Mechanism of adaptation to environmental change: parallel evolution of melanism in the peppered moth

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The genetic raw material that allows a population to respond adaptively to environmental change must come from one of three different sources: 1) genetic variation already present in the population; 2) immigration from another population that has the necessary genetic variation; or 3) spontaneous mutation within the population that produces a beneficial trait. Because a novel phenotypic trait may be produced through a variety of alternative genetic and developmental pathways, adaptive responses to the same environmental change in different populations could involve the same or different genetic changes. Furthermore, the idea that many adaptations are controlled by a restricted subset of genetic master switches or 'hotspots' implies that the origin of phenotypic novelty is to some extent predictable. This overall process influences the likelihood of a novel trait appearing in a population and therefore on the rate of adaptation. However, there are very few well worked examples of mutational origins and the existence of genomic hotspots for adaptation in natural populations.This research aims to expand our understanding of these phenomena in the context of industrial melanism in the peppered moth. This species provides excellent material for studying the mechanism of adaptation. It is widely known that in 19th century Britain till the 1970s, a black coloured variety of this moth, which had never been seen before, replaced the light coloured 'typical' form. The reason for this is that high levels of coal soot in the air caused darkening of the surfaces which peppered moths rest on during the day-time. This made the typical form much more visible than the black form to bird predators, and so the genetic variants producing the typical form rapidly declined. What is less well known is that essentially the same change happened at around the same time in the peppered moth populations of mainland Europe and eastern North America. Were these parallel changes in the separate populations made possible by the same mutation spreading around the world or by different, home-grown, mutations? If there were different mutations, did they occur in the same or different genes?For the British peppered moth population, recent genetic studies have shown that all black peppered moths (called carbonaria) are descended from just one mutant ancestor from the 19th century. Surprisingly, the obscure chromosomal region where the carbonaria gene was found turns out to also control very different colour patterns in butterflies, which hints at a deeply conserved mechanism, perhaps a hotspot. Our objective is to similarly discover the original genetic source of the black peppered moths in mainland Europe and in N America. Preliminary evidence already suggests that they are different from the UK carbonaria and also that they may be much older. This latter possibility is significant because it would mean that, in some populations, the genetic variation for the black form existed before industrialisation.We will take three complementary approaches to this problem: 1) Use parent-offspring families to work out whether the chromosomal regions controlling the black morphs are the same or different between populations. 2) Compare DNA sequence variation associated with black and typical moths in population samples, including early 20th century museum specimens through to modern day specimens. This will tell us if the black moths in Europe and N America come from different ancestors, and whether these ancestors came into being relatively recently, or have a much more ancient history. 3) Measure the activity of the genes that we find during the critical period of melanin production, shortly before the adult moth emerges from the pupa. The findings of this research will add significantly to our understanding of how natural populations adapt to rapid environmental change, which is important to anticipating the consequences of human activity.