Habitat loss and fragmentation I

Bio 415/615

Questions

1. What does FST measure?

2. How does FST relate to fire management and collared lizards in the Ozarks?

3. How are genetic drift and selection different?

4. Why is FST similar to beta diversity?

Fragmentation & Genetics• Ne decreases and distance increases• Instantaneous loss on sampling• Fast loss with reduced gene flow

(sink genes)• Slower loss with random genetic

drift, decreasing allelic diversity (rare alleles) within populations and causing populations to diverge (depends on time at low Ne and thus on generation time)

• Inbreeding increases

Processes of species loss and allele loss are similar

InstantaneousInstantaneous

FastFast

SlowSlow

Extinction debtExtinction debt

Genetic drift

Nu

mb

er

of

allele

s

Fragmentation and gene flow

• How do genes move between populations?– Movement of animals– Dispersal of plants (seeds, spores)– Movement of sperm (pollen)

• Is gene flow equal to colonization rate?– Individuals may die before reproducing (e.g.,

territoriality in animals)– Some insects invade areas where they don’t

reproduce…

Ecological dispersal is not equal to genetic dispersal

Fragmentation and gene flow

• How do we estimate the effects of fragmentation on gene flow?

Fst describes the proportion of genetic variance in the whole population (all patches) attributed to differences between patches.

Eg: if there are 10 alleles at a locus in a population of 10 subpopulations, and each has a different allele, Fst = 1

Fragmentation and gene flow

• Fst is defined more formally as the variance in allele frequencies between populations (standardized by the mean)

It is also estimated as:Fst ~ 1 / (4Ne*m + 1)Migration rate (of alleles): gene flow

Effective population size (why important?)

As gene flow goes up, Fst goes down; as local population size decreases, Fst goes up

Consider similarity to alpha and beta diversity

Population 2

Population 3

Population 1

geneflow

gene flow

gene flow geneti

cdrift

genetic

drift

genetic

drift

Gene Flow ↓ FSTDrift ↑ FST

Local adaptation ↑ FST

Consider similarity to alpha and beta diversity

Population 2

Population 3

Population 1

β diversity

α diversity

Colonization Rate ↓ β diversityIsolation ↑ β diversity

β diversity

β diversity

α diversity

α diversity

Endangered Rutidosis (daisy) in Australia: genetic diversity and NeYoung et al. 1999 Con. Bio. 13:256-265.

High Gene Flow:Fst=.17

allozymes

Rare alleles are those lost when Ne is small; heterozygosity was not related to Ne… so small populations do not appear to suffer inbreeding depression

% loci with >1 allele

Case study:

Eastern collared lizard

Missouri glades

Fragmentation

Fire suppression since 1950: glades are

disappearing, juniper expanding

Natural burn regime: every 5 years

Why do these lizards inhabit these ‘desert’-

like environments?

Fragmentation

Q: What did forest regrowth do to the lizard population?A: Some populations were lost, the lizards

stopped moving (how do they know?), and Fst (.40) indicated major genetic drift.

Q: Why do they think the high Fst was the result of drift rather than selection (local adaptation)?

A: Genetic similarity was not related to spatial distance (why does this matter?).

Q: What was the management response?

A: BURN http://www.youtube.com/watch?v=N01BygHu-T0

Response to managementThe forest opens up

Are they moving?

Conclusion

Q: What happened to the genetic structure?

A: Eventually Fst did go down (not reported here).

Q: How do the authors respond to the suggestion that population isolation is ‘good’ because it eventually produces new species (‘shifting balance theory’)?

A: It assumes that population size will again grow. Should we expect this from fragmentation?

Q: What, the authors conclude, should management focus on?

A: Maintain the ecological processes (eg burn regime).