Nothing in Biology Makes Sense Except in The Light of Evolution- Karl Kjer

A number of changes in flora and fauna have occurred over millions of years on the earth. The study of history of life forms on the earth is Evolutionary Biology.

There have been many theories of evolution out of which some were deemed important. Lamarck's theory of evolution was the theory of acquired characters or the theory of use and disuse of organ. This theory had four postulates - 

Internal forces tend to increase the size of the body. 

Formation of new organs is the result of the need or want continuously felt by organisms -Doctrine of appetency/desires. 

Development and power of action of an organ is proportional to its use. 

All changes acquired by the organism during its life are transmitted to the offsprings by the process of inheritance. 

Lamarck's theory was however discarded on the basis that physical changes acquired by an organism are not transmitted to the progeny.

Darwin's theory gained critical acclaim im the biological world later. Charles Darwin put forward "natural selection "as the basis for evolution. The source of variations according to Darwin was mostly mutation, also recombination and intermingling of populations. Darwin's theory was found weak based on the fact that he could not explain discontinuous variations.

Interestingly, 60 years before Charles Darwin's theory was presented his grandfather Erasmus Darwin wrote about evolution. 

Next in line we have the Mutation theory proposed by Hugo de vries on the basis of observations on the wild variety of evening primrose Oenothera lamarckiana.according to this theory evolution on a grand scale is macroevolution , at genetic level he called it macroevolution, units of evolution were populations and the units of natural selection -individuals. 

A very important and logical explanation for evolution was given by Hardy-Weinberg,it's called the Hardy-Weinberg principle. According to them variation at genetic level is caused by 5 processes

• Mutation
• Recombination
• Gene migration
• Natural selection

They gave an equation to elucidate their proposition -

• (p+q)square =(p)square + 2pq + (q)square.
• For a gene with 2 alternative alleles, called A and a,
• the frequency of allele A can be expressed as p and that of a as q.
• since they're only 2 alleles their sum must be equal to one.

There are many more theories for evolution, even after millions of years we are only trying to find out the reasons. We are closer to the truth but still have a long way to go.