Post date: Sep 06, 2016 4:12:11 PM
Patrik uncovered an interesting association between temperature and dark morph frequency within many Timema cristinae populations. And the nature (strength) of the association appears to depend on elevation and host. Here are his initial comments on this:
"The Timema data shows effects of both space and time on morph frequencies (I refer to melanics now) - but also potentially an interaction.
In terms of space, in my overall data base, melanics increase in frequency at lower elevations (this probably matches Aaron's finding on more limited data for higher melanic frequencies in dry and hot sites, although I have not used spatial climate data per se).
In terms of time, in my overall data base melanics increase in years with increased temperature, and do so for some individual sites (these are time series plots I sent along earlier).
However, we have enough sites with multi-year data that I was able to estimate the correlation of percent melanic with spring temperature separately for 26 localities (all those with at least 5 years of data). When doing this I noticed some sites actually showed no correlation, or even more striking, a strong reversed correlation (sign negative). I also noticed this flip in the strength or sign of the association was associated with elevation. I could infer this just from knowing the sites well but then proceeded to run a formal analysis. What the plot below shows is the strength (and sign) of the rank correlation (rho values) between melanic frequency and mean spring temperature for each site (so each point is the strength of association between melanism and yearly spring temp for a given locality), as a function of the elevation of the locality (colors are different hosts). You can see the correlation declines to zero and then even becomes negative for some of the highest elevation sites. I won't go into interpreting this now but let's chat. Here is my current thinking.
This 'complicates' things but actually could give us an edge - past temporally replicated studies tended to be of a SINGLE locality (Daphne Major for the Darwin's finches, 1 lake in a few stickleback examples), and of course most spatial studies are of one or a few years. If this trend in our data is real (sure looks real) it argues we would be fundamentally misled about the maintenance of variation if we only had data on time or only on space; in other words, time and space interact. You can't simply say hot years will increase melanics, because this depends on what part of space you are talking about. You can't predict that well the frequency of melanics in space that well without considering the year temperature. I wonder if there is a metaphoric connection to the space-time continuum in physics that would not do more harm than good - but this looks pretty fricking cool.."
My plan it to analyze this more formally.