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
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:
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 Offspring F
Or father-daughter 0.25
Half-brother - half-sister 0.125
First cousins 0.0625
Self-fertilization (or selfing) 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
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
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
Reduced Extinction Genetic
Survival & reproduction
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
The mechanisms underlying susceptibility of
island populations to extinction are controversial.
Ecologists stress the susceptibility of small
island populations to demographic and
However, this susceptibility is also predicted on
Island populations are expected to be inbred due
to low number of founders and subsequent small
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
Island populations typically have less genetic
diversity and are more inbred than mainland
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 &
Natural populations face continuous assaults from
Environmental change including:
Competitors & predators
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
Over time, genetic diversity will decline, with
Loss being more rapid in smaller than larger