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GENETIC DIVERSITYThe diversity of life is fundamentally genetic. A
variety of genetic methods have been used to investigate diversity both within and between species. Here are a few:
1. Morphological variation -- a good clue, but does not correlate perfectly with genetics;
2. Chromosomal variation -- inversions, translocations and polyploidy;
3. Soluble proteins -- blood groups, soluble enzyme polymorphism’s;
4. DNA markers -- microsatellites, “fingerprint” loci.
CONSERVATION OF GENETIC VARIATION
• The foundation of diversity is the process of natural selection shaping genetic variation.
• When genetic variation is absent (zero heterozygosity), the population (or species) has limited evolutionary potential and the risk of extinction is high.
• The conservation of genetic variation provides a hedge against extinction.
An Endangered Species: Red Wolf
• This canine family member was once found in the southeast. It disappeared in the wild by the late 1970s.
• Reintroduced into Great Smoky Mountains National Park in 1990’s.
An Endangered Species: Red Wolf
• Examination of DNA demonstrated that the red wolf is a hybrid between gray wolf and coyote.
• Expansion of coyote range and shrinking of gray wolf range resulted in gene swamping of red wolf genes by coyote genes.
An Endangered Species: Cheetah• A species that shows a
very low level of genetic variation.
• May have experienced a genetic bottleneck near the end of the last ice age (10,000 - 12,000 years ago) when many other mammal species became extinct.
• Low genetic variation in “fingerprint” loci compared to other cat species.
Population Size and Extinction Risk
• Populations are subject to chance or sampling error in getting alleles from one generation to the next (genetic drift, genetic bottlenecks, founder effects).
• Populations are subject reduction in gene flow and gene swamping.
• Small populations are particularly vulnerable to extinction due to reduction in genetic variation (heterozygosity).
CONSERVATION GENETICS (I)
• Conservation genetics is an area of study that determines genetic variation and the processes that diminish it.
• Heterozygosity is a measure of genetic variation.
• Processes that diminish heterozygosity, especially in small populations, are: 1) genetic drift; 2) genetic bottlenecks; 3) inbreeding.
CONSERVATION GENETICS (II)
• The movement of alleles from one population to another is called gene flow.
• Gene flow promotes heterozygosity by increasing the chances of outbreeding.
• Fragmentation often results in a reduction of gene flow into isolated populations.
• Gene swamping occurs when small populations are genetically assimilated by much larger populations.
Effective Population Size (Ne)
• Effective population size gives a crude estimate of the average number of contributors to the next generation (Ne).
• Always a fraction of the total population.• Some individuals will not produce
offspring due to age, sterility, etc. • Of those that do, the number of progeny
many vary.
Effective Population Size (Ne)
• A variety of ways of estimating (Ne) have been formulated.
• One that accounts for unequal sex ratios among breeding adults is:
Ne = 4(NM * NF)
NM + NF
where NM = number of males
NF = number of females
Effective Population Size (Ne)
• What is the effective population size (Ne) of one with 100 females and 10 males?
• Remember:
Ne = 4(NM * NF)
NM + NF
where NM = number of males
NF = number of females
Effective Population Size (Ne)
• What is the effective population size (Ne) of one with 100 females and 10 males?
Ne = 4(100 * 10) = 4000 = 36 100 + 10 110• Remember:
Ne = 4(NM * NF)
NM + NF
where NM = number of males
NF = number of females
Genetic Drift
• Random change in allele frequency due to sampling only a small portion of gametes from the previous generation.
• Most likely in small populations (<100 individuals).
• Least likely in large populations (< 1,000 individuals.
Genetic Drift
The proportion of genetic variation retained in a population of constant size after t generations is approximately:
Proportion = (1 -1/(2N))t
where N = number of individuals
t = number of generations
Genetic Drift
What proportion of genetic variation is retained in a population of 10 individuals after 10 generations?
Proportion = (1 - 1/20)10 = 0.9510
= .5987 or about 60% Proportion = ((1 -1/(2N))t
where N = number of individuals
t = number of generations
Genetic Bottleneck
• The loss of genetic variation when a population drops in size.
