Credit: Penn State University
Intimate Relations: The nuclear genomes of bears (black outline) suggest that polar bears and brown bears diverged from one another 4 to 5 million years ago, and that occasional exchange of genes between the two species (shaded gray areas) followed. Results from maternally inherited mitochondrial DNA (dotted line) indicate extinction (marked with an "X") and replacement of polar bear mitochondrial DNA around 160,000 years ago due to interbreeding between the two species.