Environmental Engineering Reference
In-Depth Information
Box 5.2 Genetics of
small populations
What is genetic variation?
Each gene can exist in a number of forms or 'alleles'. Recall that sexually reproducing organisms
receive one copy of each gene from each parent. These may be identical (the same allele derives
from both parents) or different. In the latter case, one form of the allele is usually 'dominant' and
expressed in the individual; the 'recessive' allele is unexpressed, but is passed on to some of the
offspring. Now consider, for example, the gene for fl ower color in a particular plant species, or the
gene for a particular enzyme in an animal species. If all individuals in a population have the same
form of the gene in question (i.e. all possess an identical allele), the population has low diversity
for that gene - all the fl owers are the same color and all the animals operate optimally under the
same physicochemical conditions. If, on the other hand, a population contains individuals that
possess one of a number of alleles of the gene (and of other genes), the population has high genetic
diversity.
Genetic variation in a population is determined by the joint action of 'natural selection' (where
the frequency of an allele in a population is related to the evolutionary advantage it confers) and
'genetic drift' (where the frequency of an allele is determined simply by chance). Geneticists have
powerful molecular tools (such as DNA fi ngerprinting) to determine genetic variation.
Loss of genetic variation in small populations
Box 5.1 explained how small populations are subject to increased demographic risks. Population
genetics theory tells us to also beware genetic problems in small populations, which arise through
loss of genetic diversity. The infl uence of genetic drift is greater in small isolated populations,
which as a consequence are expected to lose genetic variation. And populations that some time in
their history have been reduced to just a few individuals (a 'bottleneck'), or that have arisen from
a few 'founder' migrant individuals, will have particularly low genetic diversity, even if they subse-
quently increase in numbers. Migrants can be important in another respect too - populations where
immigration is a common event are likely to be genetically more diverse because of alleles con-
tributed by the migrants.
Greater prairie-chickens ( Tympanuchus cupido pinnatus ) provide a good example of the relation-
ship between population size and genetic diversity. These birds were once widespread throughout
the tall-grass prairies of Midwestern North America, but with the loss and fragmentation of this
habitat many populations have become small and isolated. Johnson et al. (2003) used molecular
(DNA) techniques to measure genetic diversity in both large (from 1000 to more than 100,000
individuals) and small prairie-chicken populations (fewer than 1000 individuals). The mean number
of alleles (per region of DNA) ranges from 7.7 to 10.3 in the large populations, but is only 5.1 to
7. 0 i n t h e small populations. It seems that prairie-chicken populations were once linked by the 'gene
fl o w ' p r o v i d e d b y m i g r a n ts, which kept genetic diversity high, but current populations are isolated
in their habitat fragments.
Why might loss of genetic diversity be a problem? Rare alleles that confer no immediate advan-
tage might turn out to be well suited to changed environmental conditions in the future. Small
populations that have lost rare alleles may therefore have less potential to adapt, increasing their
risk of extinction in the long term. Consider two populations - in the fi rst, all individuals possess
the same allele for a particular enzyme, but in the second many alleles are represented. An increase
in temperature (associated with an unusual sequence of hot years or human-induced global
warming) might lead to uniformly poor enzyme performance and a high risk of extinction of the
fi r s t p o p u l a tion. In the second population, on the other hand, some individuals with an allele that
confers good enzyme activity at higher temperatures may now be at a selective advantage and
prosper (passing their alleles on to offspring) - with a consequently reduced likelihood of
extinction.
Inbreeding depression
A more immediate potential problem is inbreeding depression. When populations are small there
is a tendency for related individuals to breed with one another. All populations carry recessive
alleles that can be lethal to individuals when homozygous (when the alleles provided by the mother
and father are identical). Individuals that breed with close relatives are more likely to produce off-
spring where the harmful alleles are derived from both parents - so the deleterious effect is
expressed. Domestic and zoo breeders have long been aware that inbred individuals may show
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