• Effective population size (Ne) after a fluctuation in population size is estimated by:
Ne = t/ sum of (1/Ni)
where Ni = size of population in generation i t = number of generations
Genetic BottleneckWhat is the effective population size (Ne) of one
that goes from 1,000 (t1) to 10 (t2) and recovers to 2,000 (t3)?
Ne = t/ sum of (1/Ni)
where Ni = size of population in generation i t = number of generations
Genetic BottleneckWhat is the effective population size (Ne) of one
that goes from 1,000 (t1) to 10 (t2) and recovers to 2,000 (t3)?
Ne = _________ 3 ________ = 3/0.1015 1/1000 + 1/10 + 1/2000 = 29 individuals
Ne = t/ sum of (1/Ni)
where Ni = size of population in generation i t = number of generations
Inbreeding
• Inbreeding occurs more frequently in isolated and small populations.
• It acts to reduce Ne. It is estimated bY;
Ne. = ____N_____
1 + F
where F is the inbreeding coefficient
or probability of inheriting 2 alleles
from the same ancestor.
Inbreeding vs Outbreeding
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Inbreeding Depression
• Prairie chickens in Illinois declined due to decreased hatching success.
• Individuals from Iowa were introduced to the breeding population and hatching success improved.
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Metapopulations Reduce Extinction Risk (I)
• Studies of the Granville fritillary show how subpopulations stabilize overall population size.
• In addition, provide opportunity for gene flow.
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Metapopulations Reduce Extinction Risk (I)
• Oerall population size remains relatively stable even when local populations go extinct.
• The metapopulation provided for increased opportunity for gene flow between local populations.
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Population Viability Analysis (I)
• PVA provides a means for estimating the likelihood that a population will avoid extinction for a given period of time.
• Freeman (2005) describes a study of how migration rates are likely to influence population viability of an endangered marsupial.
Population Viability Analysis (II)
• This endangered marsupial lives in an old-growth forest in southeastern Australia and relies on dead trees for nest sites.
• PVA was used to predict the consequences of habitat loss and forest fragmentation on this endangered species.
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Population Viability Analysis (III)
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Population Viability Analysis
• Freeman describes demographic studies of a European lizard species that is declining in some areas.
• He explains how migration maintains some local populations in spite of local extinction.
• He presents a model of how migration rates are likely to influence population viability of an endangered marsupial.
Life History Characteristics, Population Size and Extinction
Risk
• Extinction risk is related to the life history characteristics of the species in question.
• Small populations with “long-lived” life history characteristics are particularly vulnerable to extinction .
LIFE HISTORY CHARACTERISTICS
• Population attributes such as lifespan, mortality and natality patterns, biotic potentials, and patterns of population dynamics are called life history characteristics.
• Life history characteristics have important consequences for wildlife management and extinction risk.
FOUR IMPORTANT ASPECTS OF LIFE
HISTORIES • 1. Lifespan --- the upper age limit for the
species.• 2. Mortality --- the pattern of survivorship (I,
II, or III).• 3. Natality --- the age to reproductive
maturity and number of offspring produced.• 4. Biotic potential --- maximum rate of
natural increase (rmax = births - deaths).
LIFE HISTORY EXTREMES
• Short-lived.• Type III survivorship
high juvenile mortality; relatively secure old age.
• Many offspring from young adults.
• High maximum rate of population growth.
• Long-lived.• Type I survivorship:
low juvenile mortality; high mortality at old age.
• Few offspring from older adults.
• Low maximum rate of population growth.
LIFE HISTORY TRAITS FORM A CONTINUUM (I)
• Every species can be placed somewhere on a continuum with respect to the life history extremes.
• Comparisons of life histories are best done between species that show similar evolutionary histories.
LIFE HISTORY TRAITS FORM A CONTINUUM (II)
• Field mice and muskrats are rodents in closely related taxonomic families.
• Field mice (short-lived) show a Type III survivorship and produce many offspring.
• Muskrats (long-lived) have a Type I survivorship and produce few young.
LIFE HISTORY TRAITS FORM A CONTINUUM (III)
• See Freeman (2005) page 1195 for full discussion.
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Some Long Lived Species
• These have moderate juvenile mortality, low adult mortality, and low fecundity.
• They are endangered.
Whooping Crane Spotted Owl
Some Short Lived Species
• These have high juvenile mortality, moderate adult mortality, and high fecundity.
• They are thriving.
Starling House Finch