Pre-AP Biology, Period B4
22 January 2018
The polar bear is the only bear adapted for a brutal and frigid tundra-like environment, and one world’s largest land predators with one of the most advanced insulation system. They have thick fur made of white clear hairs (while some have brown, but see Single-Gene Traits section on why that is not very common), black skin that helps them soak up heat, a large storage of fat for insulation and small limbs that help retain heat. Polar bears seem to have evolved from grizzlies who lived in the northern hemisphere through speciation, as seen with the homologous structures they share (notably nearly identical structures like paws, jaws, and legs). Embryology supports this, as the embryos are very closely related. As with other bears (which further supports the argument), the vibrissae (whiskers/facial hair) adult polar bears have are vestigial organs, not serving their sensing purposes anymore.
Traffickers off of the coast of Island F were smuggling a few polar bears, but then crash into an iceberg, leading to the polar bears getting out and populating Island F. This island has a dry, harsh and very cold climate, with temperatures dipping down to -19 degrees Fahrenheit during the night. This is a founders effect, as these polar bears were the first on Island F. The polar bears thankfully find prey – harp and hooded seals – that fit their high-fat diet. These animals have evolved with the polar bear in a sort of co-evolution: the seals gaining adaptations to evade polar bears, such as the whitening of their fur when they’re young to camouflage them with the ice, and the strengthening and growth of flippers to make them faster in water, and the thickening of their skin to resist the cold and polar bear bites. The Hardy-Weinberg principle doesn’t apply to this situation, as the conditions for it to apply are not met; there is a small breeding population, nonrandom mating, mutations that cause different traits to evolve and differences in fitness between organisms due to the differences in phenotypes and to natural selection, emigration from island F (see Geographical Isolation section), and natural selection that chooses those who are fit for the environmental pressures.
After 10 thousand years
(Polar bear species 1 separates from polar bear species 2 because of migration to other parts of the island)
The polar bears start to have shorter fur, as the temperature rises due to climate change. Polar bears use their thick fur for insulation, but the rising temperature leads to overheating in a lot of thick-furred polar bears, as polar bears are prone to overheating due to their heavy insulation. Polar bears with too little fur are not insulated well enough, so the bears with a moderate amount of fur are favored in this situation. Their fur color, a single-gene trait that is very important when it comes to tracking and hunting, stays white, as this reduces heat loss and camouflages them.
2. Lung capacity
Before, polar bears could hold their breath for close to two minutes, but their prey (seals) have gone to search for food deeper and farther away from Island F, as the rising temperatures have led the seal’s main sources of food (fish such as arctic cod, and invertebrates such as krill) further into the water in search for their own food. Also, seals compete with arctic sharks for the fish in the area, so an increased lung capacity can give them an edge. What’s more surprising is that the little fish the shark eats and the shark itself are quite similar, and both evolved with convergent evolution. The bear would usually wait around a seal’s breathing hole to bite and kill the seal when it came back up or dive and go looking for seals a few feet under the ice, but this change has impacted the bears greatly, leading to the bears having to hold their breaths longer to get food. These adaptations are due to punctuated equilibrium, as the genetic equilibrium was upset by this sudden change in the environment, and needs to be reestablished.
After 100 thousand years
As the polar bears spend more time in the water, the individuals with feet that are webbed but not extremely webbed are favored. Due to the need to swim faster and more efficiently in the water, the polar bears need webbed feet, but feet that are too webbed lead to clumsiness and problems with balance or walking on land. As the middle of the curve for webbed feet is favored in this stabilizing selection, the polar bears eventually grow feet that are moderately webbed.
4. Behavior – storing food in ice
As the polar bears have to deal with less food and rising temperatures, some have started to store meat in ice to keep it cool and safe for days when they might need it. This behavioral adaptation soon spreads to all of species 1, helping their chances of survival, but species 3 doesn’t get this adaptation.
