Talk:WikiJournal Preprints/Speciation by reinforcement

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Editors:

Jong Bhak (handling editor) contact
Emanuele Natale (handling editor) contact

Reviewers: (comments)

Maria R. Servedio
Roger Butlin

Article information

Author: Andrew Z. Colvin^[i]

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↑ azcolvin429 gmail.com

This article has been declined for publication by the WikiJournal of Science.

It is archived here as a record. Discussion can be viewed below.

Update and a host of questions

Latest comment: 3 years ago3 comments2 people in discussion

Okay, so I mostly finalized the pre-print. I combined two articles from Wikipedia: w:Reinforcement (speciation) and w:Evidence for speciation by reinforcement. I removed all the links because I think it looks better without them (plus it's a pain to convert them all to wikilinks).

I am curious about how things will follow once peer reviewed. I see that normally a peer reviewed article gets essentially copied to Wikipedia; however, in this case, I would prefer it not. On Wikipedia, the article is split into two, but it wouldn't make sense to do that here, so I have combined them.

Further, I am unsure about the image gallery. It's nice to have some visuals for a lay reader; however, I am unsure of the best way to achieve this without cluttering up the document. I also would like the gallery (if included) to be symmetrical in the final publication (that is, an even distribution of images [e.g. 3x3 or 4x4]). Thoughts?

I also am not entirely sure if I have removed all the duplicate references. I see quite a few ref errors that I am unsure about how to fix. I also see a lot of red links that need removing in the refs (bibcodes, s2CID, etc).

Hopefully the tables can be formatted well to fit into the PDF after peer review. I have no idea how to do that either. Preventing inappropriate breaks is vital (as it will likely flow across multiple pages).

I still need to triple-check for grammar, typos, etc. as well. Andrew Z. Colvin • Talk 04:38, 22 December 2020 (UTC)Reply

No problem. Point-by-point info below:

You're welcome to include, or not, the wikilinks (we've example articles of either sort). We actually do have a method to convert all wikilinks to point to wikipedia (placing {{subst:convert_links| at the top of the page and }} at the bottom) in case that's useful.
We've not had a submission based on combined pages before, so once the process is finished it'll be up to your preferences what (if anything) to export and to where. We're happy to advise at the time if useful.
I think inclusion of the gallery is fine. In the final PDF it'll be trivial to format symmetrically. In the web version, Template:Parea is probably the most likely to give that effect.
I fixed the 4 with error messages. I understand that Wikipedia:AutoWikiBrowser can detect duplicate references. I've not used it before, but it may be worth downloading if you want to double-check.
The table would probably be best formatted in landscape in the final PDF (similar to this example).

Hope that helps. T.Shafee(Evo﹠Evo)^talk 10:16, 22 December 2020 (UTC)Reply

Thank you for your clarification and help! I wish I knew about the link convert trick, but honestly, I like it without. Links make reading difficult and I think Wikipedia allows too many. I should’ve removed links from WikiJournal of Science/Peripatric speciation. And yes, editors at Wiki agreed that the original article I wrote should be split into two pages, but I figured here, they are better as one. Plus I used a table instead. As for the gallery, I suppose if it looks bad in the draft, we can remove them. It’s not a huge deal.

One last thing: is it possible to change the article title to “Speciation by reinforcement”? I don’t like the parenthetical title as it exists because of the conflicting name of the psychology article on Wikipedia.

Other than that, I think I’m ready for submission for peer review. Andrew Z. Colvin • Talk 15:47, 22 December 2020 (UTC)Reply

Review 1

Review by Maria Servedio , University of North Carolina at Chapel Hill
These assessment comments were submitted on 21 Feb 2021, and refer to this previous version of the article

On the whole this is a thorough, well-researched and well-referenced article, that is pitched at an appropriate level for an audience with a relatively general background in Evolutionary Biology. The descriptions are a good mix of historical and current thinking (but see the exception about the second table, below). The article is generally clear and well-written.

I agree with the author’s decision to combine the two Wikipedia articles on “Reinforcement (speciation)” and “Evidence for speciation by reinforcement” I had noticed this separation on Wikipedia before and it has never made sense to me.

The only major conceptual addition that it would be good to include is that reinforcement-like processes can occur that are not identical to reinforcement in that they are do not specifically involve the evolution of premating isolation, but otherwise operate by almost the same principles (they are driven by selection against hybrids or hybridization and they prevent gene flow between incipient species). These include the evolution of reduced migration rates or dispersal, and the evolution of postmating-prezygotic isolating barriers such as conspecific sperm precedence.

I do think that the header “Table of examples of reinforcement occurring in nature” is too definitive. Reinforcement is notoriously difficult to prove because there are a number of other evolutionary processes that can cause the pattern of reproductive character displacement (see Howard 1992). At least several of the studies do not claim to have ruled out all other alternative explanations. In Höbel and Gerhardt’s study on Hyla, to just take one, they claim to have evidence of reproductive character displacement but fall short of claiming reinforcement per se. Many other studies in the list also just claim to support reproductive character displacement. I think a more accurate title would be “Suspected examples of reinforcement occurring in nature”.

