Download V SEMESTER ZOOLOGY DARWINISM

V SEMESTER ZOOLOGY
DARWINISM
Darwin's observations during the voyage of the Beagle made him think differently and
changed his views permanently. He started believing that species change gradually
due to the force of natural selection and started writing his ideas. In 1859, Darwin
published his famous book, “Origin of species” that gave details of the theory and
made Darwin a celebrity. Ever since, the theory is known as Darwinism or Darwin’s
theory of Natural Selection.
In his theory, Darwin gave details of his observations in different parts of the world
during the voyage of the Beagle, from which he made two deductions and in the end
concludes that natural selection takes place.
Observations
1. Overproduction: Darwin said that all organisms, without any exception, reproduce
at a much higher rate than required.
Examples:
A. Fishes lay millions of eggs during spawning. Each oyster can lay 60-80 million eggs.
B. Darwin calculated that if a pair of houseflies lays all its eggs and if all offsprings
survive and reproduce to their full potential, then in one season (March to October) it
will leave a progeny of 191,000,000,000,000,000,000 individuals.
C. Elephant is considered to be the slowest breeder. Its reproductive age is 30-100
years, during which it produces only 6 youngs. Darwin says even this is
overproduction because if all offsprings and their offsprings survive and reproduce,
then one pair will give rise to 19 million offsprings in 750 years.
D. Paramecium undergoes binary fission three times for every two days. If all young
ones survive and multiply at the above rate for five years. The volume of all paramecia
would be ten times that of total earth.
E. Pearl oyster female produces sixty million eggs per year. If all of them were
fertilized, developed into young ones and reproduce at the above rate, for five years,
the volume of all the individuals would be eight times that of the the size of earth.
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F. The population of human being is doubled for every ‘25’years. If the same rate
continues for about 1000 years, there would not be even standing place for some
humans.
2. Number is constant: In spite of overproduction by all organisms, their number in a
given area remains more or less constant. All ecosystems have a limited capacity to
support a particular species of organisms, e.g. a pond can support a fixed number of
fish and a forest can support a fixed number of tigers or deer.
This constancy is mainly due to the following facts.
They are
(a) Many eggs will not be fertilized,
(b) Many zygotes will not be developed into young ones,
(c) Some young once will die,
(d) Some will not reproduce and
(e) Some may not reproduce at the above rate.
So, there is high mortality at every stage. Even the small population may be increased;
it is kept constant due to struggle for existence
3. Variations: No two individuals are alike, even within a species, race or cohort.
There are variations among individuals in appearance, physiology and capacity to
starve, run and tide over cold or heat and many other traits.
These are two types. They are (a) continuous, small, fluctuating and hereditary
variations,(b) Discontinuous, large, sudden, non hereditable variations. Darwin said
that the evolution is mainly due to small variations but not due to large variations.
Examples:
1. The organisms which have protective coloration are better protected from enemies
than the others.
2. Sheep which have dense hair are better protected in north pole than the other
sheep from severe cold.
Darwin believed that all variations are heritable and he gave some weightage to
Lamarckism because he did not know mechanism of heredity.
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Deductions
4. Struggle for existence: From the first two points, that is, overproduction is going
on in nature everywhere but the number of organisms that can be supported by any
ecosystem has to be constant, Darwin deduced that large number of offsprings
produced will have to struggle among themselves to survive. Large number of
individuals will perish in this struggle and few will be able to reach adulthood. Darwin
believed that the struggle among the animals is always in the form of physical
combat.
The struggle is of three kinds. They are,
A. Intra specific struggle:
It is the struggle between the individuals of same species. This struggle is highly
severe as their basic needs are same. Highest severe struggle can be seen in the
individuals of same species living in the same niche. Eg-Man and Man; Dog and Dog;
Rat and Rat;
B. Inter specific struggle:
It is the struggle between the individuals of different species.
Eg — Rat and Cat; Cat and Dog; Herbivores and Carnivores. In inter specific struggle
one kind of population regulates another kind of population [Rat population is
controlled by cat population.
C. Environmental struggle:
It is also called extra specific struggle. It is the struggle between the individuals and
natural forces like earth quakes, volcanic eruptions, severe winter, serve cold etc. The
environmental struggle causes high mortality. So competition acts as a natural check
to limit the undue growth of a population.
5. Survival of the fittest: As the struggle for existence is fierce, according to Darwin
only the fittest individuals survive it and reach adulthood to reproduce and leave
offspring. As Darwin gave a lot of importance to physical strength in the struggle, he
believed that fittest is the individual which is physically strong and is able to fight for
food, space, mates and can also escape predators and survive to reproduce to become
parent of the next generation.
