Workshop on Microevolution
by Dana Krempels
I. Discuss the meaning of:
a. species
b. population
c. gene pool
d. gene
e. allele
f. heritable traits (consider "nature vs. nurture")
g. lethal alleles
h. adaptive, maladaptive, and neutral traits
i. Monogenic vs. polygenic traits
species: definition varies, depending upon which species concept one holds. The
most general definition, however, is that a species comprises a group of similar
individuals able to breed and produce fertile, viable offspring.
population: all the members of a given species living in a defined area (whether it
be a stretch of coastal chaparral or your GI tract)
gene pool: all the alleles at all loci in every member of a particular population
gene: unit of inheritance that codes for a polypeptide (or, in some cases, an RNA)
allele: one of several forms of a particular gene. Example: A GENE (the C locus)
codes for coat color in rabbits, and has four alleles: agouti, chinchilla, Himalayan
or albino.
lethal allele: if recessive and inherited in homozygous condition, this will result in
inviability of the organism inheriting it. If dominant, a single copy will result in
inviability of the organism. (examples of lethal recessives: Manx allele in cats;
yellow coat color in mice; dwarfism allele in rabbits)
monogenic trait: phenotypic expression is controlled by only one gene locus.
polygenic trait: phenotypic expression is controlled by more than one gene locus.
adaptive trait increases the likelihood that an organism exhibiting it will leave its
genes to the next generation.
maladaptive trait decreases the likelihood that an organism exhibiting it will leave
its genes to the next generation.
neutral trait has no effect on the likelihood that an organism exhibiting it will leave
its genes to the next generation.
II. Discuss the nature of each of the following in terms its effects on a the genetic
composition of a population of organisms
a. mutation
c. non-random mating
e. natural selection
b. migration
d. small population size
a. mutation will add new alleles to the population, and may result in a new
expression of a trait. (This, in turn, will be adaptive, maladaptive or neutral,
depending on the environment and its selective pressures.
b. migration increases the effective size of populations between which individuals
travel, and will allow gene flow between the populations.
c. Positive assortative mating may result in frequency of expression of a
recessive trait that is higher than that predicted by Hardy-Weinberg. Negative
assortative mating may result in heterozygosity that is higher than that predicted
by Hardy-Weinberg.
d. Small population size will increase the likelihood of a particular allele becoming
fixed in the population, to the exclusion of any other alleles at a given gene locus.
e. Natural selection is the only factor that results in a population whose members
are genetically (as reflected by phenotype) well-suited for survival and
reproduction in a particular environment.
III. Discuss the concept of Hardy-Weinberg equilibrium in terms of
a. a single gene locus and its alleles in a single member of a study population
One generally considers only one locus at a time when trying to determine whether
a population under study is in HW equilibrium. However, there's no such thing as
"Hardy Weinberg equilibrium" for a single individual. HW equilibrium is a
populational phenomenon. (I've included this because I've found that students are
often not clear on this concept!)
b. a single gene locus and its alleles all members of study population
This is the level the population biologist considers when looking at HW equilibrium
(or not) in a population: one locus (and all its alleles) in all members of a particular
population.
c. all phenotypic traits in a single member of a study population
Again, HW equilibrium is a populational phenomenon. One individual is only one
small bit of the entire picture. Just trying to keep that clear in everyone's mind.
d. all phenotypic traits in all members of the study population, considered collectively
Although every phenotypic trait is subject to the five factors that can prevent HW
equilibrium, it would be extremely difficult to monitor the HW equilibrium state of
every trait in a population. The population geneticist generally considers only one
locus at a time, though additional traits may be added in later studies of the
population.
IV. As a group:
a. Select a species of organism to design from scratch. You may select any
taxonomic group you wish. Give your organism an appropriate scientific name.
Genus:
species:
b. Define this species' natural environment, including such items as:
a. food sources
b. predators
c. space for reproduction
d. pathogens in the environment
e. other factors which affect the species' survival
c. Design this species by listing six dominant, wild type phenotypic traits that
enable it to exploit/avoid the environmental factors you have listed. (One
example has been included.)
Trait
example:
feather color
dominant (wild type)
phenotype
green
Nature of trait
provides camouflage
V. Taking turns and going around the group, create a recessive, but not lethal, allele
for each of the above traits.
