When there is limited food.
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There are really no steps in natural selection - just conditions required for it to occur.
What population? Perhaps you mean if there were no variation for natural selection to select from.
Without genetic diversity, natural selection cannot occur
Genetic variation. If there were no variation in the genes/phenotype then natural selection would have nothing to select from.
Everything from available food to climate will cause the changes we see in natural selection. Random mutations occur constantly and when those mutations are beneficial for life, the genetic code is more likely to be passed on to future generations.