Why do polyploid plants prevail in the harsh conditions of the extreme north and high mountain regions?

Why do polyploid plants prevail in the harsh conditions of the extreme north and high mountain regions? What way of speciation is observed in this case? How do such plants reproduce?

1) Polyploid forms are more viable, characterized by more lush vegetative development, increased resistance, therefore they were preserved by natural selection.
2) Speciation by polyploidization is referred to as the sympatric pathway of speciation, since speciation is not associated with spatial isolation and occurs within the range of the original species.
3) Balanced polyploids (have a paired set of chromosomes: tetraploids, hexaploids, etc.), as a rule, are normally fertile, and unbalanced polyploids (triploids, pentaploids, etc.) are infertile, reproduce vegetatively.

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