Download Micro-evolution and Allele Frequency Change in Populations

Micro-evolution and Allele Frequency Change in Populations
Objective: To identify the mechanisms causing evolutionary change in the color of individuals
within a population
Background: We will define evolution as a change in a population’s allele frequency over time.
What this means is that for any set of alleles in a population of organisms, there will be some
frequency or ratio of the characters that appear. We will use the terms alleles, characters, traits,
and phenotypes synonymously in this lab.
Mechanisms of Evolution: The factors that act on a population can come in many forms. The
following four factors are the major mechanisms that act on populations and cause them to
change in allele frequency.
1) Natural Selection: When some hereditary character provides an organism a reproductive
benefit that those without the character don’t have, it makes the holder of that trait more
fit. Fitness is a measure of potential success. Success in biological sense is how many
fertile offspring are passed into the next generation. So, if some character makes an
individual live longer, they are more likely to produce more offspring; if the trait allows
an organism to mate more often or have more offspring with each mating, the character is
directly linked to its fitness.
While it can be thought that nature selects for organisms with certain traits, it is actually a
phenomenon of numbers; the traits have no real value except that the trait allows the
individual to produce more offspring, thereby increasing the frequency of the trait in the
population. For instance, if a monkey population was
75% blue/25% red and some of those red-blind blue-eating leopards come in the area and
eat the blue monkeys, the number (or frequency) of blue monkeys in relation to red
monkeys would decrease. Blue monkeys are not at a disadvantage to red monkeys until
nature (the leopard) selected against the blue monkeys. With the leopards in the
environment, it is advantageous to be red. Red monkeys get to live longer and produce
more offspring than blue monkeys. The frequency would change in favor of the red
monkeys in a population where there are leopards.
It is extremely important for you to realize that evolution is a phenomenon of
populations. Populations evolve when the frequencies of traits change; this is evolution.
Notice no individuals have changed. One blue monkey did not become red; a red monkey
population may become blue and red through one of these mechanisms. Individuals do
not evolve, populations do. However, these mechanisms act on individuals to cause the
population change. Individuals migrate, they have a mutation that changes their
offspring, they may be more successful than the average individual, or they may be killed
quickly. Any of these factors would result in a change of the population make-up.
2) Migration: Emigration (moving out of a population) and immigration (moving into a
population) can change the allele frequency of a population. As an individual moves out
of a population, so do their genes. This will change the population’s make-up, perhaps
significantly. For instance, if a red monkey moves into a blue monkey territory and mates
with the blue monkeys, this will add new alleles to the population.
3) Mutation: A mutation is any change in genetic material. Mutations can be caused
spontaneously during DNA replication, but can also occur from environmental factors
such as UV radiation and chemical agents. It is important to note that mutations can be
positive by giving the organisms some beneficial character, negative by putting the
organism at a disadvantage or killing it, or neutral (the change is neither helpful nor
harmful under the current conditions). Furthermore, it must be noted that the effect of the
mutation may change if the conditions change; what is neutral at one time may become
negative or positive if the environment changes. For instance, a mutation that turns red
monkeys blue may be neutral, neither hurting the monkey nor helping it. However,
imagine if some predator can’t see red but can see blue. It can now see the monkey and
eat it. Under this new pressure, blue is a negative trait. It can be seen that the effect of
mutations can change over time. Mutations are the ultimate source of all variation. By
adding a neutral or beneficial mutant to a population, the frequency of characters in the
population changes from being 100% normal and having no mutants to a population
having some mutant percentage. A mutation will change the monkey population ratio
(normal/mutant).
4) Genetic Drift: Genetic drift usually refers to changes in population frequencies due to
randomness or change in events and is usually a phenomenon seen in very small
populations. For example, if one monkey in a population of 100 red monkeys has a
mutation that turns it blue, 1% of the population is now blue. If there were only 10
monkeys and with one mutant blue that composed the population, the frequency would
change to 10% blue. This is a much greater effect due only to the size of the population.
Lab Procedure: In this lab, you are going to expose your population to the different
mechanisms of evolutionary change. Your population will be a group of M&Ms. The traits will
be the different colors. In the beginning, the different colors have no real fitness difference.
Materials:
Plain M&Ms
Peanut M&Ms
Different colors of construction paper
Paper cups
Calculator (optional)
Step 1: Your instructor will give you a pile of M&Ms. This is your initial population and can be
thought of as the population at Time 1 (T1). Determine the frequency (percentage) of each color
phenotype for T1 by counting how many M&Ms you have of each color divided by the total
number of individuals (phenotypes) within the population. Put the color frequency in the chart
under T1.
Color Frequency =
# of specific color
total # of M&Ms
Natural Selection Event
Step 2: Your population will now go through a natural selection event. Put your M&Ms into the
cup provided and mix them. Randomly and without looking, select two M&Ms from your
population. With the two selected M&Ms, smash them between your fingers until you see a
crack in the candy coating. Watch closely for the slightest crack. Put the survivor into a new pile
and eat the yummy loser. Repeat this process until the cup is empty. If both M&Ms crack, eat
them both. If there is an off number of M&Ms remaining, the last one is automatically the
winner. Determine the new population color frequency and put it under Time 2 (T2).
Step 3: Replace the missing individuals by allowing the remaining colors to reproduce
themselves. In other words, for each red individual remaining, add an additional red individual;
for each blue, add a blue, and so on.
The pressure of being smashed together represents an environmental stress. In the case of
the M&Ms, hardness in the candy coating is the trait that is being selected for. Color is the way
we are categorizing the individuals. Basically, we are testing if there is a correlation between
color and candy coating hardness. Will our population evolve due to this environmental stress?
Step 4: Repeat steps 2 and 3 one more time and enter the color frequency under Time 3 (T3).
Is your population changing in color frequency? If not, why?
Migration
Step 5: Place your construction paper very close to your neighbor’s construction paper. There
should be a slight gap between the two sheets of paper. Place all of the surviving M&Ms back
into the cup and pour them onto your piece of construction paper. Do this with enough force so
that while most will land on your paper, some will spill outside the perimeter of your paper land
on your neighbor’s construction paper. Remove all of the M&Ms. Place all the M&Ms that
landed on your paper into your cup. The M&Ms that landed on your neighbor’s paper are now
added to the neighbor’s population. Those that did not land on either piece of paper become
delicious non-survivors that are eaten. Following this, your neighbor will do the exact same
procedure. Make sure you and your neighbor DO NOT put the newly arrived M&Ms into your
pile until after everyone has done the migration process!
Determine your new population frequency and enter it under Time 4 (T4). Allow the M&Ms to
reproduce the same way you did in Step 3.
Mutation
Step 6: Your instructor will now add a certain number of mutants to your population. Determine
the population’s color phenotype frequency after this mutation event and enter under Time 5
(T5).
Natural Selection
Step 7: Send your population (T5) through another kind of natural selection. This time, place all
of your M&Ms on a piece of construction paper. Eat all of those that are most conspicuous
against the background. Allow the survivors to reproduce and fill in the color frequency under
Time 6 (T6). If all are eaten, then your population is extinct.
Genetic Drift
Step 8: Pour out the remaining M&Ms onto your construction paper. Without looking, grab up as
many M&Ms as you can in one hand with one swipe over the area. Allow those in your final
population to reproduce (same as in Step 3) and enter the last frequency under Time 7 (T7). The
small population number going into this last random event may cause a significant change in the
population make-up due to genetic drift.
frequency
Red
Green
Yellow
Blue
Orange
Brown
Mutant
Total
M&Ms
T1
T2
T3
T4
T5
T6
T7