Transcript Genetic Diversity

Chapter 2 -- Genetics & Extinction
Main Point -- Inbreeding & loss of genetic
diversity are inevitable in small populations.
These factors reduce reproduction & survival in
the short-term and diminish capacity of population
to evolve in response to environmental change in
long-term.
Inbreeding -- Production of offspring by
individuals related by descent.
Genetic Diversity -- Extent of heritable
variation in a population, or a species, or across
a group of species and include:
heterozygosity
allelic diversity
haplotype diversity
nucleotide diversity
Rates of Inbreeding & Extinction Risks
Slower inbreeding allows natural selection more
opportunity to remove genetically compromised
individuals (an therefore deleterious alleles).
For fitness components such as survival & fecundity,
slower rates of inbreeding generally lead to less
inbreeding depression than fast inbreeding for
the same total amount of inbreeding.
Even slow rates of inbreeding though increase
risk of extinction.
The inbreeding coefficient (F) on an individual
refers to how closely related its parents are.
Parents
Unrelated
Brother-sister; mother-son,
Or father-daughter
Half-brother - half-sister
First cousins
Self-fertilization (or selfing)
Offspring F
0
0.25
0.125
0.0625
0.5
Response of populations to environmental and
demographic stochasticity and the impact of
catastrophes are not independent of inbreeding
and genetic diversity.
Inbreeding, on average,
reduces birth rates,
increases death rates, and
may distort sex ratios.
Inbreeding therefore interacts with the basic
parameters determining population viability,
such as population growth rate.
Adverse effects of inbreeding on population
growth rates probably occur in most naturally
outbreeding species.
Experimental populations of mosquito fish founded
From brother-sister pairs showed 56% lower
Growth in numbers than populations founded from
Unrelated individuals (Leberg 1990).
Strong reductions in pop. Growth were also found
In flour beetle populations inbred to small numbers.
They even detected adverse effects at F = 0.1
(McCauley & Wade 1981).
If populations become small for any reason, they
become more inbred and less demographically
stable, further reducing population size and
increasing inbreeding.
This feedback between reduced population size,
loss of genetic diversity, and inbreeding is
referred to the “Extinction Vortex”.
Habitat loss
Pollution
Over-exploitation
Exotic species
Small, fragmented
Isolated populations
Environmental
variation
Catastrophes
Reduced
N
Extinction
Vortex
Demographic
stochasticity
Reduced adaptability,
Survival & reproduction
Inbreeding
Loss of
Genetic
diversity
Smaller populations are expected to be more
extinction prone than larger ones for demographic,
ecological, and genetic reasons.
Since the 1600s, the majority of recorded
extinctions have been of island forms even though
island species represent a minority of total
species of all groups.
Human factors have been the major recorded
causes of extinctions on islands over the past
50,000 years.
The mechanisms underlying susceptibility of
island populations to extinction are controversial.
Ecologists stress the susceptibility of small
island populations to demographic and
environmental stochasticity.
However, this susceptibility is also predicted on
genetic grounds.
Island populations are expected to be inbred due
to low number of founders and subsequent small
population size.
There is little evidence to separate the effects
of non-genetic factors from the effects of
inbreeding and loss of genetic diversity.
Inbreeding can certainly diminish the resistance of
a population by reducing its reproductive rate and
survival such that it is more susceptible to
non-genetic factors.
Island populations typically have less genetic
diversity and are more inbred than mainland
populations.
Analysis of 202 island populations revealed that
82% had lower levels of genetic diversity than
their mainland counterparts.
Inbreeding in many island populations is at levels
where captive populations show an elevated
risk of extinction.
Relationships between loss of genetic diversity &
Extinction
Natural populations face continuous assaults from
Environmental change including:
New diseases
Pests
Parasites
Competitors & predators
Pollution
Climatic cycles
Human-induced global change
Species must be able to evolve to cope with these
new conditions or face extinction!!
To evolve, species require genetic diversity.
Naturally outbreeding species with large
populations normally possess large stores of genetic
diversity that confer differences among individuals
in their response to such environmental changes.
Due to sampling of alleles in the parental
generation to produce offspring, small populations
typically have lower levels of genetic diversity
than large populations.
Due to this random sampling process, some alleles
increase in frequency, others decrease and
some alleles may be lost entirely.
The smaller the population the more change there
will be between the parental and offspring gene
pools.
Over time, genetic diversity will decline, with
Loss being more rapid in smaller than larger
Populations.