1.   If individuals can interbreed and produce fertile offspring, they belong to the same species.

2.   Q. Can the polar bear mate with the brown bear to produce fertile offspring?

3.   Some studies show that early spawners can lay eggs repeatedly, and over time the eggs become healthier and produce more and more fertile offspring.

4.   Those fish could continue to produce fewer and fewer fertile offspring until the fishery could be in trouble of collapsing.

5.   Men with fertility problems who father children through artificial insemination are likely to produce fertile offspring, according to a study released Friday.

6.   Those fish could continue to produce fewer and fewer fertile offspring until the fishery industry is in danger of collapsing.