Peer review of: The coincidence of ecological opportunity with hybridization explains rapid adaptive (2019)



Reviewers' Comments:  
 
Reviewer #1:  
Remarks to the Author:  
I have very much enjoyed reading this paper and think that it will make an excellent and well cited 
contribution. The authors have done a great job at addressing the comments from the previous 
version. There are still some areas where I think that clarification could strengthen the arguments. 
The main issue is still just to address the confounding explanatory factors and in particular to clarify 
the role of ecological opportunity in facilitating adaptive radiation. Opportunity includes both size 
(extent or magnitude) of the lake that is being exploited (i.e., depth, area, or other factors relevant to 
the taxon) and the isolation. The authors focus on isolation, and the major environmental differences 
between lakes are mentioned, yet still not entirely integrated into the arguments. I think it would help 
clarify (and certainly not detract from) the arguments if the authors could clearly state that yes, the 
lake where they radiated (Mweru) is larger by all measurements and the larger size (depth) may (from 
what I understand of Wagner et al) be a key explanatory variable for radiation in Mweru and not 
Bangweulu.  
 
Abstract  
Depth is a critical explanatory factor that will be tested but is not mentioned. Maybe emphasize both 
depth (serving in the same way as area, presumably) and isolation. Otherwise it sounds like you have 
already made up your mind that hybridization is the answer.  
 
Line 53 Maybe “ Here, we use haplochromine cichlid assemblages in two previously unstudied African 
Great Lakes”  
 
Line 57 So the greater biotic isolation might predict more ecological opportunity in Bangweulu, but the 
reduced depth would predict less, right?  
 
Line 91 Co-occurrence of closely related lineages may also facilitate adaptive radiation by excluding 
secondary colonizers from their ancestral niche.  
 
Line 97 It would help to clarify what this paragraph is setting up. So there are two largely unknown 
lakes, Mweru and Bangweulu. I would then point out how they differ in terms of the factors that 
provide ecological opportunity – depth and isolation. Then you can first talk about depth. Then about 
isolation.  
 
Line 101 Lake Mweru is larger than Lake Bangweulu. Maybe add “in both area and depth”.  
 
Line 108 So depth is a better predictor of radiation than surface area. But then “radiations are known 
from lakes much smaller than Lake Bangweulu”. Presumably smaller in surface area rather than depth 
(otherwise the phrasing doesn’t follow). Just needs clarification  
 
Line 110 I was then a bit lost as to the parameters of Lake Bangweulu that permitted in situ adaptive 
radiation in lakes elsewhere. Presumably this means that adaptive radiation occurs in other lakes of 
the same depth, given that area is not as good a predictor of whether a radiation occurs?  
 
Line 112. You are now talking about isolation. It might help to make this a separate paragraph.  
 
Line 116. I would put the aspects of size together, and keep the aspects of isolation and connection 
together.  
 

Line 116 It would be great to know a bit more about the paleo history of the lakes at the time when 
the fish would have been diversifying.  
 
Line 122 The fish fauna of Lake Mweru is composed of Congolese and Zambezian lineages … Does this 
refer to haplochromines only? Just need to clarify relative to the next sentence where it says that Lake 
Bangweulu hosts one Congolese non-haplochromine cichlid  
 
Line 124. Take out one “large”.  
 
Lines 126-128 Maybe clarify the caveats, otherwise it is somewhat confusing as to what is being 
tested, especially as we are told earlier that ecological opportunity requires both isolation and depth. 
So when you say that the study tests predictions of the isolation and the hybridization hypotheses, it 
just seems that the depth issue has been swept under the rug.  
 
Line 440. Again, keep in mind that ecological opportunity includes space as well (you don’t find 
adaptive radiations on remote atolls).  
 
Line 483 I’m a bit lost by the statement that the smaller size of Lake Bangweulu is expected to result 
in lower species richness but cannot explain the absence of radiations. Remember that size is “the 
relative extent of something; a thing's overall dimensions or magnitude”. And we know (from Wagner 
et al) that one aspect of size (depth) is most consistently associated with radiation, while another 
aspect of size (surface area) is not. However, Mweru is larger in both surface area and (in particular) 
depth, right? (At least from what is stated on L 101).  
 
Line 486. Again, would be good to just make this really clear. Given that depth is the major predictor, 
does this mean that many lakes that are shallower than Lake Bangweulu host adaptive radiations?  
 
Line 521 “respectively” implies that lack of hybridization is more important than lack of ecological 
opportunity. Not clear what this is based on?  
 
 
 
Reviewer #2:  
Remarks to the Author:  
The authors have addressed my previous concerns. I thus think that the paper should be accepted for 
publication, and I would like to congratulate the authors to their work.