Teaching Evolution 3/5 – Non-Random Selection

This post – and activity – is a follow up from yesterday’s, looking at mutation. I’ve found that students often struggle with randomness and, short of loaning them Mlodinow’s excellent book, it’s hard to fix their problems. Instead, I focus on separating the concepts of variation and selection.

Variation always happens during reproduction. In asexual reproduction it is caused by mutations, mistakes in copying the instructions that make up the genome. For humans and other organisms that reproduce sexually most variation (in the short term) is caused by the shuffling of the genes during meiosis. We can treat this kind of variation – or the effects of it, in changing characteristics – as effectively random changes that affect an original characteristic.

Proponents of intelligent design often point to the random nature of variation as ‘proof’ that evolution couldn’t work. This is best addressed by pointing out that selection is the true friving force behind evolution, and it is clearly not random. It is true that sometimes a slow impala escapes a cheetah or a furry wolf still gets chilly, but on average those individuals with the characteristics best suited to their situation will survive and thrive. This is not random. On the contrary, it is both predictable and reproducible in statistical terms.

The first time I tried this I asked students to use the random number function of their calculators to model variation. We used even numbers for an increase in height, odd numbers for a decrease. We then ‘selected’ the offspring that were taller, imagining a situation where this would be an advantage. Over generations, the whole population became taller on average. My new version uses dice to simulate the changes but the selection works the same way. The printable activity consists of four pages:

  1. Teachers’ guide including answers to questions
  2. Instruction sheet with questions (if laminated this can be reused)
  3. Data sheet for students to write on their results
  4. Answer sheet for questions if required (could be turned into differentiated writing frame)

printable: giraffes as pdf

Please let me know if you find this useful or have any comments, especially any problems you had with it. I will give one health warning: This simulation works much faster than variation would change height in nature, in all but the most extreme environments!

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