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Conclusion
6. Natural selection: When animals overproduce, nature has a large number of
individuals to choose from. Variations and struggle for existence give it the variety
from which it selects the fittest individuals that become the parents of the next
generation. Therefore, nature selects the cream from generation to generation and
unfit and weaker individuals die out. Natural selection is a refining process, which
brings improvement in a species from generation after generation, making the species
fittest to live in a specific environment. Thus the species constantly changes and
evolution goes on forever.
Example:
1. Imagine that there are two rabbits ‘A’and’B. ‘A ’contains slightly longer legs and well
developed muscles than ‘B’. Hence ‘A’ may be escaped easily from predator than ‘B’ by
fast running. So, nature created struggle between predator and rabbits. The Nature
selected ‘A’ to survive as it had useful variations.
2. The best example is industrial melanism. The moth, Biston betularia is two
coloured. They are grey and black coloured. The grey coloured files were abundant in
England before industrial revolution. Because they were protected as they had
protective colouration (mixed with the back ground grey colour of bark). But the black
files were conspicuous to birds, they were eaten by birds. After industrial revolution,
black coloured moths are more than the grey coloured flies. Because the black
coloured moths had protective colouration. So, nature before industrial revolution
selected grey coloured flies and the nature now selected black coloured flies.
Criticisms of Darwinism
1. Darwinism mentions about survival of the fittest but does not explain about ‘Arrival
of the fittest’, which means a character needs a long time to develop before it can
become useful or fittest. For example, bird’s wings or electric organs of fish must have
taken millions of years to develop fully and during this period of growth they were not
useful organs and therefore should have been eliminated by natural selection.
2. Darwin believed that all variations are heritable, which is not true. Only genetic
variations are heritable and not the somatic ones. During Darwin’s time mechanisms
of heredity were not known and hence he tried to explain the inheritance by his theory
of pangenesis.
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3. Many useless and non-adaptive characters also persist in many animals and are
not eliminated. For example, small tails in giraffe or pig and ear muscles and appendix
in man have no selective value.
4. Darwinism does not explain over-specialization, which ultimately led to the
extinction
of
the
species.
For
example,
dinosaurs
became
extinct
due
to
overspecialization in body size. Saber tooth tiger (Smilodon) had oversized canines,
which led to its extinction in Africa. Similarly Iris deer (Megaloceros) grew such huge
antlers that they interfered in its movement through the forest and ensured its
extinction.
5. Struggle for existence is not a physical one as Darwin suggests. Most of the time it
is passive as for example many species of insects and other animals camouflaging
against the predators do not actually fight physically.
6. Most of the mutations that produce new characters are harmful and cause diseases
and therefore not useful in natural selection. Darwin did not know about mutations
and explained sudden appearance of characters due to “Spots” or “Sports”.
7. Natural selection does not operate on one character as Darwin thought.
8. Darwin believed in blending inheritance, according to which characters of both
parents blend in the offspring. But now according to the Mendel’s laws of heredity, it is
known that characters segregate in the second generation.
Supplementary theories of Darwin
Artificial Selection:
It is an additional factor. Here man (Animal Breeding and Plant breeding) acts as a
source of selection. Man artificially selects two individuals having required qualities,
breeds them and produces new variety. This method is applied in dairy, poultry,
piggery etc.
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The introduction of ‘Murrah cattle’ in Andhra pradesh leads to ‘white revolution’ by
cattle breeding . By artificial selection many hybrid varieties in crops are developed.
The artificial development of hexaploid ‘TRITICUM’ variety.
Sexual Selection:
It is also an additional factor. Darwin proposed this to explain why certain characters
are confined to one sex only. i.e., sexual dimorphism’. It does not play any role in
competition for food and shelter.
His theory is mainly based on the following assumptions.
A. In general, males are more abundant than the females. So, competition develops
among the males for copulation with females.
b. Female chooses the male having beautiful colour, appearance and physique.
eg: Female frogs initially selected the male frogs, which have small vocal sacs and
amplexsuory pads. Hence these characters gradually increased in the successive male
progeny. This ultimately lead to sexual dimorphism in frogs.
Pangenesis theory:
Darwin proposed this to explain the heredity. According to him, the every organ
consists of pan genes. These are carried to the sex organs, and accumulated in
gametes. Through the gametes, pan genes are transferred to the progeny.
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