Download Problem : How does the natural selection work

Problem : How does the natural selection work ?
1 . Statement, purpose and hypothesis : We know that over the time, species evolve, that is
to say that they change through time. For a long time, scientists have been working to find out
how it actually works. We don’t know the mechanism of evolution but we assume that the
living things must change their genetic materials to survive changing environmental
conditions.
2. Methodology :
We began our research by searching some information about Darwin’s theory of evolution.
We found a great web site which explains that theory with examples and some good
information. The web site address is :
http://www.sprl.umich.edu/GCL/paper_to_html/selection.html for the ones who want to visit
this site!
That’s a summary of what we found in this web site :
1 ) Darwin's Theory
Darwin's theory of evolution has four main parts:
1. Organisms have changed over time, and the ones living today are different
from those that lived in the past. The world is not constant, but changing.
2. All organisms are derived from common ancestors by a process of branching.
Over time, populations split into different species, which are related because
they are descended a common ancestor. This explained the similarities of
organisms that were classified together.
3. Change is gradual and slow, taking place over a long time.
4. The mechanism of evolutionary change was natural selection.
2 ) The Process of Natural Selection
Natural selection is a process that occurs over successive generations.
If all the offspring that organisms can produce were to survive and reproduce, they
would soon overrun the earth. So, there is a "struggle" (metaphorically) to survive
and reproduce, in which only a few individuals succeed in leaving progeny.
Organisms show variation in characters that influences their success in this
struggle for existence. Individuals within a population vary from one another in
many traits.
Offspring tends to look like their parents, including characters that influence
success in the struggle to survive and reproduce.
Parents possessing traits that enable them to survive and reproduce will
contribute disproportionately to the offspring that makes up the next generation.
To the extent that offspring look like their parents, the population in the next
generation will consist in a higher proportion of individuals that possess whatever
adaptation enabled their parents to survive and reproduce.
Figure 2: The Process of Natural Selection
First of all, only the ones who are able to reproduce can evolve the specie. Then,
environmental restrictions imply a competition for existence. Some organisms have heritable
variations which give them better chances to survive and others have got a mutation of their
genetic sequence which allows them to survive while others perish . The natural selection is
the mechanism which selects the best adapted to the environment. This ones will reproduce
and give a part of their genetic material to their descendants so as to give better chances to
survive to the next generations. Environment is evolving, so species evolve to survive !!
Finally, evolution occurs when the specie changes in a trait.
3 ) Natural selection requires...
For natural selection to occur, two requirements are essential:
There must be heritable variation for some trait. Examples: beak size, colour
pattern, thickness of skin, fleetness.
There must be differential survival and reproduction associated with the
possession of that trait.
Unless both these requirements are met, adaptation by natural selection cannot occur.
Some examples:
If some plants grow taller than others and so are better able to avoid shading by
others, they will produce more offspring. However, if the reason they grow tall is
because of the soil in which their seeds happened to land, and not because they
have the genes to grow tall, then no evolution will occur.
If some individuals are fleeter than others because of differences in their genes,
but the predator is so much faster that it does not matter, the specie won’t evolve.
In addition, natural selection can only choose among existing varieties in a population.
It might be very useful for polar bears to have white noses, and then they wouldn't
have to cover their noses with their paws when they stalk their prey.
Later, when we incorporate genetics into our story, it will be more obvious why the
generation of new variations is a chance process. Variants do not arise because they are
needed. They arise by random processes governed by the laws of genetics. For today, the
central point is the chance occurrence of variation, some of which is adaptive, and the
weeding out by natural selection of the best adapted varieties.
To illustrate that theory, we tried an experiment which didn’t show what it was supposed to
since it didn’t work as well as we wanted.
We wanted to understand better Darwin’s theory of natural selection. To do that, we wanted
to take some bugs and kill the ones who were the less able to survive an insecticide treatment.
Then, we looked what happened to this survivors. We chose fruit flies since they have a short
life time, so we could see the results rapidly. We called this experiment : “Fruit flies
resistance against insecticides” .
Problematic: How do fruit flies survive an insecticide treatment ?
Our first hypothesis was some flies could change their genetic sequence to build a kind of
protection against insecticides.
But after a short brainstorming time, we found that it was wrong!! So, we have searched for
some time and we found this new one:
Only the best adapted survive, the are already strong enough to fight against this poison and
the next generations become more and more resistant to insecticides since the survivors gave
them a part of their genetic material. So, lots of insecticides don’t act anymore on these new
species of fruit flies. This is natural selection.
Hypothesis : More fruit flies survive when they are descendants of strong ones who survived.
Experiment : We will give some ether ( 4 ml ) to 2 groups of fruit flies and we’ll see what
happens to them and to the next generations.
Protocol: - Make 2 groups of 25 fruit flies sleep with a cotton with 4 ml of ether on the top
of a tube.
-Count the survivors
-Place them in tubes with some food to make them reproduce.
- When the new fruit flies are adults, give them some ether like we did with their parents.
-Count the survivors and compare with the previous results.
Results :
We couldn’t count the survivors since they died before we arrived but they had laid eggs. We
kept the eggs and only 2 of them gave fruit flies. Unfortunately, they were not a male and a
female so they died some times later without any offspring. They were the strongest but they
weren’t enough to evolve the specie.
BUT we have A good result, A fruit fly was still alive when we looked at our tubes and when
we gave her ( a girl, yes) some ether, she survived so we can say that she was able to fight
against this poison and to give her ability to the next generations. The natural selection had
chosen her to survive!!
So, we can’t say if our hypothesis was right or wrong but however, we have some new
information:
- the ones who survive are the best adapted to their environment and they will make the
specie evolve
- sometimes, they are not enough to make things change!!
Applications : We can’t find an insecticide which can eliminate all bugs since they will
evolve by natural selection if we use strong ones and they will become stronger and stronger!!
Maybe we can use predators such as ladybugs.
.