PinkMonkey Online Study Guide-Biology
11.1 Darwin’s Theory of Natural Selection
Various hypotheses were suggested by scientists to explain the concept of evolution. As a result of many years of careful study and observations, Charles Darwin proposed his theory of evolution. He published a book The Origin of Species in the year 1859. He proposed that the new species came about by a process called ‘natural selection.’ While explaining his theory, Darwin gave an account of many basic facts and deductions and stated how natural selection operates and results in the evolution of a new species.
(1) Over-production. Darwin reported that all
organisms tend to increase in a geometric ratio provided there are
no environmental checks. Lower animals produce a large number of eggs
within 10 to 14 days. A female drosophila produces 200 eggs at a time.
If all flies from these eggs survive and reproduce with the same rate,
they may produce 200 million organisms within a period of 40 to 50 days.
Amongst the higher animals also there is a rapid increase in number. A
single pair of sparrows may produce as many as 275 million individuals
in a span of 10 years. Even slow breeding animals like the elephant may
theoretically give rise to 19 million descendants in a period of 750 years.
(2) Population Stability. In spite of the high
reproductive potential, the number of individuals in a species remains
relatively constant, suggesting struggle for existence.
(3) Struggle for Existence. There is an inevitable
competition between the organisms for space, food, shelter, nesting sites,
water, etc. This struggle takes place in three ways-
a) Intraspecific struggle
b) Interspecific struggle and
c) Environment struggle
a) Intraspecific struggle takes place between the individuals of the same species. This is often most severe and fatal, as the needs of the individuals of the same species are similar.
b) Interspecific struggle takes place between
the individuals of different species. In a community, some organisms feed
on others. Both animals and plants are affected by this kind of struggle.
c) Environmental struggle is the struggle
of organisms against the physical environment. Organisms struggle against
excess of moisture, drought, extreme heat or cold, lightning, volcanic
(4) Genetic variations within a species. Animals
and plants show variations in physical structure. Some of these variations
are simply caused by external conditions (environmental), such as accidents,
temperature, food abundance, etc. (e.g. animals look different when they
are starving than when they are well fed). Such somatic variations
die with the organism and not inherited. Thus, they have no effect on
Heritable variations are called genetic variations.
Such variations arising from changes in DNA are passed on within families
and to the offspring from the parents, (e.g. parents with a larger stature
body generally have larger children than parents with a small statured
(5) Natural selection, or "survival of the
fittest". Organisms struggle for existence. Those individuals
who have favorable variations have better chances of living long enough
to reproduce. They pass on their advantageous characters to the
next generation. Thus, organisms with advantageous characters
survive, while those which lack such variations perish. The advantageous
characters are passed on to the offsprings generation after generation
and the organisms become better suited for survival. Darwin described
that nature selects such organisms, i.e. there is natural selection. Herbert
Spencer referred to this as ‘survival of the fittest.’
(6) Environmental change. The physical environment
changes continuously. There are seasonal changes in availability of moisture,
light, severe heat or cold, etc. Mountain ranges are formed where
there were no mountains before. Volcanoes erupt and cause changes in the
environment. Similarly floods and fires convert plains and forests
into temporary deserts.
(7) Inheritance of adaptive characters. Because organisms are continually tested by their environment, their forms change to suit new conditions. New advantageous characters are inherited along newer generations and their cumulative effect results in an organism with new characters.
(8) Origin of species. Darwin explained that favorable variations are passed on to the offsprings down the generations. After a certain period of time the organisms appear so different from the original species that ultimately a new species is evolved.
The basic facts and deductions of Darwin’s theory are summarized as follows: