Geographic Variation

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 across island

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 still influential.

-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.

Population structure:

Same suite of changes throughout the pop. Dispersal and gene flow “enetic glue”

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 variation.

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
Apapane- Nomadic
I’iwi – juvenile dispersal

Generation of geographical variation by natural selection operating on heritable characters

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 finch)

Genetic drift (operating on neutral characteristics.. one’s that do not give the bird an ecological advantage): chance events associated with
- 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 selection

Dispersal = genetics glue

One individual only needs to enter population to make a change
Dispersal is life history character that maintains or generates geog. Variance.

22-1 Snow geese
Assortative mating offspring preferentially mate with birds parents’ color

22-1 Allopatric speciation diagram

Isolation barriers
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 interbreed.
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


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