Theory of evolution given by Robert Charles Darwin (1809-1882) is called a Darwin's theory of evolution or Darwinism or theory of natural selection.
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| Darwin |
Historical Background:
Charles Darwin Was born in Shrewsbury, England. He completed his high school
education in Shrewsbury. He went to Scotland to study medicine. He got frustrated after viewing operation of a baby as there was no anesthesia during that time. He returned back to home and got admitted in Christ College, Cambridge University, to study Theology but he spent most of the time by playing cards and drinking. He came in contact of Dr. Henslow (Botanist) and also in contact of Robert Grant (Zoologist). He used to spend most of his time with Henslow, so people use to call him man who walks with Henslow. Henslow managed a trip to Darwin in HMS (Her Majesty’s Ship) Beagle as unpaid naturalist.
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| HMS Beagle |
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| Route of Voyage |
Initially it was planned for two years but
lasted for five years from 1831 to 1836. The ship departed from Plymouth of England.
In the ship he studied book “principle of geology” by Charles Lyell which
impressed him. He visited different places in South America and explored flora
and fauna of many countries. The most famous place was Galapagos Island,
approximately 960 kilometer from west coast of mainland of South America. Darwin reached Galapagos
Island in 1835 and spent more than one month there. Darwin called “Living
Laboratory of evolution” to Galapagos Island or volcanic island (formed due to
volcanic eruption) or Guano Island (due to the presence of huge amount of
excreta of birds). Galapagos is collection of 22 islands; 14 major islands and
8 minor islands. He observed giant tortoise, lizards(Iguanas) and birds (finches).
Darwin was very much impressed by
varieties of Finches (small black birds); 14 varieties of finches; 13 in
Galapagos and 1 in Cocos Island. He studied variations in Finches which gave him
the idea of natural selection. Shape and
size of their beak was modified according to their feeding habits. There were insectivorous
tree finches, insectivorous ground finches, vegetarian tree finches, seed
eating finches and cactus eating finches. The cactus eaters were with long and sharp
beak. The insect eaters were with pointed beak. The seed eaters were with large
beak.
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| Finches |
Finches were migrated from South
American mainland to Galapagos island years ago. From the original seed eating
variety different varieties with altered form of beak were evolved due to change
in their feeding habits. In Galapagos island the finches got abundant food,
space, partners for mating and competition free environment.
The finches provide an evidence
for adaptive radiation or divergent evolution and also provide evidence for
origin of species by means of geographical isolation.
Darwin returned back in 1836 and secretly started writing his research papers by the title “problems of species”.
In 1838 he read an article “Principles of population” by Thomas Robert Malthus.
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| T. R. Malthus |
Malthus was an economist. He gave the theory of human population growth and Darwin was impressed by it. Malthus said that food supply increases in arithmetic progression while population increases in geometric progression. Sufficient food is never available for all the members of the population. When population crosses certainly limit some factors called limiting factors become operational and limit the size of population and the population remains somewhat constant. From this, Darwin came to know about the struggle for existence.
In 1858, Alfred Russell Wallace sent a paper entitled “on the
tendency of varieties to depart indefinitely from the original type” Wallace
studied the flora and fauna of Malay Archipelago, present day Indonesia and
Malaysia. It was quite similar to findings of Darwin. When Darwin told this to
his friends Joseph Dalton Hooker and Charles Lyell, they requested Darwin to publicize
his view. Hooker and Lyell arranged paper presentation for both Darwin and
Wallace in front of Linnaean society of London but both remained absent. Thus, the
papers were jointly presented by Hooker and Lyell. The papers of Darwin were
more effective, more detailed and with better evidence. Thus, Darwin got more
credit. The theory of natural selection has been jointly proposed by Darwin and
Wallace and is called Darwin-Wallace theory. However, it is commonly called as
Darwin’s theory. A.R. Wallace is regarded as forgotten evolutionist and
co-worker of Darwin.
