[Seruzawa]

Grizzlies and Polars are both types of Brown Bear. Same species.

[jbp1]

so the only way you can tell them apart is because the polar bears are bigger?

[thackney]

Polar bears are evolved from Brown Bears but are a different species.

Brown (Grizzly) - Ursus arctos
Polar - Ursus maritimus

http://www.wildlife.alaska.gov/index.cfm?adfg=bears.main

There have been hybrids found in the wild of mixed genetics between the two.