1. The marine copepod Tisbe reticulata can be reared in seawater cultures in the laboratory, although larval mortality is considerable, particularly at high population densities.  Heterozygotes V^vV^m for two codominant alleles responsible for color differences were mated and their offspring reared at low and high population densities.  The numbers of offspring were as follows:
 

Density	VvVv	VvVm	VmVm	Total
Low	904	2,023	912	3,839
High	353	1,069	329	1,751

What are the relative fitnesses of the three genotypes at the two densities?
 
 

2.  Two alleles are present at an esterase locus (Est-6) in an experimental population of Drosophila.  Each of these alleles codes for a different isozyme, an electrophoretically distinguishable form of the enzyme.  The alleles are codominant.  Thus the three genotypes seen in the flies are homozygote FF, FS heterozygotes, and homozygote SS.

First-instar larvae of the three genotypes were placed in cultures, and the numbers of emerging adults were recorded.  The results of two experiments were as follows:
 

	Larvae	Larvae	Larvae		Adults	Adults	Adults
Experiment	FF	FS	SS		FF	FS	SS
#1	160	480	360		80	240	90
#2	360	480	160		90	240	80

Assuming that the relative fitness depends only on larval viability, what are the fitnesses of the three genotypes in each experiment?  Do you think that this polymorphism may be stable?  If so, estimate the equilibrium frequencies.
 
 

3.  Two isozyme alleles, A and B,  are present at the leucine aminopeptidase (LAP) locus in wild populations of the edible mussel, Mytilus edulis.  In general, allele A allows better osmoregulation in salt water, and allele B is better in brackish water.  Heterozygotes, with both isozymes, are more adaptable to changing environments.  In one highly tidal region of the Bay of Fundy, for every 100 AB heterozygote mussel surviving to adulthood, only 40 AA and 65 BB mussels survive.  Predict equilibrium gene frequencies.

A population geneticist collected 16 AA mussels, 41 AB mussels, and 43 BB mussels from the Bay of Fundy.  Can the prediction made above be rejected?  What is the value of chi-square?
 
 

4.  Imagine now that the Bay of Fundy were dammed up to generate power from the tides.  (There have been plans to do this in the past.)  Although the mussel population described in question #3 remains panmictic, half of the mussels are in rather brackish water constantly and the other half in seawater.  Now every generation 20% of the AA mussels die, 44% of the BB mussels die, and 84% of the AB mussels die.  What is the unstable equilibrium point for gene frequencies?  What equilibrium will eventually be reached, starting with the gene frequencies you calculated in question #3?
 
 
 
 
 

ANSWERS:

(1)

Density	VvVv	VvVm	VmVm
Low	0.892	1.0	0.903
High	0.661	1.0	0.615

(2)

Experiment	FF	FS	SS
#1	1.0	1.0	0.5
#2	0.5	1.0	1.0

Appears to be frequency-dependent selection, so is stable.  Since the fitnesses appear to be symmetric, a good guess would be that equilibrium gene frequencies will be around 50/50.
 

3) p_a=0.37, q_b=0.63.   Chi-square = 1.32,  not rejected with 1 df.
 

4) p_a = 0.38, q_b = 0.62.    p_a = 0.0 at equilibrium.