Niels nearly identical structures like paws, jaws, and legs).

Niels Levy-Thiebaut

Ms. Maharaj

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Pre-AP Biology, Period B4

22 January 2018

Introduction

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)

1.      Fur

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

3.      Feet

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.

6.      Limbs

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.

9.      Claws

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.

10.  Tail

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.

Isolation mechanisms

Geographic isolation

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.

Temporal isolation

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.

Behavioral isolation

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.