EVOLUTION
Sexual reproduction and errors in DNA copying lead to variation. Its consequences are illustrated below:
Consider
a growing population of red beetles living in bushes with green leaves. Imagine
that crows eat them.
Here, the
following situations may be developed.
First
situation: Natural Selection:
A heritable
colour variation (green beetle) arises. Crows cannot see green beetles
on the green leaves and cannot eat them. So, number of green beetles increases.
But the red beetles are eaten and their number decreases.
Here, the
variation became common because it gave a survival advantage (naturally selected).
The natural selection is exerted by the crows. If there are more crows,
more red-beetles would be eaten. So the proportion of green beetles increases.
Thus, natural selection directs evolution. It results in adaptations in the
beetle population to fit their environment better.
Second situation: Genetic Drift:
A
colour variation (blue beetle) arises.
Crows
can see and eat both blue and red coloured beetles in the green leaves.
Initially,
blue beetles are few. If an elephant stamps on the bushes, most of the beetles
are died. If the survived beetles are mostly blue, their proportion increases
and the population would be with mostly blue beetles.
Here,
the colour change has no survival advantage. Blue coloured beetles accidentally
survived and changed the common characteristic of the new population.
In a
very large population, such events do not have major influence. But accidents
in small populations can change the frequency of some genes. This is called genetic
drift. It provides diversity without adaptations.
In the first & second
situations, a rare variation became common characteristic in the
population. It means frequencies of inherited traits and their genes change
over generations. It is the essence of the idea of evolution.
Third
situation: Non-genetic variation:
The bushes
are affected by a plant disease reducing the amount of leaf material. Poor nourishment
reduces weight of beetles. Here, there is no genetic change.
After some
generations, if the plant disease is eliminated and leaf food is increased, beetles’ weight again increases.
Acquired and Inherited Traits
Reduced
weight of beetles due to starvation will not change the DNA of the germ cells. i.e.,
it is not a heritable trait. So, it is not an example of evolution.
Change in
non-reproductive tissues cannot be passed on to the DNA of the germ cells. So,
the experiences of an individual (acquired traits) during its lifetime do not inherit
and cannot direct evolution.
E.g. Breeding
of mice produces all progeny with tails. If their tails are removed in each
generation, they do not produce tailless progeny because it does not change the
genes of germ cells.
So, the
study of heredity and genetics are essential to understand evolution.
Charles
Robert Darwin (1809–1882) set out on a five-year voyage to South America and proposed the
idea of evolution of species by natural selection. But he could not explain
mechanism of evolution.
Origin of life on earth
J.B.S.
Haldane suggested that life
developed from simple inorganic molecules (Chemical evolution).
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