(Species 2 becomes extinct halfway through because they did not get the lung capacity or the food storage adaptation, and consequently run out of food during a season when seals are scarce. A few thousand years later, species 3 splits from species 2 due to geographical isolation and adaptive radiation due to that isolation that led to differing environmental circumstances, which led to the quick differentiation of species and the formation of new species and subgroups as a result – see Isolation Mechanisms passage)
After 1 million years
5. Resistance to pollution/humanmade poison
As humans start to venture more around Island F, pollution and oil spills increase in that area. Polar bears having a mutation that lets them endure more humanmade poison or pollution generally live longer and have more offspring than those who don’t, leading to the polar bear species gaining an increased resistance to poison and pollution. This is directional selection, as the polar bears that have a higher tolerance of pollution and poison are fitter for their environment.
The polar bear’s limbs get longer as the temperature continues to rise and it spends more time in the water. As longer limbs help release more heat and undo the polar bear’s insulation system, as it doesn’t need it as much anymore, and overheating is a grave risk, individuals with longer limbs are favored. Also, longer limbs help the bear swim more efficiently, and therefore cover more area in less time with less effort, rendering it more efficient. Longer limbs also allow for a greater reach, aiding the bears when hunting for food. Limb size is controlled by many genes, making it a polygenic trait, so this increase in limb size leads to change in a lot of different genes.
7. Body fat
Bears finally start to have systems that do not hold on to body fat as much, as it can be dangerous for the bears in the rising temperatures. Losing some of the fat storages the bear has leads to less risk of overheating while still staying warm, and faster movements, as they have less risk of overheating and also weigh less and become more agile. This was the main change polar bears underwent in this 10 million year period, and gradualism has, over thousands of years, led to a significant loss in stored fat that was not very observable until this time period.
After 10 million years
8. Dietary adaptation – the inclusion of some berries and plants
As the polar bear now has less need for fat in their diet and seals are not as common as before, some start to include berries and plants that have started growing as a result of the increasing temperatures (and therefore longer unfrozen summer periods when vegetation grows). These plants, although still very arduous and not very sophisticated, went through a sort of secondary succession to start growing. Some polar bear starts to have a diet resembling that of a grizzly, transitioning from fully carnivorous to omnivorous, while still eating a large amount of meat. This happened in both species 1 and 3. On the other hand, other polar bears still stick with a fully carnivorous diet, but disruptive selection occurs because the bear needs a lot of calories, and bears that either get a lot of calories from plants and animals and polar bears that get a lot of calories from animals are favored in relation to those who get a moderate amount of calories from both, leading to an increase in both extremes and a decrease in the middle curve.
At first, this was caused by genetic drift in species 1 (no claw change in species 3), as claw length increased by pure chance. Over time, though, this amelioration proved to be useful. As a result of the above dietary shift, the polar bear needs longer and more dexterous claws to uproot berries and other plant, and the specialization of their claws to be more elongated while maintaining the same shape lets them uproot plants more efficiently while still being adequate for killing seals.
A mutation in species 3 has led to an increase in tail size in those polar bears. As the polar bears were not negatively impacted by the change (the tail didn’t impair movement or compromise the polar bears, but in a minute way helped the polar bear be more streamlined and move faster in water), species 3 kept this adaptation and now have elongated tails.
After about 100 thousand years, a large chunk of island F splits from the original island, leading to geographic isolation, as a part of the polar bears stay on the mainland of island F, while others are separated. This leads to the differentiation of these two groups of polar bears. The polar bears still on the mainland of island F (species 1) get all of the adaptations listed, but the polar bears that split (species 3) only get adaptations 3, 5, 7 and 8. Species 3 also eventually meets grizzly bears, and some interbreeding occurs. All of this leads to the speciation of these two groups into new species that cannot interbreed.
Polar bears’ breeding season is usually around springtime, and the females seek out maternity dens (similar to a cave – land where snow accumulates, can be along coastal bluffs, river banks or pressure ridges on sea ice) in October and November. Mothers give birth to small bears during their November-December hibernation, and nurse them until they’re about 20-30 pounds (which takes mothers about 5-7 months). Species 1 stuck to the same schedule, but species 3 changed the schedule by a few months, leading to temporal isolation and the inability of the two species to breed, as they search for mates at different times.
There is a common behavior among the polar bears of species 1 to follow the tracks of a female when looking for a mate. This mating ritual is not practiced by the bears of species 3, who lost that ritual after splitting off. This leads to behavioral isolation, as the males and females from species 1 are not attracted by the males and females of species 3, and vice-versa.