natural selection

1. Modern concept of natural
selection

2. Selection
• In a population some individuals are more successful
than others in passing along their genes into the next
generation. This may occur for many reasons.
• Some organisms die before they reach reproductive
age.
• Some organisms fail to find a mate.
• Among those individuals that do breed, there are
differences in the number of offsprings produced.
• The result of these differences means that for one
reason or the other there is a differential rate of
reproduction among members of a population, we call
this selection.

3. Selection patterns
• Both in natural selection and artificial selection ,
the principle is the same: some organisms breed
more prolifically than others, thus, increasing the
frequency of some genes and decreasing the
frequency of others.
• The following three patterns of natural selection
have been recognized by population geneticists:
• Directional,
• Stabilizing and
• Disruptive (diversifying).

4. 1. Directional Selection
• The directional selection describes the change that
occurs when a population shows a particular trend
through time
• Such selection process favours individuals that are best
adapted to new situations or to new ecological
opportunities
Under directional selection, the advantageous allele
increases as a consequence of differences in survival
and reproduction among different phenotypes. The
increases are independent of the dominance of the
allele, and even if the allele is recessive, it will
eventually become fixed.

5. • In fact, directional selection transforms the
gene pool of a species or population toward
the highest level of adaptedness that can be
reached in the new environment.
• E.g Industrial melanism in moth.

6. 2. Stabilizing Selection
• Stabilizing selection is a type of natural
selection in which genetic diversity decreases.
• It describes the change when extreme
individuals are eliminated from the
population. So the intermediate values for a
given trait are favoured over the extreme
values. The result of this process is a reduced
variability in the population.

7. • A classic example of this is human birth
weight. Babies of low weight lose heat more
quickly and get ill from infectious diseases
more easily, whereas babies of large body
weight are more difficult to deliver through
the pelvis. Infants of a more medium weight
survive much more often. For the larger or
smaller babies, the baby mortality rate is
much higher.

8. 3. Disruptive Selection
• This form of selection occurs when the
extreme values have the highest fitness and
the intermediate values are relatively
disadvantageous in terms of reproductive
effectiveness.
• For example, shell patterns of limpets (marine
molluscs) .

9. • Limpets typically dwell in one of two distinct
habitats, attaching either to white goose-neck
barnacles, or to tan-coloured rocks. As might
be expected, the light-coloured limpets seek
the protection of the white barnacles,whereas
tan limpets live by choice almost exclusively
on the dark rocks. Limpets of intermediate
shell patterns are conspicuous and are
intensely selected against by predatory shore
birds.

11. Levels of selection
The genetic makeup of a population is altered
through an interaction with the ecology of the
organism. We refer to this interaction as the
process of natural and sexual selection. The
fundamental premise of Darwinian selection is
that natural selection acts on the individual, or
more properly, differences in phenotype
among individuals within a population

12. • In recent years a number of authors have
argued that selection might act at a number of
different levels and these levels of selection
are loosely structured according to heirarchies
of biological organization:
• genes -> individuals -> kin -> groups ->
species

13. Labortory and field examples
regarding action of natural
selection

14. 1. Melanism in Moths or Industrial
Melanism
• The peppered moth (Biston betularia) is a moth which flies during
the night and rests on trees during the day, where it is camouflaged
to hide from birds. The two most common forms are called typicall,
a pale speckled moth which is well-disguised on light-coloured
lichens growing on trees, and carbonaria, a black (or melanic) moth
which is easy to see on the same background. Both are different
forms of the same species, like humans with blonde or brown hair.
Before the Industrial Revolution, when there was much less
pollution, many trees were covered in lichen and the typical form
was well-camouflaged when resting on them. Because
the carbonariaform stood out against this pale background, birds
foundcarbonaria moths much more easily than typical,
so carbonariawere more likely to be eaten, and so less likely to
survive to pass on their genes. This meant that the carbonaria form
was rare.

15. • As coal-burning factories were built, air pollution
increased significantly, which killed off the lichens and
blackened the trees with soot. On this dark
background, the pale typical moths were no longer
well-camouflaged and were easily caught by birds, but
the carbonaria moths were harder for birds to see, so
more carbonaria survived to breed and pass on their
genes, and this dark type of peppered moth became
more common. By 1895, 98% of moths in heavily-
polluted Manchester were carbonaria. Armed with
Darwin’s new theory of natural selection, J.W. Tutt, an
English entomologist, hypothesised in 1896 that the
change in colour was due to selection pressure based
on how often birds were able to spot the moths.

17. 2. Australian Rabbits
• In 1859, a small colony of 24 wild rabbits (Oryctolagus
cuniculus) was brought from Europe to an estate in
Victoria in the south eastern corner of Australia. From
such modest beginnings, the rabbits multiplied
enormously and by 1928 had spread over the greater
part of the Australian continent.
• According to an estimate, the number of adult rabbit
was over 500 million in an area of about 1 million
square miles. The rabbits caused extensive damage to
sheep-grazing pastures and to wheat cropfields

18. • For controlling the population explosion of
these rabbits, the Australian government
spent huge sums of money for many years.
Trapping, rabbit-proof fencing, poisoning of
water holes, and fumigation all proved to be
largely inadequate. Then, beginning in 1950,
outstanding success in reducing the rabbit
population was achieved by a biological
control method, i.e., inoculating rabbits with a
virus that causes the fatal disease
myxomatosis

19. • The deadly myxoma virus was implanted into
the tissues of rabbits in the southern area of
Australia. In a remarkably short period of
time, the virus had made its way, aided by
insect carriers (mosquitoes), into most of the
rabbit-infested areas of the continent. By
1953, more than 95 per cent of the rabbit
population in Australia had been eradicated

20. • But, after their drastic decline in the early
1950s, the rabbit populations began to build
up again.Evolutionary changes have occurred
in both the pathogen (i.e., myxoma virus) and
the host (rabbit).Mutations conferring
resistance to the myxoma virus have
selectively accumulated in the rabbit
populations. At the same time, the viruses
themselves have undergone genetic changes;
less virulent strains of the virus have evolved
(Frank J. Fenner, 1959).

21. Labortory example
• Gjedrem (1979) showed that selection of Atlantic salmon (Salmo
salar) led to an increase in body weight by 30% per generation. A
comparative study on the performance of select Atlantic salmon
with wild fish was conducted by AKVAFORSK Genetics Centre in
Norway. The traits, for which the selection was done included
growth rate, feed consumption, protein retention, energy retention,
and feed conversion efficiency. Selected fish had a twice better
growth rate, a 40% higher feed intake, and an increased protein and
energy retention. This led to an overall 20% better Fed Conversion
Efficiency as compared to the wild stock.Atlantic salmon have also
been selected for resistance to bacterial and viral diseases.
Selection was done to check resistance to Infectious Pancreatic
Necrosis Virus (IPNV). The results showed 66.6% mortality for low-
resistant species whereas the high-resistant species showed 29.3%
mortality compared to wild species.