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| A. R. Wallace |
In November 1959, Darwin published
his theory of natural selection in a book “origin of species by means of
natural selection: the preservation of favoured races in the struggle for life”.
1250 copies of the book were published, and all copies were sold on the first
day and brought a revolution in the field of Biology.
Salient Features of Darwin’s
Theory of Natural Selection:
The theory of Natural Selection can be explained
under following headings:
* Over
production
* Struggle for existence
* Variations
* Natural selection or Survival of the
fittest
* Inheritance of useful variations and
* Origin of species.
* Over production (Rapid Multiplication):
All organisms possess enormous power of
reproduction (fertility). Their number increases in geometric ratio. Some
examples are:
The paramecium divides three times
in 24 hours by binary fission and if the rate of reproduction remains the same
and all paramecia survived then they will produce about 280 million Paramecia in a month and mass equal to 10,000 times of the volume of
the earth at the end of 9000th generation (about five years).
A salmon produces 28 million eggs and a star
fish produce about a million eggs in one season. If all of them survive and
continue to reproduce, a single species will fill all the sea after few
generations.
A single pair of English sparrows would be the
ancestor of 275 million individuals in 10 years if they and all their descendants
could reproduce at their natural rate without any check.
A female rabbit gives birth to six young ones
in one litter and produces four litters in a year. Six- month-old rabbit is
capable of reproduction. If all the rabbits survived and multiplied at this
rate, their number would be very large after some time.
Even slow breeders like elephant
has high power of reproduction, an Elephant become sexually mature at about 30
years and during its life span of 90 years produces only about 6 offsprings. At this rate, if all Elephants survive a pair
of elephants can produce about 19 million Elephants in 750 years.
From these examples it is
confirmed that every species can increase their number within few generations
and occupy all the available space on Earth, provided all survive and repeat
the process. So, the number of species will be much more than can be supported
on the Earth.
*Struggle for Existence:
Despite of rapid multiplication of all types
of species, food and space and other resources remain limited. They are not
liable to increase. Due to rapid multiplication of population but limited food
and space there starts an everlasting competition between individuals having
similar needs. In this competition, every living organism desires to become
superior over others. The competition between living organisms for needs like
food, space, mate etc. is called struggle for existence and is of three types:
(i) Intraspecific
Struggle:
It is the struggle between the individuals of
the same species because their requirements like food, shelter, breeding
places, etc. are similar. Two dogs
struggling for a piece of meat. Many human wars are the examples of
intraspecific struggle. Cannibalism (eating the individuals of its own species)
is another example of this type of struggle.
(ii) Interspecific Struggle:
It is the struggle between the members of
different species. This struggle is normally for food and shelter. For example,
a fox hunts out a rabbit, while the fox is preyed upon by a tiger. Predation,
Parasitism is example of this type of struggle.
(iii) Environmental Struggle:
It is the struggle between the organisms and
the environmental factors, such as drought, heavy rains, extreme heat or cold,
earthquakes, diseases, etc. Thus, climate and other natural factors also help
in restricting the number of individuals of particular species.
Among these three types of struggle the intra-specific
struggles are the strongest type of struggle because the needs of individuals
of same species are quite similar with each other.
Thus, struggle for existence acts as an
effective check to ever increasing population of a species.
*Variations:
Variation is the law of nature. No two
individuals, except the identical twins are similar and their requirements are
also not exactly the same. It means there are differences among the individuals.
These differences are called variations. These variations result due to the everlasting
competition among organisms. Due to the variations some individuals would be
better adjusted towards the surroundings than the others.
Adaptive modifications are caused through the
struggle for existence. According to Darwin, the variations are of two main
type continuous (gradual) which passes from generation to generation and discontinuous
(called sports by Darwin) which appears suddenly and disappears frequently. On
the basis of their effect on survival of organisms, the variations may be
useful, neutral and harmful. Darwin proposed that living organisms tend to
adapt to changing environment by useful and continuous variations and would be
passed on to the next generation, while the others disappear.
