ANSWER KEY—Chapter 23 Discussion & Practice Qs
1. Differentiate between the following terms: microevolution, population genetics, gene pool,
and gene locus.
Microevolution – changes in gene frequencies in a population over time
Population Genetics – the study of genetic traits in a population
Gene Pool – the sum of the available alleles for a trait in a population
Gene locus – the location of a genetic trait on a chromosome
2. In a population of 200 mice, 72 are homozygous dominant for brown coat color (BB),
96 are heterozygous (Bb) and 32 are black and homozygous recessive (bb).
a) Calculate the frequencies of the dominant allele B and the recessive allele b in this
population.
B = .6 and b = .4
b) Calculate the genotypic frequencies in this population.
BB = 36%, Bb = 48%, bb = 16%
3. Assume that the population in #2 is in a Hardy-Weinberg equilibrium. If the population
increases to 1000 individuals:
a) How many brown mice should there be? How many black mice?
Brown – 840, Black 160
b) Have the frequencies of the dominant and recessive alleles changed and is this population
“evolving”?
If the population is in H-W equilibrium, the gene frequencies will not change.
4. Identify and briefly explain the five Hardy-Weinberg assumptions.
 Large population – population must be over a certain size
 Isolation – no movement/migration of individuals or genes in or out of the
population
 No net mutations – no new alleles are added to the gene pool
 Random mating – no sexual selection or mate selection
 No natural selection – all offspring are equally likely to survive and
reproduce
5. Based on your personal observations of populations of plants and animals, discuss if you
expect the five assumptions to hold up over time in most natural situations. Describe what
must happened to the populations over time based on your discussion.
– No, Most organisms do not mate randomly, migration is common,
mutations occur, natural selection is present due to selective pressure from
the environment, etc.
– The gene frequencies will change and the population will be evolving
6. The population of Mexican wolves fell to less than 50 individuals in the last century.
Although the population numbers have been increasing this species may be doomed in terms
of microevolution responses to its environment. Discuss why this is likely to be so.
- The population underwent a bottleneck effect. The small population size
limits genetic diversity. Without genetic diversity, the population is limited in
what evolution can work with, possible dooming the population over time.
7. Explain what can be inferred if an allele persists in a population over time.
- The gene must have or had some adaptive value. Otherwise it would be
selected against in the population over time.
8. Many plants can reproduce sexually and asexually. If a population of plants stopped
reproducing sexually and now only reproduces asexually, is this population evolving?
– Asexual reproduction generally does not change the genetic variation of a
population. Evolution would essentially stop for this population and it
should not change over time unless new random mutations were to take place
allowing for evolutionary change.
9. Consider a population in which heterozygotes have a selective advantage such as in sickle
cell anemia in malaria infected areas. Explain if this situation represents directional,
disruptive or stabilizing selection.
– stabilizing selection
– selection is for the average (heterozygote) and away from the extremes
(homozygous dominant and homozygous recessive).
10. Describe to an AP Reader your understanding of the importance or value of the HardyWeinberg principle in Evolutionary Biology.
– Any of the following ideas: HW allows evolution to be tracked over time. It
allows the calculation of a gene frequency in the gene pool at any given point
in time. It predicts that evolution should not occur if the assumptions hold
(or it predicts that evolution must occur since it is nearly impossible for the
assumptions to hold), by looking at which assumption has failed, it allows the
study of the driving force or cause of evolution in a population.