that was good.Tautology is making a claim which says nothing and makes no prediction at all. The statements of which the claim is comprised reinforce each other in a way that the statement becomes a meaningless circular argument. For example if a person claim that i predict that it will rain tomorrow or it will not rain. One can see that both the statements "It will rain" or "it will will not rain" are just reinforcing each other and the claim is making no prediction at all and the statement is basically meaningless though it is necessarily true.This is a simple example of tautology.
The main mechanism of the Darwinian evolution is natural selection. natural selection is the driving force of the flow of evolution and in simple words it states that the nature selects the fittest. Those species who have a better compatibility with nature so that they can get enough food for their survival will survive and the others will perish gradually. For example if we have two animals A and B and they have different morphology and physiology and they are competing for survival in the "harsh" world of nature. Which of them will survive? The one that is fit will survive but wait ,which one is fit? The one that survives is fit. This tautology of natural selection can be summarized as below.
1. The one that is the fittest survives
2. Which one is the fittest?
3.The one which survives is the fittest
Natural selection is nothing more than a tautology as it makes no prediction and its a tautology is its core. As no testable predictions are being made , therefor the claim is unfalsifiable and as it is unfalsifiable it can not be taken as a scientific theory.