Lecture 9 : Migration Flashcards
Conservation of the Florida panther (jeckyll effect example)
in the early 1990s the Florida panther population was near extinction, with 2 dozen individuals in the population and an effective population (Ne) of 2-10. Because the population was so low, they were very susceptible to inbreeding, causing a ton of health problems. So managers took panthers from texas and brought them to florida. This reintroduced lost alleles into the population, reducing the effect of inbreeding, and allowing the population to grow.
What is migration (3)
-Migration is anything that causes alleles to be moved from one population to another
-Causes gene flow
-“Realized dispersal” where individuals are able to move into and subsequently reproduce with a different population
Gene flow (2)
- The movement of alleles from one population to another
- the “jekyll and Hyde” of evolution
2 components of gene flow
-Gene movement
-Gene establishment
Migration vs Gene flow
Migration is the Action that causes gene flow. Gene flow is the actual effect
Migration and HWE (4)
picture the island model, where lots of genes are flowing from the mainland to the island, and a few genes are flowing from the island to the mainland.
-The alleles arriving on the island represent a relatively large proportion of the islands gene pool
-The Alleles arriving on the mainland represent a relatively small proportion of the mainland’s gene pool
This changes the allele frequencies in HWE. For example, the island has a A1 frequency of 1.0 (800 A1A1 individuals), then 200 A2A2 mice migrate there. The resulting frequencies become 0.8 for A1 and 0.2 for A2
The Island model vs stepping stone model
-The island model is when you have one large population (the mainland) migrating to one or more smaller populations (islands)
-the stepping stone model is when you have say 4 smaller populations. gene flow happens between pops 1 and 2, then 2 and 3, then 3 and 4.
Migration and population differentiation
Recall: Population differentiation = Fst = variance(p) / pBAR (1-pBar)
Fst also = 1/(4Nm+1)
Recall
N= effective population size
m = migration rate (proportion of migrants per generation)
But
Nm together = # of migrants per generation
Variation in allele frequencies with population age (Giles and Goudet) (3)
-When islands are young they have the largest variation across different populations, as the populations are unique
-intermediate aged islands will have the lowest variation of alleles across different populations due to to gene flow
-older islands will have medium allele variation across different populations, as gene flow is still happening, but competition excludes new immigrants
Migration and Allele frequency (p’)
-p’ = pSink(1-m) + pSource(m)
-q’ = qSink(1-m) + qSource(m)
where:
p’ and q’ are the allele frequencies after 1 gen of migration
sink = receiving population
source = source population of migrants
m = migration
Migration and Allele frequency example
ex frog color, where Red =A1 allele and yellow = A2
-mainland (source) has a p = 0.85 (mostly red)
-Island (sink) has a p = 0.15 (mostly yellow)
-m = 0.04
-p’ = pSink(1-m) + pSource(m)
-p’ = 0.15(1-0.04) + 0.85(0.04) = 0.178
0.178 > 0.15 therefore the population of red frogs (and A1 allele) has increased
Benefits of Migration (Jeckyll effect)
Migration from other populations should reintroduce alleles that have been lost and reverse effects of drift and inbreeding
Cost of migration (Hyde effect)
Migration may prevent or reduce local adaptation by bringing deleterious alleles.
Lake Erie water snakes (Hyde effect example)
Water snakes in lake Erie can either be banded or unbanded, or somewhere in-between, with the banded allele being dominant. On the mainland, snakes are almost all banded as it allowed them to be better camouflaged in the forest, however most of the snakes on the island were unbanded as they were better camouflaged against the limestone. Because of migration, more snakes on the island started to become banded, and because the rate of gene flow was stronger than selection, the population started to decline as more banded alleles became prevalent.
Migration overall (3)
-Homogenizes allele frequencies across populations (reduces Fst)
– May alleviate effects of drift (and inbreeding) = “Jekyll” effect
– May prevent adaptation to local conditions by bringing unsuitable alleles = “Hyde” effect or “migration load”
Drift vs selection what’s stronger (3)
-Drift leads to random loss of alleles, whereas selection maintains one or more alleles.
What happens when a favored allele is at low to moderate frequency in a small population?
-drift overrides selection when s «_space;1 / 2N
-selection overrides drift when s»_space; 1 / 2N
recall s = strength of selection, N = population size
Heterozygosity and population differentiation
recall: Fst = population differentiation, and can be calculated with allele frequency, or migration
-Fst can also be calculated with heterozygosity
fst = Ht - Hs / Ht
where:
Ht = expected heterozygosity averaged over total populations (mainland and Islands)
Hs = expected heterozygosity averaged over subpopulations (islands only)
