This is exactly wrong. Lamarck believed in the inheritance of acquired traits (e.g. if you cut off a mouse's tail, then its babies would be born with shorter tails).

Darwinism, conversely, is about selection from within a range of traits already present (i.e. beak size).

These frogs come in a variety of colors. Dark colors have a better fitness for the highly radioactive environment. Those are the ones that reproduce. This is textbook Darwinian evolution.