DIVERSITY Outline
GENETIC AND
EXTINCTION
Genetic diversity and small population size
Minimum viable population
Immigration/emigration
Genetic consequences inbreeding
Lemur
Golden lion tamarin
Small Populations
Genetic Diversity
Central issue:
how small populations can maintain genetic integrity
intraspecific variation or gene diversity
studied by geneticists
- focus on population consequences of rareness and the
ability of small populations to survive
Genetic Issues
The maintenance of genetic diversity in rare and
endangered species
Relationship between heterozygosity and fitness
Genetic Diversity
Genetic considerations
small population size
inbreeding
genetic drift
Loss of genetic diversity from small populations
Demographic considerations
immigration/emigration
Minimum Viable
Population Size (MVP)
What factors can cause a species to go
extinct?
When species consist of a few or even a single local
population/s, protecting these populations is key to
preserving these species
the smallest isolated population having a 99 percent chance of
remaining extant for 1000 years despite the foreseeable effects
of demographic and environmental stochasticity, and natural
catastrophes.
Defined by M.L. Shaffer
What sizes of populations are needed to save a species?
Must be large enough to:
cope with chance variations in demography (births and deaths),
environmental changes, genetic drift, and catastrophes
The actual MVP for a species depends on the life history of the species and the ability of
individuals to disperse
Minimum Dynamic Area (MDA)
Minimum Viable Population
Minimum Dynamic Area
is the area of suitable habitat necessary for
maintaining the minimum viable population
Focus: on top predators
Why?
Based on the home-range size of the individuals, family groups, or colonies
Rational:
Home
range
Size of species
Big horn sheep:
How does population size affect
persistence?
Protecting Populations Is the Key to
Conservation Efforts
Summary:
study of bighorn sheep persistence in the deserts of the
southwestern United States (J. Berger)
120 populations were studied
all populations with 50 individuals or less went
extinct within 50 years
all of the populations with 100 or more individuals
persisted over the same period
Species at Risk
endangered species
- those that are in danger of extinction throughout all or a
significant portion of their range
threatened species
- those likely to become endangered in the for-seeable future
vulnerable species
- those that are becoming threatened; vulnerability likely caused
by habitat loss and destruction
a documented case of MVP in bighorn sheep
Endangered Species
(low numbers)
Spekes Gazelle
Genetic Consequences
of Small Population Size
Loss of genetic diversity:
Population bottlenecks
Founder effects
Genetic drift
GENETIC CONSEQUENCES
OF SMALL POPULATION SIZE
Inbreeding Depression
decreased fitness that occurs as more
deleterious recessive traits are expressed in
the phenotypes of homozygous recessive
individuals
reductions in viability and fertility
inbreeding -
Inbreeding
- mating between relatives reduces the frequency of heterozygotes and
increases the frequency of homozygotes in each generation
- affects genotype frequency, but does not change allele frequencies
% of initial
heterozygosity
Generations
Demographic considerations:
Immigration between patches
Immigrants can increase
genetic variability
Inbreeding depression in Lobelia cardinalis
Computer simulations for affect of Immigrants on
genetic heterozygosity
Immigrants
per
generation
% initial heterozygosity
N = 120
5
2
1
0.5
0.1
none
Inbreeding Species:
Inbreeding Depression
Endangered Depression
2 processes:
many recessive alleles: represent loss of
function mutations
many genes involved in fighting disease
have a heterozygote advantage
F
r
e
q
u
e
n
c
y
Inbreeding Depression:
Cheetah
Restricted to two wild populations in southern
& eastern Africa
A survey of 52 proteins indicated complete
monomorpism (had 1 form of each protein).
Other cats have 8-21% of loci polymorphic
Cheetah (Acinonyx jubatus)
Variability at the MHC (Major Histocompatability Complex: genetic
locus in mammals that directs synthesis of antigens on the
surface of most cells)
Skin graft experiment:
Allografts: from unrelated animals of same species no
rejection
Autografts: from self no rejection
Xenografts: from another species rejection
Conclusion: cheetahs must have identical genes at the MHC
Scientific Amer. 254(5)84-92
Scientific Amer. 254(5)84-92
Cheetah
Inbreeding Depression:
Wolves of Isle Royale
Consequences:
high juvenile mortality
low spermatozoal counts
low genetic variation: likely resulted from a historical
bottleneck in population size
Deformed vertebrae:
pain
partial paralysis
inhibited locomotion
year of birth
1 = deformed bones
Congenital bone deformities
0 = normal bones
Scientific Amer. 254(5)84-92
Reintroduction Is Necessary to Reestablish
Populations of Some Species
In some cases, the direct action of ecologists through transplants
and reintroductions may be the only way to save a species from
inevitable extinction:
White rhino
Reduced to near 50 at the beginning of the 20th century
Operation Rhino transported surplus numbers of individuals to
protected areas
Numbers have recovered to an estimated 7500 individuals
Genetic Diversity
Different populations of the
same species have different
deleterious traits, and
therefore their crossbreeding
will not result in
homozygosity in most loci in
the offspring.
This outbreeding
enhancement, is practiced by
conservation managers and
zoo captive breeders to
prevent homozygosity.
Raikkonen et al. 2009. Biol. Conservation
Reintroduction Is Necessary to Reestablish
Populations of Some Species
Black Rhino: sustaining genetic diversity
driven to near
extinction by human
hunting:
low genetic variability
Genetic Diversity: Dilemma
intermixing two different
populations (or subspecies)
may give rise to unfit traits
in outbreeding depression,
yielding offspring which
lack the genetic adaptations
to specific local
environmental conditions.
Sustaining Genetic Diversity
Black rhino
Rhinos are scattered in populations too small to be
self-sustaining or in large populations vulnerable to
disease, poaching, or overpopulation
Relocation
Peregrine Falcon
Reintroduction Is Necessary to Reestablish
Populations of Some Species
Introduction of individuals from captive-bred populations
requires pre-release and post-release conditioning:
Successful outcomes:
Whooping crane
Hawaiian goose
Peregrine falcon
California condor
California Condor
Population disappeared east of the Mississippi by
1960
captive propagation is a
temporary measure
DDT was responsible
DDT was banned in 1970
Captive breeding program started using falcons from other parts of
the country
Success story
Environmental Ethics Is at the Core of
Conservation
The evolutionary argument is based on genetics
The mass extinction of modern-day species limits the
potential evolution of species diversity in the future
original range: British
Columbia to Mexico
San Diego condor breeding facility
as of 2003: 81 wild
condors in California
and Arizona