How do you know that the groups Ursus Maritimus and Ursus arctos are closely related?

Question

How do you know that the groups Ursus Maritimus and Ursus arctos are closely related?

Answer 1

Because Ursus, which means bear, refers to the Genus, so they are both bears!

Maritimus, which is a Polar Bear, is the species, while arctos, which is a European/Asian Brown Bear, refers to a slightly different species!

Answer 2

One fairly convincing piece of evidence biologists have that Ursus maritimus (i.e., polar bear) and Ursus arctos (i.e., brown bear) are closely related genetically is that these 2 species can interbreed to produce hybrid, fertile offspring.

Traditionally, the concept of a biological species has been defined to closely align with 'cross-breeding' barriers (i.e., two organisms were thought of as belonging to one species if they can interbreed).  Anthropologists have used this fact to identify the wild species which gave rise to the domesticated plants and animals we have today (e.g., teosinte/maize, wolf/dog, mouflon/sheep).  While it is not a hard-and-fast rule that only members of the same species can interbreed (c.p., horse/donkey), two organisms can only naturally interbreed if their genetic materials exhibit a high degree of compatibility (the details of which are slightly beyond scope).  in some cases, when members of two species (A and B) mate, offspring will only be produced if a male A mates with a female B, but not if a female A mates with a male B (you see this with U. arctos and U. thibetanus).  In other cases, the offspring may exhibit different characteristics depending on whether the mother was A or B (e.g., mule vs. hinny).  Additionally, the hybrid offspring may or may not be fertile (i.e., able to produce its own progeny).  Arguably the most successful form of interbreeding is one in which either s*x of species A can pair with the opposite gender of species B to produce fertile offspring both in the wild and in captivity.  Yet polar bears and brown bears exhibit precisely this level of compatibility.

Given that U. maritimus and U. arctos live in adjacent habitats (arctic and tundra) but are specialized to predate separate ecological niches (one eats arctic fish, the other eats freshwater fish and other forest things), it is quite conceivable that, at some point in the not-so-distant evolutionary past, the polar bear and the brown bear were one and the same species.