Download Natural Selection

Natural Selection:
Assignment: Choose ONE animal species. Your goal is to describe the way in which an
evolutionary change might occur for a particular characteristic (trait) of that species as a
result of natural selection. The characteristic could be something like coloration pattern, length
of the limbs, or size of the teeth or beak, or any measurable trait that is inherited.
The organism you choose should be a real one but the evolutionary change you describe can
be real or theoretical.
In your own words,
a.
Explain how the change in this trait occurs in terms of the four postulates listed
above and in the book. You will describe what the population starts out like at time
zero and then what it will look like at a later time, as a result of natural selection.
I am choosing the peppered moth (Biston betularia), and I’d like to talk about its coloration
patterns and how natural selection works on the frequency of different color variations. The
evolutionary change that I want to use is the example of the change in frequency of light vs
dark moths in pre-industrial, industrial revolution, and current times.
The population of the moths that I am discussing are in England. (There are other populations
in Europe, America, and Japan.) There are several variations in the population, but the one
that we’ll be talking about is the colored morphs (colored forms) of the moth. One morph is
the light colored moth; the other form has dark coloration of its body and wings. There is
one gene, with two alleles, that code for color – so the coloration is inheritable. (The ‘dark’
gene, that codes for increased melanin, is dominant over the lighter version.) (Postulates 1
and 2)
The moths rest in trees during the day, and the males fly in search of females at night. Birds
prey on the moths as they rest in the trees, and the ones that can hide better and avoid
predation are more likely to reproduce. The color morph is directly related to how well they
can hide, and therefore how well they can reproduce. (Postulates 3 and 4)
Prior to the Industrial Revoultion in England, most trees were lighter in color – a combination
of their bark and the lichens that grew on them. Most of the moth population was the
“typical” or light/speckled morph. Darker (“carbonaria”) forms were far less common, making
up less than 10% of the population. Call this Time Zero.
During the Industrial Revolution, massive coal burning released large quantities of soot, which
coated the trees, killing the lichens and making the trunks much darker. The lighter morph
stood out more (could be seen better by the birds), while the dark version could hide better.
During this time, the frequencies of light to dark morphs changed dramatically – with more
than 90% of the population in the soot-covered areas of England being the dark morph. Call
this Time One.
As the Industrial Age has advanced, pollution controls have become much more stringent; and
fuels other than coal have come into use. As a result, there has been less soot coating the
trees, and less pollution killing the lichens. The trees have gotten lighter in color again. The
light colored moths are better able to hide against these cleaner trees than the darker morph.
The frequency of the light/dark morphs has changed again. At this time, call it Time Two,
the percentage of light colored morphs has increased, and the percentage of dark morphs has
decreased.
b.
Make sure to explain the process by which the changes occurred. What is the
selective pressure? Explain why the change might confer more fitness to a particular
environment?
Since birds prey on the moths as they rest in the trees, their color is important in helping
them avoid visual detection by the birds. It is important for the color of the moth to match
the color of the trees in which they are hiding. If the color of the tree changes, we would
expect the color of the moth to affect its fitness (its ability to survive and produce more
offspring. At a genetic level, the frequency of the light / dark alleles would change, depending
on the ability of the moth that carried those genes to survive and reproduce.)
At Time Zero, with light colored, lichen covered trees, the light morph was better able to
blend in against the tree trunks. Dark morphs stood out, and were much easier for birds to
see, and so eat. Dark morphs were selected against, and the dark allele was a very low
percentage of the gene pool.
At Time One, the trees became very dark colored, and the situation reversed: Light morphs
stood out against the dark trees, and were easily seen by the birds (and eaten.) Dark morphs
were much harder to see against the dark background – so the dark morph was able to
survive and reproduce more often, therefore increasing the frequency of the allele that coded
for more melanin production. The gene frequency changed, with a greater proportion of the
“dark allele”.
At Time Two, trees have become lighter again (as soot washed off, and as older trees died and
newer one grew). The darker morph does not have as strong of an advantage in hiding
against these lighter colored trees, and so is being selected against by increased predation
pressure. The gene frequencies are again changing, with the lighter colored allele (and so the
lighter colored moth morph) increasing in frequency, and the darker colored allele decreasing
in frequency.
Since, at its most basic level, evolution is simply a change in gene frequency, we can see how
Natural Selection (the ability to survive predation) has driven a change in the gene
frequencies for moth body color in the peppered moth (Biston betularia) in England.
Tips: Make sure to include the following in your answers to a and b above:
Clearly state the name of the SPECIES you are going to be talking about for this
assignment. (Make sure you know what is meant by a species.)
Also, clearly state what population of the species you are going to talk about by
specifying its geographic location. (Make sure you understand what a “population”
is.)
Clearly state the characteristic (trait) of the population that you are going to be
talking about and explain how this trait varies among individuals.
Explain what change occurs in the environment that causes a change in the above
characteristic (trait)?