What basic modifications have been made in evolutionary theory since darwin's time?

Knowledge of genetics did not yet exist in Darwin's time, so genetic theory has since been used to explain natural selection. In addition, Darwin believed that evolution progressed at a slow, steady rate. However, that idea has been replaced by the punctuated equilibrium model, in which major changes in brief spurts.

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because they are eaten by the frog and the frog getseaten by the snake and the snake by the eagle

condenser is used to focus in the light microscope.