David M wrote:Why are your Swiss Speckled Woods like our British ones, rather than the rich orange brown spotted French versions?
padfield wrote:Switzerland is on the dividing line, David, which runs from about Brittany to Italy. Both forms fly here, and some intermediates too, like this one, photographed this April:
Guy
David M wrote:Interesting stuff, Guy.
What happens if they interbreed? Do you just get a mélange?
padfield wrote:Anywhere along the 'border' you can see intermediate individuals, from the Channel Islands, through France, to Switzerland, and for that reason I have always naturally assumed they do interbreed. But once you get well within the territories of each form you only see one kind (in my experience). It's a strange situation, I admit! Presumably it is maintained in part by the very sedentary behaviour of this species.
Guy
David M wrote:They can't be that sedentary, Guy; they've recently colonised the Isle of Man (presumably from Cumbria). I was going to ask you if you'd ever seen an example of one form mating with another, but then I suddenly realised that I've never seen Speckled Woods mating, which I find odd given that other members of the 'Brown' family are commonly observable (Grayling, Meadow Brown, etc). Do they go up to the canopy for this?padfield wrote:Anywhere along the 'border' you can see intermediate individuals, from the Channel Islands, through France, to Switzerland, and for that reason I have always naturally assumed they do interbreed. But once you get well within the territories of each form you only see one kind (in my experience). It's a strange situation, I admit! Presumably it is maintained in part by the very sedentary behaviour of this species.
Guy
padfield wrote:It's true that speckled woods readily colonise new areas, and have been particularly active in that respect of late, but that doesn't count against the predominant behaviour, within an area where the species is well established, being sedentary. It has been observed that colonising speckled woods are larger than sedentary ones and there may be a trade-off between fecundity and flight (see http://www.jstor.org/pss/3592338, though you have to pay to get the full article). If so, it would mean that where one form attempted to colonise a previously colonised area it would be outmatched, reproductively, by the established, non-dispersive population. Those same dispersive individuals moving to an unpopulated area would be able to reproduce with less competition, when the genes for non-dispersive individuals would be expressed in a new, sedentary colony.
The species never flies terribly far, though, even in dispersal mode. You never (to the best of my knowledge) get migrant speckled woods of the 'other' form very far away from the boundary.
All good questions, and I don't know most of the answers; but there is a lot of research going on in these areas at the moment, as if there is continued climate change the ability of various species to move into newly suitable habitat will be very important.
Guy
PS - Here is the abstract from the article linked to above:
"During recent climate warming, some species have expanded their ranges northwards to keep track of climate changes. Evolutionary changes in dispersal have been demonstrated in these expanding populations and here we show that increased dispersal is associated with reduced investment in reproduction in populations of the speckled wood butterfly, Pararge aegeria. Evolutionary changes in flight versus reproduction will affect the pattern and rate of expansion at range boundaries in the future, and understanding these responses will be crucial for predicting the distribution of species in the future as climates continue to warm".
David M wrote:That sounds highly plausible, though surely there would be occasions when one form mates with another? In the case of continental Speckled Woods though (where thera are no sea borders) is there some kind of Maginot line which separates the two populations? Does anybody know why there are two forms in such close proximity which are quite different to one another in coloration?padfield wrote:...it would mean that where one form attempted to colonise a previously colonised area it would be outmatched, reproductively, by the established, non-dispersive population. Those same dispersive individuals moving to an unpopulated area would be able to reproduce with less competition, when the genes for non-dispersive individuals would be expressed in a new, sedentary colony.