Have any of y’all seen the woodlands of Alaska? Compare Russia's vast birch forests to the ecosystems in Alaska. The Siberian Tiger would quickly dominate in this ecosystem. Why no Siberian Tigers in Alaska? They would absolutely have PRIME food there in Alaska; it’s population would have flourished.
If there was ever a land bridge ~ the Siberian Tiger would certainly be there. The absence of the Siberian Tiger is in direct opposition to the survival of the fittest crowd's standard line. This one thing alone is enough to dispute a land bridge ever having been present almost without question.
About 100 years ago, my Grandpa's family traveled across the Bering Strait to settle in Alaska. Is it only possible for migrants to have traveled to Alaska by land? Don't think so. The rest of those migrants came over in the same manner ~ boat.
Are you seriously using the argument that since there were no Siberian Tigers in Alaska it's proof that there wasn't a land bridge?
There were certain times during the Pleistocene Epoch when the temperatures were cold enough that most of the Earth's waters froze (also known as Ice Age(s)). The sea level dropped more than 300ft around the world, and in such places such as the Bering Strait, the land was exposed because the Strait lies in relatively shallow waters. Through this "land bridge" and glaciers, humans and animals were able to cross from Asia into North America.
If you really want some examples of some of the animal species that crossed via this land bridge then you only need to look at the Saber-Tooth Tiger (Smilodon populator), the Wooly Mammoth (Mammuthus primigenius), Giant Cheetah (Acinonyx pardinensis), North American Lion (Panthera leo atrox), North American Cave Lion Panthera leo spelea).
Now, about your "Siberian Tiger" theory...the reason why there weren't Siberian Tigers in Alaska, there were no Siberian Tigers at the time of the Bering Strait/Alaska Land Bridge. It's really that simple, otherwise it may have crossed along with the other animals. The Cave Lion's range was wide spread, stretching from Europe, Asia and North America. The only way it could have gotten to North America is by crossing the land bridge during one of the Pleistocene's ice ages. Same with the other animals that I listed.
Here is a site that lists some of the animals that came across.
A Pleistocene Bestiaryack-ack