Trait
recessive
Proportion of individuals in the
phenotype
population expressing
recessive phenotype
example:
yellow
0.25
feather color
VI. Let's assume there are 1000 organisms in your study population. Using the
Hardy-Weinberg equation, and for each trait, calculate the number of homozygous
dominant, heterozygous, and homozygous recessive individuals in this population.
If 250 of these birds have yellow feathers, then they have 500 recessive alleles
among them.
q2 = .25
q = .5
solving for p: (1 - q = p) we get
1-0.5 = 0.5
plug into HW equation: p2 + 2pq + q2
.25 + .5 + .25
In this population, 250 should be homozygous dominant, 250 should be
homozygous recessive and 500 should be heterozygous for feather color, if the
population is not evolving (i.e., is in Hardy-Weinberg equilibrium).
VII. Select one of the traits you have created above. For each of the possibilities
below, create a scenario that might act to change the proportion of wild type to mutant
alleles now present in the population. If there's time, do this for another trait or two.
mutation: (example: new mutation at feather color allele results in white feathers)
a mutation could produce a third allele. It might be dominant or recessive to
either of the existing alleles, and its phenotype would be subject to selection.
migration:
If a neighboring deme to our study population had a different initial proportion of
dominant and recessive alleles, immigration from that deme could change the
allele frequencies in our study population if immigrants breed with locals.
small population size:
random sampling error is more likely to affect genotype frequencies in a small
population.
non-random mating:
you get the idea. See the description of non-random mating above
natural selection:
And again. You can figure this one out and create many different scenarios that
would change allele frequencies in our population.
Discussion Questions
1. Describe some conditions under which a mutation generating a phenotype
different from the wild type might confer a selective advantage.
You may get as off the wall as you wish here. So here goes. Let's take the
example of the recessive white-feathered mutant. Unless this was lethal, this new
form is likely to remain in the population in heterozygous form, and rarely
(depending on how many population members carry it) show up in the phenotype if
it's inherited in two copies. If these birds nested in white rock cliffs, a colorsensitive predator might be better able to home in on green or yellow birds that
didn't match the rocks, whereas the white-feathered birds might be more likely to
escape detection. This could lead to their leaving more offspring. (Yes, there are
all sorts of "but what if's..." you could add here. And you certainly should do that, if
you know enough about birds, their parental care, etc.)
2. Discuss how immigration or emigration of individuals from a particular population
might change allele frequencies in that population.
See above.
3. What might happen to the allele frequencies of a population in the event of a
natural disaster that randomly wiped out a large proportion of the individuals?
This would result in Bottleneck Effect, a form of genetic drift. A major shift in
genotype and allele frequencies is far more likely to occur in a small population,
due to random sampling error.
4. If a small group of organisms were blown to an island during a hurricane, and
could not return to the population from which they had come, what might become
of the allele frequencies of that small population, relative to the original
population?
Similar effect as that in #3, but this time it's called Founder Effect.
5. What might happen to phenotypic frequencies in a population if organisms with
similar phenotypes had a higher likelihood of mating with each other?
See the description of positive assortative mating above.
6. What might happen to phenotypic frequencies in a population if organisms with
dissimilar phenotypes had a higher likelihood of mating with each other?
See the description of negative assortative mating above.
7. What is sexual selection? In which of the five categories above does it fall? How
might it result in sexual dimorphism?
Sexual selection is the result of one sex in a species showing a preference for
members of the opposite sex who have a particular phenotype. It is more
appropriately considered a form of natural selection than a form of non-random
mating, since in this case, some members of the population might not get to mate
at all.
Sexual dimorphism--the existence of different morphologies between the two
sexes--could evolve if a particular trait present only in one sex confers a selective
advantage to members of that sex. This dimorphic trait may either make the
opposite sex prefer those who express it, or it might confer a competitive
advantage to the individual competing with members of the same sex for
territories, food resources, etc.
8. If one particular (monogenic) trait of the several you listed is in the process of
changing allele frequencies due to one of the five factors that may disrupt HW
equilibrium, what does this imply about the allele frequencies of the other loci?
Again, since you are monitoring only one trait at a time, it is not possible to
accurately predict what will happen to other traits in an evolving population.
However, traits that are chromosomally linked or otherwise closely associated
with a trait that is subject to natural selection may also be affected. You can
probably think of ways this could occur, and you might want to ask your workshop
students what they think about this.