natural selection

1. Modern concept of natural
selection

2. Natural selection
• Natural selection is the differential survival
and/or reproduction of organisms as a
function of their physical attributes. Because
of their phenotypes,which are due to the
amalgam of traits that make up an individual,
some individuals do better than others.

3. Selection patterns
• Selection is defined as some sort of functional
relationship between fitness and phenotype
and we can easily describe fitness in terms of
three kinds of curves:

4. 1.Directional selection in which the trait is
linearly related to fitness,
2.Stabilizing selection in which there is an
optimal value for the trait of interest, and
3.Disruptive selection in which individuals with
the smallest and largest values of the trait
have the highest fitness and individuals with
intermediate values are at a fintess
disadvantage

5. 1. Directional Selection
• Favors variants of one extreme.
NumberofIndividuals
Size of individuals
Small Large

6. 2. Stabilizing Selection
• Acts upon extremes and favors the intermediate.
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of
Individuals
Size of individuals
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7. 3. Diversifying Selection
• Favors variants of opposite extremes.
Number
of
Individuals
Size of individuals
Small Large

8. • Each mode of selection alters the mean or variance of
the phenotypic trait in a population or species. In the
long term, directional selection can have the most
dramatic impact on the evolution of a species.
Directional selection can lead to the formation of a
new type from an existing type. This contrasts with the
action of stabilizing selection which maintains the
existing type without change in mean over long periods
of time. Stabilizing selection eliminates the extremes in
a distribution of phenotypes, and as such it leads to a
refinement of the exisiting type. By eliminating
individuals from the center of the distribution,
disruptive selection favors the individuals in the tails or
more extreme values of the phenotype. Disruptive
selection can lead to the formation of two new types
from a single exisiting type.

9. 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

10. • 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

11. Laboratory and field examples
regarding action of natural
selection

12. 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.

13. • 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.

15. 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

16. • 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

17. • 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

18. • 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).

19. 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.

20. Action of natural selection leading
to
convergence,radiation,regression
and extinction

21. Radiation
• Natural selection can ultimately lead to the
formation of new species. Sometimes many
species evolve from a single ancestral species.
Fourteen species of Darwin finches evolved from
one ancestral species. Such an evolutionary
pattern, in which many related species evolved
from a single ancestral species, is called adaptive
radiation. Adaptive radiation most commonly
occurs when a species of organisms successfully
invades an isolated region where few competing
species exist. If new habitats are available, new
species will evolve.

22. Extinction
• Natural selection is a process through which the traits that
are helpful in the survival of an organism are passed on to
the next generation while those that can hinder survival are
eliminated. Natural selection then only allows the very well
adapted species to reproduce. This is done to ensure
survival in the ever-changing environmental conditions.
Due to the evolutions that have been taking place, the
habitat of organism change and to cope up with these
changes, the organisms develop some genetic
characteristics that would enhance their survival. However,
even with natural selection, studies have shown that some
organisms, both plants and animals have become extinct.
Dinosaurs became extinct sixty five million years ago.
Another example is the thylacine tiger of Australia.

23. • The minority of the organisms in the environment
are the ones that become extinct together with
those that are unable to survive the changes in
the environment. The minority are unable to
compete with the majority for survival means.
Moreover, when new species are being formed or
modified they exert pressure on the existing ones
and finally eliminate them. The introduction of
new breeds of animals leads to the extinction of
the inferior ones.

24. • The main reason why extinction still occurs is
that natural selection is good at maintaining
and even improving species adaptations while
on the other hand it eliminates those species
that are the minority and those that are not
well adapted. It is then clear that natural
selection does not only play a role of
preventing extinction, it also supports
extinction of the species that are not well
adapted for survival. This being the case then,
extinction is bound to prevail even if natural
selection is still there.

25. Convergent Evolution
• Species from different evolutionary branches
may come to resemble one another if they live in
very similar environments.
• For example, flight has evolved in both bats and
insects, and they both have wings, which are
adaptations to flight. However, the wings of bats
and insects have evolved from very different
original structures

26. Regression
• Natural selection may favor Regresion.
• Regression is a process of partial or complete
reduction of organs that have lost their adaptive
significance.
• For example, replacementof the notochord by a
cartilaginous skeleton and later by a bony
skeleton in the process of
vertebrate evolution,replacement of gills by
lungs when vertebrates emerged onto dry land.

27. Co-evolution
• Evolutionary change, in which one species act
as a selective force on a second species,
inducing adaptations that in turn act as
selective force on the first species.
• In evolutionary biology, mimicry is a similarity
of one species to another which protects one
or both.

28. Examples of coevolution: mimicry
• The classical examples of mimicry illustrate nicely different types of
co-evolutionary interactions.
• Müllerian mimicry describes the convergence of unpalatable
models to a similar phenotype, i.e. reciprocal evolution between
species all of which are distasteful. It is thus characterised as a +/+
mutualistic interaction, i.e. all involved species benefit
• Batesian mimicry describes the convergence of a palatable species
to unpalatable models. A non-toxic, edible species mimics the
warning colour of a toxic, noxious model. This is a 0/+ interaction,
because only one species benefits . The system works well as long
as the mimic does not become too frequent. Otherwise, the
noxious model may have a disadvantage (i.e. the interaction may
turn into antagonism, –/+), because predators do not avoid the
warning colour any more.