English

Natural Selection

High schoolBiologyOne-gene selection model, dark allele dominant
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About this experiment

Before the Industrial Revolution almost every peppered moth in England was pale and speckled, hidden on lichen-covered bark. As soot blackened the trees, birds found the pale moths more easily, and within about fifty years most moths near industrial cities were dark. After clean air laws in the 1950s the trend reversed. This simulation follows the dark allele generation by generation so students can see natural selection change a population, and test how the strength of predation and the starting frequency affect the speed of change.

Learning goals

  • Explain natural selection in terms of variation, selection pressure, survival and inheritance
  • Describe how the environment decides which variation is an advantage
  • Distinguish between the share of dark moths (phenotype) and the frequency of the dark allele
  • Explain why a rare recessive allele is removed very slowly by selection

How to use it

  1. 1Leave the bark sooty and press Play. Note the generation when dark moths pass 50%.
  2. 2Reset and change the extra bird predation to 10% and then 40%. How does the speed change?
  3. 3Run on sooty bark until about generation 50, then switch to clean bark and watch the light moths return.
  4. 4Compare the dark moth curve with the dark allele curve. Why are they different?

The model

Dark-allele frequency in the next generation. p and q are the frequencies of C and c.
Moths that stand out on the bark survive less often. s is the extra predation.
Hardy-Weinberg: CC and Cc moths are both dark because C is dominant

Assumptions

  • Moth icons, bird flight and cohort transitions are illustrative. Numerical frequencies come from the population model; individuals do not change their inherited color.
  • The population is very large and mates at random, so chance (genetic drift) plays no part and the result is the same every time.
  • Wing color is controlled by one gene with two alleles. The dark allele C is fully dominant, as in the real carbonaria form.
  • Bird predation is the only difference in survival. There is no migration or new mutation.
  • One generation is one year, which matches the peppered moth life cycle in Britain.

Try this in class

  1. Q1Did any moth change its color to match the bark? What actually changed in the population?
  2. Q2Why does the dark moth share rise faster than the dark allele frequency?
  3. Q3Start with 99% dark moths on clean bark. Why do the last dark moths disappear so slowly?
  4. Q4If the bark stayed sooty for ever, would the pale allele vanish completely in this model? Why or why not?

Frequently asked questions

Is the peppered moth story still accepted?

Yes. It was questioned in the 1990s, but a large field study by Michael Majerus, published in 2012, confirmed that birds eat more of the moths that stand out on the bark.

Where does the dark form come from in the first place?

From a mutation. In 2016 researchers traced it to a piece of DNA inserted into a gene called cortex, which arose in the early 1800s, shortly before the first dark moth was recorded in Manchester in 1848.

Why is there no randomness in this simulation?

The model tracks allele frequencies in a very large population, where chance events average out. This isolates selection so the effect of each setting is clear. Genetic drift needs a separate small-population model.