Small populations
behave differently from larger populations. Often this has various
harmful consequences for the survival of that population.
Demographic effects
The influence of
stochastic variation in
demographic (reproductive and mortality) rates is much higher for
small populations than large ones. Stochastic variation in
demographic rates causes small populations to fluctuate randomly in
size. The smaller the population the greater the probability that
fluctuations will lead to extinction. They are subject to a higher
chance of
extinction because they are
more vulnerable to
genetic drift,
resulting in stochastic variation in their
gene pool, their
demography and their
environment.
One demographic consequence of a small population size, the
probability that all offspring in a generation are of the same sex,
and where males and females are equally likely to be produced (see
sex ratio), is easy to calculate: it is
given by 1/2^{n-1} (The chance of all animals being females is
1/2^n; the same holds for all males, thus this result). This can be
a problem in very small populations.
In 1977, the last 18
Kakapo on a Fiordland
island in New Zealand
were all male, though the probability of this would
only be 0.0000076 if determined by chance (however, females are
generally more predated than males and kakapo may be subject to
sex allocation). With a
population of just three individuals the probability of them all
being the same sex is 0.25. Put another way, for every four species
reduced to three individuals (or more precisely three individuals
in the effective population), one will go extinct within one
generation just because they are all the same sex. If the
population remains at this size for several generations, such an
event becomes almost inevitable.
Environmental effects
Stochastic variation in the environment (year to year variation in
rainfall, temperature) can produce temporally correlated birth and
death rates (i.e. 'good' years when birth rates are high and death
rates are low and 'bad' years when birth rates are low and death
rates are high) that lead to fluctuations in the population size.
Again, smaller populations are more likely to go extinct due to
these environmentally generated population fluctuations than are
large populations.
Genetic consequences
Conservationists are often worried about a loss of genetic
variation in small populations. There are two types of genetic
variation that are important when dealing with small
populations.
- The degree of homozygosity within
individuals in a population; i.e. the proportion of an individual's
loci that contain homozygous rather than heterozygous alleles.
Many deleterious alleles are only harmful in the homozygous
form.
- The degree of monomorphism/polymorphism within a population;
i.e. how many different alleles of the same gene exist in the gene
pool of a population. Polymorphism may be particularly important at
loci involved in the immune response.
There are two mechanisms operating in small populations that
influence these two types of genetic variation.
- Genetic drift - Genetic variation
is determined by the joint action of natural selection and genetic drift
(chance). In small populations the relative importance of genetic
drift (chance) is higher; deleterious alleles can become more
frequent and 'fixed' in a population due to chance. Any allele,
deleterious, beneficial or neutral is more likely to be lost from a
small population (gene pool) than a large one. This results in a
reduction in the number of forms of alleles in a small population
and in extreme cases to monomorphism where there is only one form
of the allele. Continued fixation of deleterious alleles in small
populations is called Muller's
ratchet, and can lead to mutational meltdown.
- Inbreeding - In a small population,
related individuals are more likely to breed together. The
offspring of related parents have a far higher number of homozygous
loci than the offspring of unrelated parents.
There are two types of potential consequence of loss of genetic
variation:
- Inbreeding depression -
Inbreeding depression is usually taken to mean any immediate
harmful effect, on individuals or the population, of a decrease in
either type of genetic variation. Inbreeding depression can almost
never be found in declining populations that were not very large to
begin with; it is somewhat common in large populations
becoming small though. The reason is purging selection, most efficient in
populations that are strongly but not dangerously inbred.
- The ability of the population to adapt/evolve to changing
conditions, “without variability evolution is impossible” . It is
obvious that the absolute size of a population limits the absolute
degree of allelic diversity. On the other hand, should an
advantageous mutation arise, it is likely to show its effect sooner
and more thoroughly.
The
effective population
size is commonly lower than the actual population size.
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