Lecture Notes 33: Gene Flow and Population Genetics

To maintain Hardy-Weinberg equilibrium, certain assumptions must hold true:

Random Mating
- Includes inbreeding and assortative/disassortative mating.
Infinite Population Size
- No sampling effects, allowing for stable allele frequencies.
No Migration (Gene Flow)
No New Mutation
No Natural Selection
- All genotypes have equal survival and reproduction.

Predicts genotype frequencies given allele frequencies and the assumptions above.

Definition: Incorporation of alleles into a population from another population.
Important Note: Not all migration leads to gene flow.

Gene flow homogenizes genetically divergent populations.
- For five populations of equal size: allele frequencies p = 0, 0.25, 0.5, 0.75, 1.0 with migration rate m = 0.1.
The power of gene flow to cause evolution is dependent on:
- Amount of initial genetic differentiation between populations.
- Rate of gene flow (m), defined as the proportion of alleles derived from migrant parents.

Definition: Patterns of genetic differentiation among populations of a species.
Scenarios illustrating levels of gene flow can be analyzed:
- High variation within each population and low variation among populations (Scenario A).
- Low variation within each population and high variation among populations (Scenario B).
Understanding gene flow requires examining patterns of variation in multiple populations.

The measure of genetic differentiation among populations is represented by Fst.
- Fst = 0: Populations are genetically identical (same alleles, same frequencies).
- Fst = 1: All populations are fixed for one allele or another, indicating complete divergence.
Formula: $F{st} = rac{HT - HS}{HT}$ where:
- $H_T$ = expected heterozygosity calculated for allele frequencies pooled across populations (Total population).
- $H_S$ = the weighted average of expected heterozygosities calculated from each individual population (Subpopulation).

For two populations with:
- Population A: 2N=100 alleles where p=0.6, q=0.4.
- Population B: 2N=200 alleles where p=0.8, q=0.2.
Heterozygosity for:
- Population A: $2pq = 0.48$
- Population B: $2pq = 0.32$
Calculation of $HS$ : $HS = rac{(50)(0.48) + (100)(0.32)}{50 + 100} = 0.373$
Calculation of $HT$ : $HT = 2pq = 2 * rac{60 + 160}{300} * rac{40 + 40}{300} = 0.391$
Final calculation:
$F{st} = HT - H_S = 0.391 - 0.373 = 0.018$
- Indicates low genetic differentiation for these two populations.

Populations with increased gene flow should be differentiated and have a Fst:

Direct Methods:
- Example: Mark & recapture techniques.
- Use of marker traits/alleles (e.g., dominant and recessive seed color traits in plants).
- Detecting heterozygous seeds reflects gene flow.
- Example: Gene flow by pollen in maize observed up to 40 meters.
Disadvantages of Direct Methods:
- Migration does not equal gene flow.
- May not represent other populations, seasons, or environments.
- Important rare, long-distance events are challenging to detect.
Indirect Measures:
- More common, easier to perform, and often more accurate for assessing current levels of gene flow.

Species: Crotaphytus collaris, predominantly found in the Southwest US and Missouri glades.
Genetic differentiation example:
- Ozark collared lizards: Fst = 0.4
- Southwestern collared lizards: Fst = 0.1

Literature indicates that human populations show little evidence of population structure across most loci.
Subtle population structure detected within Europe, with Fst = 0.004, becomes evident using >100K SNPs.
Analysis of polymorphic sites in Maasai and Inuit populations indicates that most genetic differences are shared among these groups.

Essentialist Concept of Race: Views human species as divided into distinct groups based largely on physical characteristics.
Population Concept of Race: Identifies races as clusters of populations differing genetically, acknowledging overlap.
Actual genomic data supports that race is a social construct, not a scientific one.
- African populations are highly diverse and encompass nearly all genetic variation.
- Most variation found among non-African populations shaped by historical colonization dynamics.

Assumption: Genetic markers are selectively neutral; selection does influence Fst.
- Selection favoring different alleles in divergent populations increases Fst.
- Selection favoring same alleles across populations reduces Fst.
Important influencing factors include:
- Spatial environmental heterogeneity
- Heterozygote advantage
- Spread of new high-fitness alleles.

Comparison of allele frequencies can identify loci with greater differentiation due to selection.
Locus under Selection: Expected genomic scans show differing patterns of demographics relative to selected loci.

Tibetans adapt to life at high altitudes without increasing hemoglobin levels as a response to environmental stress, preventing potential complications such as blood thickening.
A whole-genome study between Tibetans and Han Chinese highlighted selective pressures in genes related to oxygen regulation.

Gene flow plays a critical role in defining species and their evolution, shedding light on evolutionary processes.

Cyanogenesis Cline in White Clover.
Squirrel Example: Fst in squirrels around the Grand Canyon, illustrating vicariance events.

Understanding the interplay between gene flow, selection, and genetic drift is crucial to grasp the dynamics of population genetic structure and differentiation.