Believe it or not, tigers are not the largest big cat. Ligers are (you might remember ligers from Napoleon Dynamite). Why? It has to do with the weirdness that occurs when you hybridize across two lineages which have been distinctive for millions of years, but not so long so as not to be able to produce viable offspring (in fact, many ligers are fertile as well).
Here’s the explanation:
Imprinted genes are under greater selective pressure than normal genes. This is because only one copy is active at a time. Any variations in that copy will be expressed. There is no “back-up copy” to mask its effects. As a result, imprinted genes evolve more rapidly than other genes. And imprinting patterns — which genes are silenced in the eggs and sperm — also evolve quickly. They can be quite different in closely related species.
Lions and tigers don’t normally meet ...
