Building the 1st Model

Figure 2 shows the flow diagram for an introductory model of two moth populations. The black population corresponds to the melanic phenotype; the white population corresponds to the typical phenotype. Weaver and Hedrick (1995, p. 52) describe a third phenotype, the darkly mottled insularia. Let's ignore the third phenotype to keep the model simple.

Figure 2. Introductory model of the moth populations.

The first model adopts identical assumptions for the black and white moths. They both spend 10 months in a preadult phase and 2 months as adults. Each population has 50% females; each female has 15 eggs per brood; eggs are deposited in July; eggs from white females lead to white progeny; and eggs from black females lead to black progeny. (These final assumptions make sense if the moths are pure breeding adults that self fertilize.) A single conveyor is used to represent their early months as ovum, larva and pupa. During the preadult stage, their loss fraction can range from a low of 50% to a high of 99%. The exact loss fraction depends on the total in the preadult stages:

total_in_preadult_stages = black_ovum_larva_pupa+white_ovum_larva_pupa
loss_fraction_in_preadult_stages = GRAPH(total_in_preadult_stages)
(0.00, 0.5), (200, 0.6), (400, 0.7), (600, 0.8), (800, 0.9), (1000, 0.99)

After 10 months, the moths mature into the adult phase where both blacks and whites are exposed to a 50% bird loss fraction.