Learning from the Fossil Record
Grade 8 Science
Part 1 of Performance Assessment Task – Skull Model
1.
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How is it possible for the recessive short skull and jaw trait to change to a dominant long skull
and jaw trait and stay that way permanently?
Since long, skinny tail is recessive over fat, skinny tail, since all the organisms in the deepest
fossil layer have long and skinny they must be homozygous recessive (tt), because once you put
a capital letter into it becomes a fat, short tail.
In the next fossil layer, there were some organisms with long, skinny tails and short, fat tails. The
long, skinny tailed organisms were offspring of the deepest layer. (tt x tt = tt all offspring) The
organisms with short, fat tail cannot be genetically explained because they require at least one
dominant trait (Tt or TT). Since this is not possible, coming from homozygous recessive parents
The reason for this trait must be due to a change in genes
In the next fossil layer all the organisms have the dominant short fat tail trait, this is most likely
came about by a cross between TG x tg. This resulted in all the offspring coming out Tt.
In the final fossil layer all the organisms had the dominant short fat tail once again, this had to
come out due to the Tt x Tt cross resulting in seventy five percent dominant trait (short fat tail)
and 25 percent recessive trait (long skinny tail). For some reason the long and skinny tail
organisms could not survive beyond birth(probably due to something in their natural habitat
where they needed short fat tails)leaving only the dominant organism to live to adult hood.
Eventually all organisms would end up TT only, since the chance of having tt became extinct and
they did not survive after birth.
Learning from the Fossil Record
Grade 8 Science
2. How is it possible for the dominant long front legs trait change to a recessive short front legs
trait and then turn into flippers?
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Since Long front legs is dominant over short front legs, since all the organisms in
the deepest fossil layer have long front legs they have the dominant trait,
homozygous dominant.
In the next fossil layer, there were some organisms with long front legs and short
front legs. The long front tails organisms were offspring of the deepest layer
(TTxTT=TT all offspring) The organisms with the short front legs cannot be
genetically explained because they require two recessive traits. Since this is not
possible from homozygous dominant parents the reason for the change must be
because of what the organism needs.
In the next fossil layer all the organisms have front flippers. This is change is
reflected on the environment they are in and or a deformity.
In the final fossil layer all the organisms have front flippers once again. This is
because the organism’s habitat changed so therefore they changed so that they
could survive. Once the habitat changed the long front legs and the short front
legs changed to flippers so that their race could continue to live and wouldn’t die
out.
No Mutation
 In an environment with mostly medium trees, long legs would be a great advantage
over where the animal lives. With long front legs the animal could easily survive by using
its long legs to climb up the trunk of trees to grab food, and also maybe some small
animals that live in the tree. This trait is key in this environment and the animal would
not have to change. I believe that this place is the creature’s natural habitat.
 When the environment changed to smaller shrubs, the animal needed to adapt to its
habitat. Since this animal has such long legs, it would be easily spotted by other
predators because it is so tall. That is why this animal had to shorter legs to survive, and
that is why it changed. With shorter legs this animal could move throughout the smaller
shrubs and be hidden from larger animals that might try to harm it.
 In the animals last environment, which changed to smaller creeks, this animal must have
had to change once again. This animal now had to change to a tail that would make it
easy to swim. With this environment with mostly small creeks, it would be much easier
for this animal to travel by water. Therefore it had to adapt. With a tail like this, you
could move much quicker in the water which would be good for catching food, are
hiding from predators in the water. All these changes were do to natural selection, not
a mutation. When the environment changed around the animal, every time the animal
reproduced something about it changed a little bit to suit the environment. That is
natural selection, where your genes are given to you so you are able to survive in the
environment you live in.