I am always confused when I get evolution part.
In cliff's natural selection is the differences in survival ans reproduction in a population as a result of their interaction with the environment. But it is also said that natural selection is the increase or decrease in allele freq due to the environment.

I am always confused when I get evolution part.
In cliff's natural selection is the differences in survival ans reproduction in a population as a result of their interaction with the environment. But it is also said that natural selection is the increase or decrease in allele freq due to the environment.

Yes, you are correct.

Natural selection is termed fitness. Fitness is the ability to reproduce successfully. This would increase (successful, high fitness) or decrease (unsuccessful, low fitness). Fitness can increase with the organisms interaction with the envirnment, i.e, if a organism can matabolize available energy resources faster, they will reproduce more and survive longer -> longer living means you can reproduce more and reproducing more means your are more 'fit.'

Make sense? The two definitions you stated are essentially saying the same thing