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Physically, there are some characteristics that can be found in all organisms within the Family Ursidae. They have:

large bodies with stocky legs

relatively short snouts

rounded ears

small eyes

short tail

elongated skulls

vestigial lacrimal bone

massive forelimbs and claws

strong jaws

Behaviorally, bears tend to have some similarities as well. Their vision and hearing is not well-developed (about the same as a human), but have a keen sense of smell and use their sensitive lips to locate and maneuver food. Males will sometimes use chemical cues to locate receptive females around them. Home range boundaries, individual identity, and sexual condition may be advertised, both visually and chemically, by tree-scratching and by urinating and defecating on boundary trails. Aside from mating, males and females normally don't associate all that much. Bears have an omnivorous diet which allows their food options to vary throughout the seasons. Although they are four legged animals, bears walk on the soles of their feet, rather than their toes.

Although bears tend to be most abundant and diverse in temperate and northern regions, they live in a wide range of environments around the world.

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Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Carnivora
Family: Ursidae
Genus: Ailuropada
Species: melanoleuca

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Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Carnivora
Family: Ursidae
Genus: Ursus
Species: maritimus

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Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Carnivora
Family: Ursidae
Genus: Tremarctos
Species: ornatus

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The taxonomy of these organisms alone shows you how related they are. The three only differ in Genus and Species. The more groups an organism shares with another organism, the more closely related the two are. Being within the same family is still showing that the organisms come from some of the same ancestors, or they are related pretty well.

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One of the most prominent splits within this family is between the brown bear and the polar bear. It is unclear of when polar bears actually diverged from the species of the brown bear, but it was more recently. Scientists think that over time,brown bears moved to the Arctic, where they adapted to their surroundings and began developing fur that would help them blend in with the harsh, white Arctic ice. Proving their genetic compatibility, brown bears and polar bears can mate and produce viable, or fertile, offspring. It is this reproductive viability that establishes that an animal belongs within a given species. They would’ve had to come from the same branch somewhere, or else that couldn't have happened.

Below is a picture of the mixture between a polar bear and a brown bear, also known as a "pizzly".

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Panda Bear                       (Ailuropoda melanoleuca)

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Panda Bear                       (Ailuropoda melanoleuca)

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Physical Characteristics:
Pandas have white with black ears and teardrop circles around their eyes. Their arms and legs are also black along with a band extending from their front arms around their back. These colors (black and white) are designed to blend in with the black mountain rock terrain and white snow. Pandas have teeth that are designed to penetrate the fibrous bamboo stalks. Extending from their wrist bone is a unique thumb like feature to grasp bamboo stalks (in addition to their five fingers). Their digestive systems are more similar to that of a carnivore than an herbivore, and so much of what is eaten is passed as waste.

Behavioral Characteristics:
Pandas have a very specialized diet consisting nearly entirely of bamboo (between 30-45 pounds a day). They will spend 10 to 16 hours a day foraging and eating. Between feedings, they will typically nap for two to four hours at a time, snoozing on their side, back, or belly, either sprawled or curled up. Pandas rely on extremely detailed vocalizations to express all shades of mood from amorous to angry with their fellow pandas. Adult giant pandas tend to be generally solitary, but they do communicate periodically through scent marks, calls, and occasional meetings. Aside from the 7-15 individuals in their group, they mostly avoid interactions with bears in other groups.

Habitat:
Panda bears tend to live in remote mountainous areas that have dense forests of bamboo (China).
They will often remain in areas of very high altitude, where there aren’t any humans around, but in the winter, some will drop to some lower altitudes, still staying out of sight of humans.

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Polar Bear                         (Ursus maritimus)

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Polar Bear                         (Ursus maritimus)

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Physical Characteristics:
Polar bears have an outer layer of fur is hollow and reflects light giving the fur a white color (white fur helps the bear remain camouflaged in icy and snowy habitat). Underneath, their skin is black; this black can only be seen on the nose. The footpads of a polar bear's’ paws have a kind of “non-slip” surface allowing them to get traction on slippery ice. They have wide feet to distribute weight while walking on the ice (prevents it from breaking) and webbing between their toes that help with walking on ice and swimming. Polar bears also have a think layer of fat under their skin to keep them warm, especially when they are swimming in freezing water.

