to be mobile or not to be mobile

First and foremost, a big thanks to you all for reading this blog. I know that it is probably not the most riveting blog to read but I think it is a good experiment in how we go about shaping our ideas and hypotheses. I believe in a community of science. That means all of you. I feel that it is invaluable to reach across labs, across research topics and across universities to overcome a lot of boundaries that previous academic generations have built. I am not sure this will make me a good scientist but this community interaction will make the process more bearable for me. If I get to the end that great PhD tunnel, I hope this leads to future collaborations.

Next I would like to answer some of the questions raised about my last two entries. They are all really good questions because either I was unclear or I just have not been able to get to the next nuance of local adaptation. This is where Julie is particularly helpful because she works on host-race formation, which on my concept of an adaptation continuum, would be the next ‘big step’ (depending on where biotypes fit in).

A population or deme can be adapted but not locally adapted. When I mean adapted to extant conditions, I mean that they have a range of fitness in their present environment that is the same mean fitness as other population in the surrounding environment. For example, in the following schematic of circles, imagine the white circles are adapted populations, the red circle is a maladapted population and the blue circle is a locally adapted population (I know that you are thinking about gene flow, but hold off on that for a moment).



In this schematic, the white populations all have around the same mean fitness. Let us say for these populations we are using the fitness measure of # eggs laid by a female. The white populations all lay an average of 60 eggs. The maladapted red population is not doing very well and only lays an average of 25 eggs. The blue population has ‘reached’ a higher level of fitness due to some phenotypic adaptation that may only be genetic changes at a single locus and difficult to find with genome-wide genetic analysis. This adaptation means that this locally adapted population of females lays 100 eggs on average. This is what I meant by the graph below:



Is the only difference between locally adaptation and host-race formation gene flow. First let me quickly explain host-race formation in brief so we are all on the same page. Julie, feel free to jump in on this because you are much more an expert than I am.
Julie and I both agree that the best paper to explain host-race formation is one by Dres & Mallet 2002. If you think that local adaptation is controversal than host-race formation is an Africa-hot topic. This is a simplified list of the criteria for host races according to (Dres & Mallet 2002):

1. The species uses different hosts in the wild and some individuals seem to “prefer” one host.
ex. the moth caterpillar feeds on two trees, box elder and willow. Some caterpillars prefer to feed on willow, some prefer to feed on box elder OR some female adult moths prefer to lay their eggs on box elder, while others choose willow.

2. The populations are found where the 2 host trees exist together.
ex. the moth caterpillar is found in ranges where both box elder and willow are found side-by-side or near to each other.

3. The individuals on one host have significant genetic differentiation from the individuals on the other host.
ex. the caterpillars on box elder are significantly (defined as more than one locus) genetically different from the caterpillars on willow.

4A. There is a connection between the preference for the host and the preference for a mate who likes the same host.
ex. moths that like to lay their eggs on box elder also like to mate with males who grew up eating box elder.

4B. There is some gene flow between the individuals groups on the different hosts. Because there are still the same species, there needs to be some mating between moths on box elder with moths on willow but not enough so that the genetic differentiation is lost.
ex. box elder moths occasionally mate with willow moths.

5. This is the same criteria as local adaptation. Individuals of the different host races have higher fitness on their own (natal) plant. ex. box elder caterpillars have higher fitness on box elder relative to their fitness on willow.

Okay, so now that I have gotten host-races out of the way, let’s get back to whether the only difference between host-races and local adaptation is the amount of gene flow.

Certainly in the Dres & Mallet paper they say the only difference between host-race and host-associated groups is gene flow (are host-associated and locally adapted the same thing? Initially I would say yes…). I would argue that it is not just gene flow. It really depends on what and how strong the selection pressure is. If there is a very strong parasitism pressure on the caterpillars on box elder, e.g. if you are willow caterpillar and you end up on box elder you are going to get parasitized, than the system can tolerate relatively high gene flow. So, yes, gene flow is important. In order for a group or population on a host to become a host-race, they must be genetically differentiated and so gene flow intuitively would be low. But then this connection between gene flow and host race formation (or local adaptation) would beg the question, are insects with low mobility more likely to be locally adapted (or host races)?

Well, Astrid was right on top of that question! It is true that the classic case of local adaptation was with scales which are pretty non-mobile (Edmunds & Alstad 1978). In my last post I referred to a meta-analysis examining 17 independent studies on local adaptation (Van Zandt and Mopper 1998). In this study they were interested in several trends in who was and who wasn’t locally adapted. One question they did look into was whether insects that were more sedentary were more likely to be locally adapted (Van Zandt and Mopper 1998). Well, the results were surprising… of the 12 insects used in the meta-analysis (4 mobile, 8 sessile), they did not find that to be the case. An insect was equally likely to be locally adapted if it was sessile, as if it was mobile.
Now I did a little eyebrow raising here because I have read enough papers on gene flow (a favorite topic of mine) to know it is very difficult to measure gene flow. In addition, quantifying gene flow can get particularly murky when you are dealing with an insect with a sessile caterpillar and a mobile adult or a sessile female and a mobile male, e.g your favorite and mine – Orgyia spp.(see poor wingless female below).


photo by Jeremy B. Tatum

Van Zandt and Mopper’s study was done over 8 years ago now. It would be interesting to look at where we are now with evidence of locally adapted species. Maybe we would get a different picture with respect to mobility. But with the information up to this point, I cannot say that there is evidence that sessile species are more likely to be locally adapted, although intuitively we would think this would be the case.

On a side tangent – If local adaptation is a necessary step before host race formation, then maybe it would be worthwhile to see if the same life history characteristics that are found to be correlated with local adaptation would also be correlated to host-race formation. It would be interesting to discover that only a subset of characteristics in locally adapted species are found to be present in host races. Then maybe one could predict what subset of locally adapted species would have a higher probability of becoming host-races. This is just an idea that I have been toying around with and will discuss with my host-race pal, Julie.

Now I have only begun to scratch the surface of some of your questions. I still need to address questions about spatial scale. But I imagine that you are as tired as I am. So I will leave that for another day.