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Lesson · Grade 11 · Life Sciences

Evolution as Changing Allele Frequencies

  • Grade 11
  • Life Sciences
  • 2 min
  • British Columbia
  • BC11-SC-154

A population of 100 beetles has a gene pool of 200 colour alleles, 120 B and 80 b, which gives allele frequencies of 0.60 and 0.40, shown on a bar chart. As birds eat most of the light beetles, B rises to 0.68 and then 0.74 over three generations, which shows that evolution is any change in a population's allele frequencies over generations, called microevolution at this small scale.

Concept: Population genetics: evolution as changing allele frequencies

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In one sentence

The question this lesson answers

When a population of beetles evolves, what exactly is changing?

Here are 100 beetles. They form a population: one species living in one place.

Transcript

What the lesson says

When a population of beetles evolves, what exactly is changing?

Here are 100 beetles. They form a population: one species living in one place.

Body colour is set by one gene. Its versions, called alleles, differ slightly in their DNA: big B for dark, small b for light.

Each beetle has two of these alleles. Big B is dominant, so one copy makes a beetle dark. Only two small b alleles make it light.

The gene pool is all the alleles in a population. 100 beetles with two each carry 200 colour alleles.

Of these 200 alleles, 120 are big B and 80 are small b.

An allele's frequency is its share of the gene pool. For big B, 120 divided by 200 is 0.60. For small b, 80 divided by 200 is 0.40.

A bar chart shows these two frequencies for generation 1.

Now birds spot light beetles more easily and eat most of them, so fewer small b alleles are passed on.

In generation 2, big B rises to 0.68 and small b falls to 0.32.

The birds keep hunting, so in generation 3, big B rises to 0.74 and small b falls to 0.26.

This is evolution: any change in a population's allele frequencies over generations, whatever the cause.

Each allele is a stretch of DNA, so this is a small change in the population's DNA, called microevolution.

So, evolution is any change in a population's allele frequencies over generations.

Curriculum

Where this fits

British Columbia, Grade 11

Life Sciences 11, Evolution and genetics: microevolution: changes in DNABC11-SC-154

Previous: Mutations: DNA Changes That Help, Harm or Do NothingNext: Three patterns of natural selection

The outcomes and strands follow the British Columbia curriculum.

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