Behavioral Characteristics:
Polar bears are almost purely carnivorous, feeding on different types of seals, walruses, and whale carcasses. Because polar bears (and their populations) are so large, most other food sources are not plentiful enough for the species. Polar bears hunt seals by waiting for them to come to the surface of sea ice to breathe. When the seal nears the surface, the polar bear will bite or grab the seal and pull it onto land to feed.
Similar to humans talking, polar bears will use grunts, growls, roars and squeals to communicate with each other. Polar bears breed in the late spring as the temperatures begin to rise.

Habitat:
Most live north of the Arctic Circle to the North Pole, but others live in places like Alaska, Canada, Russia, Greenland and some northern islands owned by Norway like Svalbard. They are normally on the sea ice in these places.

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Spectacled Bear     (Tremarctos ornatus)

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Spectacled Bear     (Tremarctos ornatus)

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Physical Characteristics:
The Spectacled Bear (also known as the Andean Bear) is one of the smallest of the Ursidae family. Their fur is usually black, although can sometimes have dark red-brown tones on the upper-parts.
They commonly have white to pale yellow markings around the muzzle, on the neck and the chest (also around the eyes which is the reason for it’s name).

Behavioral Characteristics:
Like most other bears, spectacled bears tend to be solitary, but sometimes can be found in packs if there is a plentiful food source.

They will form pairs for reproduction between March and October, which shows that they aren't able to reproduce at different times of the year. Spectacled bears have an omnivorous diet, although they are fairly specialized in fruit and different parts of several plants.

Habitat:
Spectacled Bears are found in South America in the tropical Andes (Venezuela to Northern Argentina, including Colombia, Ecuador, Peru and Bolivia). According to some researchers, the greatest number of bears is to be found on the borders between Colombia, Ecuador and Peru.
They are most commonly in cloud forests (also known as Andean forest) and high Andean moorland.

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This tree shows how although my organisms are related, some are much more related than others. The closer two organisms are together on a tree, the more related they are. For example, you can see that the panda and the spectacled bear are much more closely related than the panda and the polar bear.

This tree can also show you how fast the different branches of evolution occurred. The divergent evolution of the polar bear from the brown bear occurred extremely quickly compared to the branching off of the giant panda from the other bears. This is basically the difference between punctualism and gradualism equilibrium within the family.

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This karyotype is my biggest proof for evolution of bears. There are two different sets of chromosomes being shown and compared in this picture: chromosomes from a panda and chromosomes from spectacled bear. In the picture, you will see that there are five small arrows. These arrows are pointing to the centromeres that have been displaced in the spectacled bear karyotype compared to the homologous panda chromosomes by an inversion. Aside from this few mutations, the chromosomes are almost identical.

Over time, there were mutations that occurred that made the two species branch off from one another. They came from a common ancestor, which led them to having similar chromosomal pairs. The closer the two pairs are, the more closely related they are. This karyotype is proof that the two are pretty closely related.

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Here is a jaw bone fossil of a short-bear (Arctodus simus). It's at least 150,000 years old. As you can see, it is similarly structured to the fossil in number one. This is once again proof of how the structure was favorable in the family and was continued to be passed down through onto current organisms in the Family Ursidae.

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Here, we have a fossil of a jaw and tooth of a polar bear. It is around

110,000 to 130,000 years old

(which is more modern than the short-faced bear, but not too recent). This is great evidence of the evolution of bears. Fossils from other bears (extinct and living) are almost identical in structure. These structures show how the powerful jaws were one structure that was advantageous to bears.

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One thing that all bears have in common is the vestigial lacrimal bone. This skull bone works as a pathway for the tube that transports tears. It transports tears to the nasal cavity from the eyes. This bone has been long out of use for the family for thousands of years, yet they all have it. In order for them to ALL have it, it obviously would've been passed down over time through a common ancestor. It lost its purpose at some point, but never completely went away and lingered within the family.

While it was previously mentioned that the jaw was one thing that carried through with the family,  it's an important structure that all bears have. Bears have extremely powerful jaws, and although different species may utilize them for different purposes, they are still beneficial to the organisms. Pandas can use them to break into the bamboo, while polar bears can use them to rip apart the flesh of their prey.

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This fossil is showing one of the earliest species of pandas found, compared to a more recent one. As you can see, the prehistoric panda's teeth and jaw indicate that the species, Ailuropoda microta, was already eating a diet of bamboo, just like the pandas of today. The skull is remarkably similar to the more modern skull, just about half the size. This shows that the species had evolved into a much bigger version of itself over time.

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This is the skull of a short faced bear (Arctodus simus). It lived around 800,000 years ago and went extinct around 12,500 years ago. This shows how over time, the snouts of bears how gotten much longer. Their faces generally aren't pushed in anymore like this species. This also relates to in number 6, where modern bears foreheads slope more gradually, rather than just drop off.

