1. Look at the phenotype chart. Why are there more with gray tails?
  2. Start with sexual and asexual the same, 4 individuals: WW, BB, RR, GG (all of the "pure" genotypes).
    Can an asexual organism ever produce a "mixed" genotype from the above individuals?
  3. Reproduce one cycle.
    a) Why are there more asexual (8) than sexual (6)?
  4. Liver Fluke Infestation! All GGs die.
    a) one sexual dies, depending if M or F died then total = 7 or 8
    b) two asexual die, no more G genes in population. total = 12
  5. Predator Eats All RRs. (that flashy red tail was too visible!)
    a) no more R genes in asexual, total = 16
    b) one sexual dies, depending if M or F died then total = 8 to 11
  6. Blue Tail Rot Kills All BBs.
    a) Only WW asexuals left! total = 16
    b) one sexual dies, depending if M or F died total = 9 to 13
  7. No One Will Mate with a Yellow Tail
  8. Viral Plague kills all individuals with a WW gene combo.
    a) no asexual individuals left!
  9. Cycle of reproduction
    a) most gene combinations still present in sexual population.

Comments on Scenario #1


  1. The "wild type" gene is dominant. Note: in the non-wild type gene pairs the combination of the two genes produces an intermediate color.
  2. Probably unrealistic to start with homozygous individuals, but this lab cannot run for several million years to make the points either!
  3. The "male" cost shows up as they cannot reproduce themselves, whereas ALL of the asexuals can.
  4. First infestation. All of the GG genes are removed from the asexual population, forever. In the sexuals, the gene can remain "hidden" in a heterozygotic combination. Even though more asexuals have died as a result of the infestation, they breed faster and are still way ahead in numbers.
  5. The asexuals loose 4 this time, but are still in the lead.
  6. Now the asexuals are all one strain, a very dangerous situation. Unless there was a mutation (a rare event) they could be totally wiped out.
  7. When a preference gets set, like in this case, it can lead to speciation. The asexuals don't care because they don't mate. In a way each individual can be thought of as a separate species.
  8. & 9. The asexuals are gone. By not being able to recombine their genes, they have succumbed. After the plague/ infestation/ predator/ rot has passed (no food left for that strain of parasite/predator), the heterozygotics can then re-introduce the missing strains, thus maintaining the genetic diversity.

Let both sides reproduce several cycles with no diseases or predation. Which side reproduces faster? Why?

Comments on Scenario #2

Without predation, asexual reproduction does better. The cost of "males" is high under these conditions. Unfortunately, these "easy" times are rare. The "ideal" organism would be one that could use either method, depending on circumstances.

A variation on this scenario would be to have limited random predation. Some individuals are still lost, but because it does not result in "selection" pressure [no preference for a particular phenotype], asexual still comes out ahead.