This moth is famous as one of the most-studied
examples of "industrial melanism". In its usual form it is well hidden
when it rests on a lichen-covered tree trunk. Occasionally an almost black (melanic)
form, carbonaria, appears, which is conspicuous and is quickly snapped
up by birds. Furthermore, the melanic gene is dominant over the normal gene.
This means that a heterozygous individual, having both the melanic and normal
genes, will be black. In turn that means that the melanic gene doesn't have
a chance and is quickly extirpated from a population. Then came the industrial
revolution in Europe. Lichens were killed and tree trunks were blackened by
soot. Suddenly the melanic form had a huge advantage, the "normal"
form became conspicuous, and populations in industrial areas became predominantly
black. It is one of the most rapid textbook examples of
variation and natural selection on record.
Here on Vancouver Island I have so far come across only "normal" adult moths. The caterpillars come in several colours, and it might be of interest to learn if that has any significance. They are to be found in August on willow and alder. They are one of our largest geometrids, and they have two triangular bumps on the head, giving their heads a resemblance to a pussy cat. Winter is spent as a pupa.