Why is more than one reproductive isolation barrier needed to completely isolate a species?

What is a cordon and where is it found

Type the correct answer in the box. use numerals instead of words. fiona has a lovely flower bed in her yard. she wants to add a fence along one side of the flower bed. the fence material is sold in meters. fiona knows that her flower bed is 200 centimeters in length. how much fencing material does fiona need in meters? fiona needs meters of fencing material for her flower bed.

When analyzing tree rings, what do scientists assume a thin ring indicates?