Dahirel and colleagues present the results of an experimental study to examine the effects of connectivity and density dependence on trait selection during range expansion. The purpose of the study is well motivated and clear. Based on a previous experiment and theory for pushed vs pulled expansions, we would expect that the high connectivity (reference) landscapes would have higher fitness at the leading edge than the core, and/or negative or non-density dependent dispersal. The reduced connectivity landscapes should have lower fitness at the leading edge and/or positive density dependent dispersal. Although the results were somewhat complicated, the figures were clear. The authors did find that the density dependence of dispersal varied between treatments in the manner we would expect given high connectivity landscapes induced pulled waves, and reduced connectivity resulted in pushed waves. The manuscript would benefit from being more strongly grounded with theory – both clarifying in the introduction and stronger links in the discussion. Below I provide more specific feedback to improve the manuscript.

1. L67-71 I think this is a bit misleading, “spatial sorting” is typically used to refer specifically to traits that enhance dispersal, and there are a wider range of traits potentially under selection during spread that are not described by this term (the authors describe some of these in lines 74-75). Related, I disagree with the use of ‘spatial selection’ at L83 to refer to all of these processes – if the authors wish to equate spatial and natural selection (as it’s arguably still debated in the literature), they will need to explicitly define this. Otherwise, I think it would be more suitable to use natural selection for traits like fecundity.

2. I was surprised the authors did not incorporate any of the literature that explicitly considers landscape connectivity in shaping range expansions. There are a number of previous studies that look at both density dependence during spread in fragmented landscapes, as well as how landscape connectivity alters evolutionary trajectories of leading edge populations. I’ve provided some suggestions below for papers that should be highly relevant, but I would suggest the authors incorporate more of the previous literature on this topic to better motivate their work. (e.g. Dowdall et al. 2018, J. of Mathematical Biology; Lutscher & Musgrave 2017, Ecology; Gralka & Hallatschek 2019, eLife; Williams et al. 2016, American Naturalist & Science)

3. In some places, the methods need clarifying to help the reader follow the complex story. Adding a summary of the traits studied and from which generation would help – this could be in the text (~L 155 – 168) and/or in a table. It wasn’t clear why traits were tested at different generations, and not the latest one (12th generation, I believe?). I’m not sure it really matters for the results but I found it confusing. Please also add an explanation of why two different measures of dispersal were used, how are they related to each other, and what the expectations were for each treatment. (I’d also recommend putting these two sections next to each other in the methods). 

4. For the measurements of wasp size, where 8 core-edge populations were selected at random, how did you choose from across the isolines? Maybe I missed something, but I thought the original design had 8 replicates of each of the three types.

5. Throughout the methods and results, I found the use of the word ‘context’ hard to keep track of (in particular the contrasts between core and edge and the two connectivity treatments). Upon revisiting this section of the methods, I see now that context is used more generically to mean both core vs. edge and reference vs. reduced connectivity. Since specific hypotheses are laid out for each, to me it does not make sense to combine them into one word. To me it would be clearer to give each more precise names, or at least occasionally redefining context.

6. I struggled with some of the language in the results section around whether or not two groups were different from each other. What does it mean by ‘some indication’ or the ‘only potential effect’? Is there an alternative way to make it clearer that the groups are not significantly different (using the appropriate Bayesian language here, of which I am not an expert), but the mean is higher in one treatment than another?

7. L 381-384 This was interesting to me and I wish the authors gave more information in the main text. From Figure S.2.1: wasps were smaller after the experiments than stock populations, and edge populations in reference landscapes were smaller than core populations as well as those from the reduced connectivity landscape. But there were no differences between edge and core in the reduced connectivity landscapes? Earlier, the authors state that body size is linked to fitness, which I assumed to mean that larger individuals reproduced more. It wasn’t immediately clear why they expected the differences in body size they reported to be most important in the context of dispersal-competition trade-offs. I wondered if the authors could provide some further insight on how body size relates to competition (if at all) and perhaps whether this could be indicative of selection in the reference landscapes? Further in L385-400, can you relate any of these trait changes to the changes in speed from the first experiment?

8. In a few places in the discussion, the complexity of the results gets washed over (e.g. L377-379, 404-406) – while I appreciate trying to simplify, the arguments would be more powerful if the nuance were included.

9. L410-424 – can you provide an alternative hypothesis? What if the egg number-egg size tradeoff was not supported with further work?

10. L445 – In this paragraph, I think more can be said from theory in the metapopulation context that makes predictions about why dispersal might be lower when patches are more dispersed.

11. Overall, the discussion would benefit from a paragraph describing the limitations of this experiment to help put it into a larger context. There is just a little on L471-473.

12. The supplementary material sections include several sections – it would be clearer in the main text if the authors referred to specific figures rather than referring the reader to the entire section.

A few minor comments:

L24. The reader does not yet know what ‘treatments’ refers to – please be more precise here.

L92. The use of the word surprising in this case seemed a bit strong. To me, the lack of studies is not surprising – this is a new field and these kinds of experiments are very difficult to do. Instead, what about something like, “the next step’ or ‘the way forward’ is to…

L 98-88. What does ‘appears to move edge populations away from’ mean?

L203. What is a tracklet?

L391. What does “but in the other in the second” mean?

L410. Appearance would be more appropriate than apparition here.

I just completed the review of the manuscript with the title “Landscape connectivity alters the evolution of density-dependent dispersal during pushed range expansions”. It is well-written and describes adequate and well though off experimental procedures to answer an interesting research question about species range expansion in the context of environmental change.

I have an overall question concerning the main conclusions of the preprint: could it be that, as a species expands its range, the lower densities at the expanding range edge cause an initially pushed expansion to move towards a pulled one?

Introduction

The introduction provides an adequate framework for the study. It is well structured, explaining the research context and knowledge gaps and it is based on relevant research. The motivation of the study, as well as the objectives, are clear and relevant in the current context of environmental change.

Materials and methods

Data and code were made available to the reader. The description of the methods is detailed enough to allow replication and the statistical analysis are adequate. I think that the methods would benefit from a new figure, describing schematically the experimental design. As such, it is my recommendation that the authors consider creating a figure were the experimental steps are schematically represented.  

Results

Paragraph around line 355 – I understand what the authors meant. However, when they write “…leading to negative density-dependent dispersal…” that is not always absolutely right, from what I can understand. In the wasps from the “core” in the “reference” landscape and the wasps from the “edge” in the “reduced connectivity” landscape, there is a positive density-dependence. The authors should clarify this paragraph.

References

References are adequate.

Discussion

Lines 390-391 – Please clarify this sentence.