The idea that species evolve at a slow constant rate is called

The correct answer is -

Gradualism.

Gradualism can be described as an evolutionary theory according to which evolution of organisms/species proceeds continuously at a slow and steady rate that is without any sudden formation of new biological features or sudden formation of new species from one generation to the next generation. The approach in which Charles Darwin proposed the emergence of new species (over many generations) from already existing species is attributed to gradualism.

In other words, species evolving at slow and constant rate is known as gradualism.

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