* Natural Selection or Survival of the Fittest:
The organisms which are provided with useful
and continuous variations would survive, because they are the fittest to face
their surroundings, while the others are discarded by nature. Originally it
was an idea of Herbert Spencer (1820-1903) who used the phrase ‘the survival of
the fittest’ first time while Darwin named it as natural selection.
It is to be noted that only survival of the
fittest is not enough. But organisms should also adapt or change themselves
according to the changed conditions of the environment as environment is always
changing. To explain the phenomenon of survival of the fittest, the extinct
reptiles can be cited as an example. During the evolution of reptiles, giant
reptiles, the dinosaurs etc., appeared.
Majority of them were herbivorous, but due to
certain climatic changes, the vegetation disappeared and, therefore, most of
them became extinct. However, small animals who could change their feeding
habits from herbivorous to carnivorous diet survived, because they could easily
get adapted to the changed environment.
Thus, natural selection is an automatic and self-going process which checks on the animal population. It acts as
restrictive force to check population.
Wallace called it “Survival of the fittest”.
* Inheritance of useful variations:
The organisms after getting fitted to the surroundings
transmit their useful variations to the next generation so that they are born
fit to the environment. The non-useful variations are eliminated. Darwin could
not differentiate between continuous and discontinuous variations. In this
respect, Darwin agreed with Lamarck’s views, because according to Darwin
acquired characters which are useful to the possessor could be inherited.
* Speciation (Formation of new species):
Darwin considered that useful variations are
transmitted to the offspring and appear more prominently in succeeding
generations. After some generations these continuous and gradual variations in
the possessor would be so distinct that they form a new species.
Criticism
of the Natural Selection Theory:
(Objections against the Theory of Natural
Selection):
1. Inheritance of Small Variations:
According to this theory only useful
variations are transmitted to the next generation, but sometimes small variations
which are not useful for the possessor, are also inherited. It is beyond
understanding that if the appearance of small wings in birds could help them in
flying.
2. Inheritance of Over-Specialized Organs:
Some organs like tusks of elephants, antlers
of deer have developed so much that instead of providing usefulness to the
possessor, they often give hindrance to them. They are also inherited. This
theory cannot explain these facts.
3. Vestigial Organs:
Why vestigial organs are present in some
animals when they have no function? According to the Natural Selection Theory,
vestigial organs should not be present.
4. Arrival of the Fittest:
The theory only explains the survival of the
fittest but, is unable to explain the arrival of the fittest.
5. Degeneration of Organs:
The theory does not account for the
degeneration of certain organs in animals.
6. Discontinuous Variations:
The theory fails to explain the cause of
sudden changes in the body. The main drawback of Darwin's theory was lack of
the knowledge of heredity and that is why he could not explain that how the
variations are caused.
Darwin himself was conscious of the
inadequacies of his theory, when he remarked that, “I am convinced that natural
selection has been the most important but not the exclusive means of
modifications.”
Evidences in Favour of Natural
Selection:
1. Rate of Reproduction:
Rate of reproduction is many times higher
than the rate of survival in all organisms.
2. Limitation of Resources:
Food, space and other resources are limited.
3. Struggle for Existence:
Competition or struggle for existence is seen
in all organisms.
4. Abundance of Variations:
Variations are so abundant in nature that no
two individuals of a species are similar, not even the monozygotic twins (they
possess some dissimilarity due to their environment).
5. Production of New Varieties of Plants and
Animals by Artificial Selection:
When man can produce various new varieties of
plants and animals in a short period, nature with its vast resources and long
time at its disposal can easily produce new species by selection.
6. Mimicry and Protective Colouration:
They are found in certain animals and are
products of natural selection.
7. Correlation between Nectaries of Flowers
and Proboscis of Insects (Entomophily):
The position of nectary in a flower and the
length of proboscis in pollinating insects are wonderfully correlated.
8. Pedigrees of Some Animals:
Pedigrees of horses, camels and elephants
also support the Natural Selection Theory.