A Cryptic Northern Comment...

After publishing my past post I came across a comment on Tzedakis et al (2002)’s paper. Stewart (2003) first draws attention to the studies that have presented evidence for cryptic Northern refugia in Europe and points out that the species response model that Tzedakis et al (2002) base their study on ‘does not provide the only explanation of the empirical data relevant to the location of temperate refugia during the ice ages—nor does it represent the most likely areas from which postglacial recolonisation took place’.

Stewart (2003) then notes, in contrast to Tzedakis et al (2002), that the area in which they conducted their study is more generally considered to be a refugium for the ‘pan-European temperate taxa’. He then goes on to say that it is difficult to see how the area could act as a refugium and an ‘isolated area that produced allopatric endemics’. He suggests, instead, that, seeing as though there has been so much evidence of temperate species responding to climatic change in the way that the Northern refugial hypothesis lays out, that the genetic diversity Tzedakis et al (2002) discussed could have originated in these Northern refugia and then just moved down into more Southern areas (rather than having originated here as Tzedakis et al (2002) suggest).

Finally, Stewart highlights the possible importance of parapatric speciation and urges scientists to consider both allopatric and parapatric evolution as well as the role of both Northern and Southern refugia in order to gain a more informed understanding of how genetic diversity could have been created in relation to refugial areas.

Tzedakiset al (2003) were quick to respond…

They acknowledged the growing literature on these new ‘cryptic’ Northern refugia and noted that it the species ranges are likely to have been made up of larger Southern populations as well as ‘smaller, Northern peripheral ones’. They suggest that the existence of these Northern refugia can be evaluated using ‘continuous pollen records spanning the last glacial North of the Alps’. If small populations of temperate tree species did in fact survive in Northern glacial refugia then they would have been able to expand their ranges during the ‘2000-3000 year’ interstadial phases when the climatic conditions became favourable again – no such expansion is evident in the French pollen records.

Tzedakis et al (2003) are only referring to pollen records here and we know that there has been mammalian fossil evidence showing records of Northern glacial responses and animal species couldn’t have survived alone without plants. It seems that there is a mismatch between the different types of data, leading to different theories as to how species’ responded to climatic change. This doesn’t take away from the fact that refugial areas play a role in the creation of genetic diversity, but it does question the role of Northern refugia in this process. I am inclined to agree with Stewart (2003) and say that we have to look at the role of both Northern and Southern refugia in the creation of modern European biota, and not discount the importance of either.