
If individuals disperse can upset that.
Individuals needs to make it from point A to point B but needs to reproduce.
Theory: one individual per generation makes it into other population
that pop. will not loose an allele. Can look at dispersal as geographic
22-4 Males did not move much, nestlings moved a lot.. females tended
to disperse farther than males.
Amakihi – juveniles go somewhere else
Akepa Juv.- Are not going anywhere.. show great changes from one island
to the next.. they have much less dispersal of the kids
I’iwi – juvenile dispersal
Generation of geographical variation by natural selection operating on
Quantitative genetics of
1. morphological traits
2. behaviorial traits
Measure heritable characteristics operated on by natural selection
Measure non-heritable characteristics ? caratenoids in plumage (house
Genetic drift (operating on neutral characteristics.. one’s that
do not give the bird an ecological advantage): chance events associated
- traits begin to look different
- no envt. Variation
- no envt. Selection
- just chance events of whose surviving and who is not
If weak selection, genetic drift can overcome it
All evolutionary processes come into play with geographic variation
Downy woodpecker: latidutinal cline
Galapagos finch competition – Santa Cruise: interspecific competition
Daphne major: Intraspecific competition
Population structure is geographic structure is maintained by natural
Dispersal = genetics glue
One individual only needs to enter population to make a change
Dispersal is life history character that maintains or generates geog.
22-1 Snow geese
Assortative mating offspring preferentially mate with birds parents’
22-1 Allopatric speciation diagram
EX: Australian robins
Aracari in Amazon basin…. Vicariant event (glacial period) reduced
the forest to patches resulting in Aracari’s being divided ? speciating
and then brought back together during interglacial (now)? they can not
Mannikins – rivers are isolating barriers
22-9 Divergence temporary in Yellow-rumped warbler/Myrtle warbler now
being re-joined – barrier gone
22-10 Orioles (hybrid zone)
Two theorys in static hybrid zones (see previous page)
By Rob Nelson
The Wild Classroom