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Conservation Genetics:
Lessons from
Population & Evolutionary Genetics
I.
Definition
Conservation Genetics:
The science of understanding how genetic issues
affect the conservation and restoration of populations
and species.
II. Major Issues (from Frankham 1995)
-Inbreeding depression
-Accumulation of deleterious alleles
small
-Loss of genetic variance in small populations population
-Genetic adaptation to captivity and effect on
size
reintroduction success
-Fragmentation of populations
-Taxonomic uncertainty (unique?, novel?, hybrid?,
hybridize for successful reintroduction?)
III. Taxonomic Uncertainty
Example: Dusky Sea Side Sparrow (Ammodramus maritimus nigrescens)
Avise and Nelson 1989
IV. Small Population Size
-Most threatened/endangered species exist in
Small Isolated Populations
Gaston et al. 1997 (ECOGRAPHY)
Newton 1997 (ECOGRAPHY)
Must focus on consequences of small population size
Genetic Consequences of Small Population Size:
-Loss of Genetic Variation
-Inbreeding Depression
-Accumulation of Mutations
All as a result of Drift and Fragmentation
V. Drift
History: Natural historians, including Darwin, noted that
some variation among individuals would not
result in differences in survivorship and
reproduction
e.g., Gulick, Hawaiian land snails exhibited great diversity
of shell color patterns
Changes in pattern across generations arises by chance
Drift (population genetic translation- Wright):
Evolutionary process by which allele frequencies change
by accidents of sampling
VI. Origin of Accidents of Sampling
Assume diploid population with 2 alleles at a locus
A with frequency p
a with frequency q
Zygote = union of 2 independent gametes or union of 2
independent events
Thus genotype frequencies represent binomial probability
distribution:
(p + q)2 or AA= p2, Aa = 2pq, aa = q2
Assume: finite population size (N)
Zygotes are a sample of gametes:
A or a
with frequency p and q
Thus random sampling process will introduce variation
of allele frequencies across gernation of
Variance of binomial: pq/N
Diploid organisms: pq/2N
Loss of Heterozygosity is proportional to 1/2N or 1/2Ne
(Population Geneticists use Ne because loss of
heterozygosity is often greater than the census number)
Effect of sampling variation after many generations
Change in allele frequencey of Drosophila melanogaster populations
VII. Consequences of Drift:
-allele frequencies fluctuate randomly
-populations vary by chance
-increase variation among populations
-decreased heterozygosity in populations
-increased homozygosity in populations
-increased genetic relatedness in population
-SELECTION NOT AS EFFICIENT
NeS < ¼
then deleterious alleles and new deleterious
mutations will become fixed by drift (more later)
VII. Consequences of Fragmentation
A. Wahlund Effect:
All of the same consequences as Drift
decreases heterozygosity within populations
increases homozygosity within populations
increases genetic relatedness within populations
Natural History Examples of Fragmentation
(From Hamrick and Godt)
# of
P
species (within population)
pollen dispersal
animal
wind
seed dispersal
gravity
wind
164
102
199
105
36
50
30
43
Gst
(among pop)
0.2
0.1
0.3
0.1
P = % of loci with > 2 alleles
Gst = proportion of genetic variation distributed among pop.
FRAGMENTATION  LOSS OF GENETIC DIVERSITY WITHIN POPULATIONS
B. Further consequences of Fragmentation
Allee Effect: As density decreases, ability to find mates
also decreases
e.g. Oostemeiger, Arnica montana, Netherlands
Visitation rates in small and large populations:
Small
Large
High Density
Large
Low Density
IX. Consequences of Inbreeding
A. Inbreeding depression
Low
High
Heterozygosity
Low
Extinction Rate
High
B. Loss of Genetic Variation
Lakeside Daisey (hymenoxys acaulis var. glabra)
M. Demauro, 1994
Last remaining population in Illinois
Lakeside Daisey is Self Incompatible
Number of Mating Groups
Selection of D. melanogaster for resistance to
ethanol fumes in Large vs. Small populations
Resistance (minutes)
Weber, 1992
L = Large
S = Small
Generation
Consider response to global climate change!
C. Mutation Accumulation
NeS < ¼
1. Fixation of ancestral mutations (From Lynch and Burger, 1995)
2. Introduction of new mutations
3. Extinction Risks Due to Mutational Meltdown
R = Reproductive Rate; K = Carrying Capacity
Consequences of Mutations for Small Populations
Critically Depend on:
Mutation Rate
Distribution of Mutation Effects (all deleterious?)
X. Genetic Manipulation to Counteract Small Population Size
A. Purging of “bad” mutations
Husband and Schemske, 1996
Natural History Examples:
Drift led to both the
fixation and extinction
of deleterious alleles
Purging critically depends on genetic basis of
inbreeding depression:
Inbreeding depression: expression of recessive
deleterious alleles in homozygous condition
Dudash and Carr, 1998
Inbreeding depression due to recessive alleles
B. Crossing Programs to Restore Genetic Variability
Case Study: Fenster and Colleagues
Chamaecrista
fasciculata
XI. Conclusion
Small population size may lead to lower genetic fitness
through fixation of deleterious alleles
XII. Future Directions
We Need:
-Better estimates of mutation rates and effects
-Field based experiments to determine if a population
can be purged of deleterious mutations
-Studies to quantify effect of adaptation to captivity
-Better understanding of the genetic basis of adaptive
differentiation