The Sorby Natural History Society, Sheffield

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North/South Hybrids

The following article was written by Sorby member Bill Smyllie. It is presented here in unedited form. The illustrations were provided by Bill Smyllie.

The Brown Argus butterfly is a very variable species. For well over 100 years there have been disagreements as to the cause or causes. In 2009, Butterfly Conservation took the unusual step of declaring a consensus for the species: in Britain it consisted of a southern race, a northern race, and hybrids. To enable this to happen there were three separate aspects which had come together, In datal order, firstly a paper by me which was published in the Entomologist in 1992 and concluded that the butterfly consisted of a range of hybrids. Upper forewing lunules from specimens in 23 museums had been recorded to provide a large data bank covering the whole of Britain. This was followed in 2002 by a large project via nuclear genetic analysis which again covered the whole of Britain and crucially, agreed hybrids with compositions ranging from 50/50 to 90S/10N. Although the work was known about, it was not published until 9 years later (Mallet et al, 2011) and this held up the consensus. Lastly, during the present global warming the butterfly migrated north through Yorkshire between 1997 and 2006. Its passage was well charted by records from many Butterfly Conservation volunteers and as the species recorder for Yorkshire I completed the annual reports over a period which included these years. This type of information had never been previously available and produced some startling information – the northward migration route went roughly south to north and stuck to low ground: the migrants did not impinge on either the Peak district or the Yorkshire Dales, both on high ground further west. Also, southern race colonies in the Peak district and further north to the north Yorkshire Moors could be identified by their ability to have second broods in better summers. However, the BC records showed that none of these southern race colonies provided any individuals which took part in the present northward migration.


The map published in Nature in 2007 (left above) shows wildlife following high ground south in an ice-age and low ground north during global warming.  Brown Argus races have been superimposed.

Another part of the complex situation is explained by the right-hand chart taken from Lamb and Sington, Earth Story, BBC books 1998.  This shows that up to around 1 million years ago the earth’s mean temperature had been consistently warmer than at present for some 290 million years.  Subsequently there have been 4 mini ice-ages in Britain, the last one being the most severe and having its glacial maximum at around 17,000 years ago.  Since then temperatures have reverted to reasonable, and have included a global warming around 10,500 years ago.

It is important to understand the conclusions from the above information because the butterfly is one species in an eco-system, and all species in the same area have experienced the same climate.  Since 2010 I have been looking at other species, including plants, and consider that information showing parallels to the Brown Argus is available.  The situation is as follows:

Around one million years ago there was one Brown Argus ancestor with mid-brown upper wings and 6 fore-wing and hind-wing lunules (orange marginal spots).  In the course of the 4 ice-ages temperatures have sunk and for considerable periods of time the whole eco-system has not been warm enough in Scotland to maintain two broods each year.  Warmer temperatures in south and central England have allowed bivoltine emergence to continue.  In order to assist in warming up in the scarcer warm periods in Scotland, the butterflies darkened, lost their lunules, and became obligate univoltine.  Undersides, which have a different camouflage function, are largely similar in the different races. The photos show a southern race male with a full complement of 6 upper fore-wing lunules from Coombs Dale, Peak district on the right, whereas the male on the left has lost its lunules and lighter colour.  This specimen was taken in the Yorkshire Dales, the furthest south area to have the northern race and is a rare example.  Hybrids contain many variants including the occasional ones similar to their ancestors.

At the glacial maximum the northern race would have been 100% as in the left-hand photo.  However, since then in the more normal temperatures there has been very slow diffusion of the southern race into the northern and vice versa, something under 10% in the 17,000 years involved.

The Peak district is agreed via nuclear genetic analysis and lunulation data to be the furthest south area for 50/50 hybrids.  However, there is a mixture of hybrids and southern race present.  While hybrids are at Longstone Edge, southern race is at Coombs Dale and at points they can be within 1km of one another, a most important point because it indicates how sedentary the butterfly must be.  The two colonies have the low-order diffusion like everywhere else, and this lack of extra mixing has a similarity much further north at sea level where at Burnmouth (a little north of Berwick-on Tweed) there is a hybrid colony, the furthest north reached in the previous global warming.  Less than 2km further north at Eyemouth and thereafter, it is all northern race.  This information means that during an ice-age this species (and no doubt several others) must have survived for long periods of time frozen in situ.  If the above short distances cannot be bridged to equalise the different races in c17,000 years, there is no way the species can at the same time be forced from many other extant colonies in an ice-age, and then miraculously re-appear from somewhere else.


Lathkil Dale and Long Dale have southern race, like Coombs Dale, while Cressbrook Dale has hybrids.  It appears that at the last glacial maximum, the northern race kept to high ground and hybridised with the resident southern, while the same southern race on lower ground remained intact.

The Brown Argus has a life-span of 3 weeks, but its flight period is over 8 weeks because the northern hybrid portion has a slower development rate.  In the Sorby Record No 29 there is an article with a Table which shows this important point for the southern race at Coombs Dale.  There are other aspects, but suffice it to say that the various aspects of the butterfly are complex and specific.  Since 2010 I have been looking into parallel situations in other species which have distributions up to the north of Scotland and have found several, including plants.

The approach is to look for some type of visual variation.  If one occurs, see if any of the variations can be easily quantified.  For example, 4 orchid species all have spikes which vary in colour and it is possible to classify into approximately white and darker.  The great advantage of these spikes is that some, not all, will show a darkening of colour going up the spike, demonstrating an increase in the northern portion of the hybrid through the flowering period in one photo.  The mechanisms in hybrids are extremely complex and a good camera will prove to be the most satisfactory approach.  In the butterfly I have found that it is necessary to have photos from 20 or more specimens to provide adequate information which will allow classification into south, north or c50/50 hybrids.  Information on Common Rock-rose and Orchids is available via the blog Coherent Evolution.

The approach of checks via multiple specimens is necessary for hybrids, and this type of different approach should not put people off.  It is not necessary to deviate from normal field visits, merely to be aware that hybrid signs may pop up anywhere.  For example both Cowslip and Lesser Celandine are possibles which show variations and are being looked at.  Comments from the field would be appreciated and will expand our knowledge, among other things, of one type of evolution if this is described as creating visual differences, which (from the point of view of south and north races) have developed over a period of time and continue today.

Please bear in mind that records throughout any flowering or flight period will play their vital role in checking whether characteristics change.  Also, since what is around today has not necessarily been dominated only by the last 1 million years, other aspects may turn up.  These would expand our overall knowledge, rather than being considered a disaster.  For north/south hybrid checks, the present combination of sequential change plus differences between low-lying and higher sites provide jointly an adequately complex identity proof.

Bill Smyllie  30.4.16