Transcript Populations & Conservation Genetics

Conservation of Populations
I. Defining Populations
II. Demographics – growth and decline
III. Conservation genetics & populations
I. Defining populations
A. Spatial disjunction
–
Distribution pattern, groups are separated by location,
regardless of other similarities
B. Genetic disjunction
–
All individuals in one group share genetic attributes with
one another, but not with individuals from other groups
C. Demographic disjunction
–
Individuals from different groups have different
demographic properties; birth rate, death rate, sex ratio,
age structure
Important characteristics of a
population:
Population Density – number of individuals
per unit area or volume
– Sampling to estimate density
1) Absolute Density
2) Capture-Recapture Method
3) Quadrant techniques
Dispersion – pattern of spacing
among individuals
– Limited by abiotic
factors
Abiotic factors:
• Patchy environment will
affect dispersion within
population
– Limited by biotic
factors, species
interactions
Biotic Factors
DISPERSION PATTERNS:
Clumped pattern – individuals aggregated
in patches
Uniform pattern – evenly spaced resulting
from direct interactions
Random pattern – occurs in the absence
of strong attractions or repulsions among
individuals
II. Demographics – growth and
decline
Recruitment, fecundity
Immigration
Emigration
Death
survivorship
Natality (avg. per capita birth rate)
Mortality (avg. per capita death rate)
Density dependent fecundity in fingernail clams, Musculium securis
B. Life histories and population size
•
Traits that affect an organism’s schedule
of reproduction and survival
1) Semelparity reproduction
2) iteroparity reproduction
•
Limited resources and trade offs
– Red deer in Scotland
– Insect species
– Perennial plants
Different life histories represent a resolution of
conflicting demands. When an organism engages in one
activity other activities are constrained!
limited
resources
time
energy
nutrients
Population Growth Models
increases if birth rate > death
rate
A. Exponential
– unrestricted growth due to
abundance of resources
(food/space)…
– the larger the population, the
faster its potential for growth.
– ΔN = rmaxN
Δt
B. Logistic
– ΔN = rmaxN (K – N)
Δt
K
C. Population growth factors
1) Density Dependent
– Negative feedback
2) Density Independent
3) Top-down effects
4) Bottom up effects
5) Indirect effects
III. Conservation Genetics
A.
B.
C.
D.
What is Genetic Diversity?
Why is Genetic Diversity Important?
Genetic threats to populations
Case studies
A. What is Genetic Diversity?
1)
2)
3)
4)
Among species
Among populations
Within populations
Within individuals
B. Importance of genetic diversity
a) Evolutionary potential
b) Loss of fitness
c) Instrumental value
C. Genetic threats to populations
1) Small Population Size


Effective population size
Drift & Bottlenecks
2) Inbreeding depression


Loss of genetic variation
Accumulation of harmful mutations
3) Introgression and hybridization

Outbreeding depression
Small populations are subject to rapid decline due to:
1) Loss of genetic variability and related problems
2) Demographic fluctuations due to random variations in birth
& death rates
3) Environmental fluctuations due to variation in predation,
competition, disease, natural catastrophes, etc.
Effective Population Size (Ne)# of breeding individuals
Vs.
Census Population Size (Nc) – actual number
of individuals in a population
Unequal Sex Ratio
Unequal production of offspring
a) Bottleneck: drastic reduction in population size
•
Founder effect: when a few individuals establish a
new population that has less genetic variation than
the larger original population
b)
Genetic drift: random fluctuations of allele frequencies
•
•
A loss of certain alleles, especially rare alleles & fixation of others
Reduction in the amount of variation in genetically determined
characteristics, decline in heterozygosity (H)
the rate at which new (neutral) mutants are fixed is 1/u ; this
rate is INDEPENDENT of population size, N.
Due to the fixation of certain alleles, heterozygosity will decline in the case of
genetic drift in small populations.
Ht = HO [1 – 1/(2N)]t
Ht
Population A (50)
Population B (10)
H0
0.500
0.500
H1
0.495
0.475
H2
0.490
0.451
H3
0.485
0.429
H4
0.480
0.407
H5
0.475
0.387
2. Inbreeding Depression
• Inbreeding = Mating between relatives
• FI - is the probability that two copies of the
same allele are identical by descent (IBD)
• Example: FI of the offspring of a mating between
full sibs is ¼
– F is the proportion by which heterozygosity is
decreased, relative to that in a random mating
population with the same allele frequency
Normal lion sperm
Abnormal lion sperm from an isolated, inbred
population in Tanzania
Inbreeding leads to the expression of recessive
deleterious alleles that are suppressed in
heterozygotes
3. Introgression and hybridization
•
•
When mating occurs between individuals that
are too genetically dissimilar
Loss of fitness results –
– “Swamping” of locally adapted genes – adaptive
gene complexes in native populations are being
displaced by the immigration of genes that are
adapted to another environment
– Breakdown of biochemical or physiological
compatibilities between genes in the different
populations.
Case studies
Due to severe over hunting, by 1892
somewhere between 8 and 20 were left.
Since then there has been an almost
exponential increase, especially in the
northern colonies. In 1957 there were
13,000 elephant seals, in 1976: 48,000.
The population is still not at equilibrium
(Boveng et al, 1988). In 1991, the total
population was estimated at 127,000,
with 28,164 pups born that year and
there appears to be a 6% annual increase
(Stewart et al, 1994)
Marsh rat
Silver rice rat
1975, 20 individuals translocated to the island (4 males
& 16 females)
Total population by 1999 = 650
Molecular markers show that these island sheep are
much less genetically diverse than those found on the
mainland (H = 0.67 as compared to H = 0.42)
• Insular populations in Sweden exhibited
inbreeding depression symptoms
• Abnormal scales
• Decreased litter size
• Increased # inviable progeny
- Capra ibex ibex (Austria), C. i. aegagrus (Turkey) & C. i.
nubiana (Sinai)