![](https://steemitimages.com/DQmcguJkXLDvwzWHG52TyHeNgCqdLCZJbMkgcKp4hyPWpE9/image.png)
Two Pairs  G+C and A+T account for the diversity in species. G can only make pair with C and A with T. Ratio between them is not equal or not same between two pairs in different species.G+C is more favorable but the question here is <b> why </b> ?.  If scientist want to know why there is different in composition of DNA they have to find answer of this question.

<h2><center>G+C More Favorable Than A+T</center></h2>

 G+C is more favorable but it is opposed by a pressure which is called mutational pressure. This pressure is applied by  various strength in the opposite directions. There is different G+C composition in different organism human contain 41 percent while corn have 47 percent.   

<h2><center>The Difference is Selection </center> </h2>

 Scientist have experimented with 40 different species and they found out that the difference is due to selection but they are unable to find out why selection produce this difference if they will be able to find the answer of this question than this will be the miracle of history.