POPULATION GENETICS
1. Evolution occurs by what mechanism according to Darwin? Natural selection What
must be true for this to occur? Some individuals must have greater fitness than
others, change must occur between generations, populations must have genetic
variation, and more offspring than can be supported by the environment must
be born (i.e. competition must take place).
2. Natural selection acts on the (individual), while evolution acts on the (population).
3. In microevolution, population—a localized group of organisms of the same
species—is studied by looking at their gene pool, or total composition of genes (at
that time).
4. The phenotype of a population of humans is non-crazy (CC), semi-crazy (Cc) and
totally loony-tunes (cc). This is an example of incomplete dominance.
What is the total number of alleles in the population? 200 (100 people x 2 alleles
each) How many alleles for this character does each person have? 2 What are the
allele frequencies? Fill in the table below with each phenotypic frequency.
Phenotypes
Number of
Individuals
Frequency
Non-crazy (CC)
Semi-crazy (Cc)
Loony-tunes (cc)
60
30
10
0.6
0.3
0.1
C: (60 + 60 + 30) / 200 = 0.75
c: (30 + 10 + 10) / 200 = 0.25
What do the frequencies add up to? 1 What must happen for the above population to
be evolving? There must be a change in allelic frequency in the next generation.
5. If a population is under Hardy-Weinberg equilibrium, then what’s happening (or not
happening)? What 5 conditions must the population meet for this to occur? The
population is not evolving. The population must have a large size, no migration,
no mutations, no natural selection (so, no resource competition) and random
mating.
6. What does the equation p2+2pq+q2=1 represent? It represents the total phenotypic
frequencies.
p2 = dominant homozygote frequency
2pq = heterozygote frequency
q2 = recessive homozygote frequency
7. There are 4 causes for microevolution. Describe the two below and how it affects
populations. Which causes are random? Does either one allow species to adapt?
Genetic drift: This is random and does not allow the species to adapt. It mainly
affects small populations. It is simply the changes in allele frequency due to
chance.
Natural selection: This is not random and does allow the species to adapt. The
environment acts on the individual, selecting for which individuals of a species
survive to reproduce. It attempts to increase the fitness (or reproductive
potential) of a species. Thus some individuals die off and do not pass their genes
on—only the “fittest” are able to pass on their genes.
8. Sexual reproduction (increases) (genetic) diversity within a population. What two
mechanisms are responsible for that? Crossing over (during Prophase I of Meiosis
I) and independent assortment.
9. Natural selection (works on current) diversity, (decreasing) (phenotypic) diversity
within a population. (An example: brown mice can hide in fields better, while
white mice are obvious and get eaten easily. Thus, natural selection kills off the
white mice, thereby decreasing the phenotypic (and thus genotypic) variation in
that population of mice.) What 3 mechanisms work against natural selection?
Diploidy, heterozygote advantage and neutral variation.
Diploidy: organisms can have recessive genes that don’t affect the phenotype of
heterozygotes and thus natural selection cannot act on it.
Heterozygote advantage: when the heterozygotes have the advantage, allowing
dominant and recessive alleles to exist.
Neutral variation: genetic variation not displayed in a species’ phenotype. It can
diversify but natural selection cannot act on it because it doesn’t show in the
phenotype.