11 February 2014
Chapter 18
Practice Problems
18.1 Figure 18.3 shows the decline in the frequency of the silver morph of fox in eastern Canada
due to its preferential harvest because of it more valuable coat. Heterozygotes (Rr) are
phenotypically similar to the red (RR) fox. Why do you think the frequency of heterozygotes
declined even though they apparently had the same fitness as the red fox, which increased in
frequency?
18.2 Some authors have suggested that the large population sizes of harvested marine fish make
even collapsed populations resistant to the loss of genetic variation by genetic drift because
population sizes of even collapsed stocks are in the thousands or millions (e.g., Beverton 1990).
Recent studies, however, have found that there is evidence for the loss of genetic variation in
overharvested populations of marine fish P(Pinsky and Palumbi 2014). How can you explain
these unexpected observations given our understanding of genetic drift and effective population
size?
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Assignment Problems
18.3 Pinsky and Palumbi (2014) compiled data from 11,049 microsatellite loci across 140
species of harvested fish and found that allelic richness was 12% lower in overfished
populations. Moreover, heterozygosity was on average 2% lower in overfished populations.
Do you think that these small reductions in genetic variation are likely to be important for the
long-term sustainability of these fish populations?
About 400 tons of Inca scad (Trachurus murphyi) caught by a purse seiner off the coast of Peru
in 1997. This is one of the overfished populations included in Pinsky and Palumbi (2014).
Photograph by C.O. Rojas, National Oceanic and Atmospheric Administration, U.S. Department
of Commerce.
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18.4 The figure below is a schematic representation of the age- and stage-structured population
dynamics model for harvest of Arctic cod off of the coast of Norway (Heino 1998). Fish can be
harvested either on the spawning grounds or on the feeding grounds. As discussed in Guest Box
18, the fishery on Atlantic cod has resulted in earlier age of sexual maturity which is caused at
least partially by change in allele frequencies at the Pan I locus. Would you recommend that cod
be harvested on the spawning or the feeding grounds to minimize genetic change in response to
harvest?
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