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This is yet a fossil of a jaw bone of a cave bear (Ursus spelaeus) that lived during the Pleistocene between 300,000 and 10,000 years ago. This is just more proof of the same thing as the other two fossils previously shown.

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Emigration can be seen in the Ursidae family when the movement of brown bears to the Arctic, causes the evolution of the polar bear. The brown bears left their community and migrated into new territory. Once they moved, they adapted new traits and split into their own species. When organisms separate themselves from the rest of the community be moving to another place, the likelihood of creating a new species seems to be pretty high.

According to one of my sources, the decline in the population of pandas can be seen as a microevolution. Their population decreasing is a result of the deforestation of bamboo, the primary source of food for the creature. Eventually, due to this new struggle for them, they will either adapt and find a new food source, or will die off and become extinct.

The diverging of polar bears from the brown bear species can also be seen as microevolution. As  the mutation causing white fur occurred in a geographically  isolated population of organisms, and has not spread into related species, it is an example of microevolution. The trait was favored (dominant) and then was passed down until the entire population of that species was solely white furred.

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This is the skull of a cave bear (lived between 300,00 and 10,000 years ago. The biggest evidence of evolution shown from this fossil is that modern bears have more gradually sloped foreheads, compared to a sudden drop off. Somewhere over time, their faces changed shapes to become more angled. Otherwise, you can see that the general skull structure is pretty similar to that of modern bears.

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The family Ursidae as gone through gradualism rather than punctuated equilibrium. Although there have been some quick bursts, it seems as though overall, the changes have been pretty subtle and slowly changing. The general structure of each member is similar, but they have made a few individual adaptations to their environments in order to survive. For example, there haven't been a ton of big changes throughout the group. They are generally all recognizable as bears due to their body and facial structure. If you refer to the phylogenetic tree, each branch (except for the split between polar bears and brown bears) takes a decent amount of time to split.

One exception to the family going through punctuated equilibrium opposed to gradualism would be when the brown bears that moved into the arctic suddenly became white. Unfortunately, I'm not sure for a fact that it was punctuated instead, but I can infer based on my research that the polar bears who were lighter in color and darker in skin were the ones that survived. The darker colored bears with skin that was not heat absorbing would either freeze, or they would die of hunger.

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Descent with Modification:
Descent with modification is easily seen in this family. Certain traits, such as the jaw bone, will get passed down with small alterations, while others may get passed down and pass due to natural selection. The more favorable traits will stick, and the less favorable ones will be weeded out eventually. If you take a look at the homologous structures from this group, or even just look at the characteristics they have in common, and you can tell which traits were advantageous]. They have a keen sense of smell, so that means that those with the not so great sense of smell would die off quicker, and those that did have it would breed and produce offspring that had it as well. When polar bears diverged from brown bears, those with the lighter-colored and thicker coats would survive, and therefore would reproduce. The process would repeat itself with those that are most fit to survive in the environment reproducing and passing on their traits. The outcome would eventually result in today's modern polar bears.

Stabilizing Selection:
Through time, there have been bears of all sizes, from the extremely monstrous short-faced bear (Arctodus simus), to the tiny ancestors of our modern pandas. Now, it seems as though those that were too big and those that were too small were the ones who didn't make it. Although some bears (like the spectacled bear) are smaller than the others, they tend to be more moderate. The extremes didn't make it because they were either too small and fragile to be predators, or they were too massive to be able to find food sources that would provide an adequate amount of nutrients for them. The middle of the spectrum is the one that will survive and continue to produce and make more bears that are like them.

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Mitochondrial DNA has been playing a big part in how we see the evolution of organisms such as polar bears. Scientists will trace the mitochondrial DNA back, and try to find close relatives. In the case of the polar bear, recent studies have shown that the polar bear matriline (mitochondrial DNA) evolved from a brown bear lineage since the late Pleistocene period. This means that is possibly indicating rapid speciation and adaption to arctic conditions.

In another case, scientists studied the mitochondrial DNA of the  North American short-faced bears (Arctodus), to discover that their thought to be close relatives, the South American short-faced bears (Arctotherium), were just convergently evolved, rather than branching directly from one another. The North American short-faced bear was actually more closely related to the more modern spectacled bear when looking at the mitochondrial DNA.

In conclusion, by looking at the DNA, we can see how much more related organisms are and figure out the ways in which they evolved into the species they are today.