Natural Selection in Action

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Two forms of Biston betularia (peppered moth)

(melanic)

Slide 12

Biston betularia

Differ by only one allele of the gene forming dark pigment (melanin)

Both forms fly by night

Both forms rest on trees during the day

Slide 13

Prior to Industrial revolution

Pre 1800s

Light form common throughout Britain

Dark arose by mutation

-very rare

Slide 14

Light peppered moths

In non-polluted areas

-tree trunks covered with pale coloured lichens

Moth well camouflaged against pale background

Dark form easily seen and eaten by predators

Slide 15

Survey in the 1950s

Pale form most abundant in non-industrial areas

Dark forms most abundant in areas suffering from heavy air-pollution

Slide 16

Reason for change?

In polluted areas

-toxic gases kill lichen

-soot particles darken tree trunks

Dark coloured well hidden and favoured by natural selection

Light coloured moth easily seen

Slide 17

Frequencies of two forms of peppered moths in the 1950s