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
Number of alleles
Instantaneous
Fast
Slow
Genetic drift
Extinction debt
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
gene flow
genetic
drift
Population 1
Gene Flow ↓ FST
gene
flow
Population 2
Local adaptation
↑ FST
genetic
drift
Population 3
gene flow
Drift ↑ FST
genetic
drift
Consider similarity to alpha
and beta diversity
β diversity
α diversity
Colonization Rate
↓ β diversity
Population 1
β diversity
Population 2
α diversity
Population 3
α diversity
β diversity
Isolation ↑ β
diversity
Endangered Rutidosis (daisy) in Australia:
genetic diversity and Ne
Young et al. 1999 Con. Bio. 13:256-265.
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
allozymes
High Gene Flow:
Fst=.17
Case study:
Missouri glades
Eastern collared lizard
Fragmentation
Natural burn regime:
every 5 years
Fire suppression since
1950: glades are
disappearing, juniper
expanding
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 management
The 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).