The “Table of Laboratory Studies of Reinforcement” is quite out of date and could use an update. (one newer example to include is Matute 2010).

Additional Specific comments

Under “History and Terminology”

4th para – “several studies using highly complex computer simulations” – for most of these models it is an exaggeration to call them “highly complex”. They often assumed a small number of loci underlying the problem and as such were relatively simple.

5th (last) paragraph – The distinction between the way that Butlin vs. Howard demined reinforcement is could be made mode clearly. As stated, Butlin’s distinction was made on whether there was already complete reproductive isolation between the species that came into secondary contact (e.g., whether speciation was already complete). “Reinforcement” was used when there was initially only partial reproductive isolation whereas “reproductive character displacement” was used when isolation was already complete. In contrast, Howard drew a disctinction between reinforcement as the process of the evolution of premating isolation due to the existence of postzygotic isolation, whereas reproductive character displacement was the resulting pattern of more divergence of characters involved in reproduction between species in sympatry rather than in allopatry.

Under “Models” The sentence “Butlin laid out four primary criteria for reinforcement to be detected in natural or laboratory populations” (and the subsequent list of his suggestions”) does not really seem to fit in this section. Actually the first two or three sentences of this paragraph are also of questionable fit. The sentences in the middle about the forces acting on reproductive isolation are thoroughly appropriate though.

I would actually recommend having a brief new initial paragraph where you briefly introduce the purpose and concept of addressing this question with models. This could include the sentence “Genetic models often differ in terms of the number of traits associated with loci;[27] with some relying on one locus per trait[24][28][29] and others on polygenic traits.[21][20][30]” from the “Genetics” subsection, and as well as a brief discussion of the forces acting or reproductive isolation, since these are what is considered in models of reinforcement.

Under “Models: Genetics”

While I’m very familiar with the Felsenstein 1981 model on which this paragraph is based, I’m not clear on what is meant by the sentence “An alternative model exists to address the antagonism of recombination, as it can reduce the association between the alleles that involve fitness and the assortive mating alleles that do not”. I think it is referring to Felsenstein’s one-allele model, but the distinction between these types of models is subtle and I don’t think that it would be worth going into it here. I would recommend cutting this sentence. Instead I would mention that most models of reinforcement consider allelic combinations that lead to low fitness, as in this Felsenstein model, but these are often in the form of Dobzhansky-Muller incompatibilities rather than locally adapted ecological combinations.

Under “Alternative Hypotheses”

1st paragraph – “There is no single, overarching signature of reinforcement; however, there are two proposed possibilities”. This is true, but reproductive character displacement is pretty universally acknowledged as the most telling signature of reinforcement. This is of course mentioned up above, but I’m not sure why it’s not mentioned here. Also to start this paragraph, and in a few places below, you mention “observed patterns”. Do you mean reproductive character displacement when you say this? This could be clearer.

Under “Alternative hypotheses: Fusion”.

Something about the sentence “This hypothesis does not explain the fact that individual species in allopatry, experiencing consistent gene flow, would not differ in levels of gene flow upon secondary contact” is throwing me off. I am not following it – please rephrase.

Under “Arguments against reinforcement”

The argument you present in the second paragraph is an argument for why reinforcement should be difficult, but it is not accurate to call it a concern “about hybrid fitness playing a role in reinforcement” (having selection against hybridization is part of the definition of reinforcement, so the current opening sentence of that paragraph does not make sense). Additionally, all of the models about reinforcement already take the effect of recombination into account, so they are not prohibitive.

Changes that I have already made on the wiki:

In the first paragraph of the section “Reinforcement”

1. “…where during secondary contact, the two populations mate” to “…where during secondary contact, individuals from the two populations mate”. This is of course more accurate.

2. “Natural selection results from the hybrid's inability to produce viable offspring; thus members of one species who do not mate with members of the other have greater reproductive success.” To “Natural selection results from the hybrid's inability to produce viable or fertile offspring; thus members of one species who do not mate with members of the other have greater reproductive success.” Reinforcement has traditionally also been considered to be driven by hybrid sterility.

Under “History and Terminology”

Changed “The Wallace effect” to “The term "the Wallace effect"”

Changed “referring to incomplete isolation as reinforcement and completely isolated populations as experiencing reproductive character displacement.“ to “referring to the evolution of premating reproductive from incomplete isolation as reinforcement and from completely isolation as reproductive character displacement.“

Under “Models”

1st paragraph - 2nd sentence. Changed “This pattern” to “This fact”

Changed “This process of speciation influenced by natural selection is reinforcement, and can happen under any mode of speciation ” to “The influence of natural selection against hybrids on the evolution of premating isolation is reinforcement, and it can happen under any mode of speciation”

Changed “as the exchange of genes between individuals leading to hybrids cause the genotypes to homogenize” to “as the exchange of genes between individuals leading to hybrids cause the genotypes to homogenize across the incipient species”

Under “Models: selection”:

Changed “ In direct selection, the frequency of the selected allele is favored to the extreme” to “Under direct selection, the alleles causing increased premating isolation diverge because they directly affect viability; the evolution of premating isolation in this case can mimic reinforcement but is arguably a different process.”

Changed “In cases where an allele is indirectly selected, its frequency increases due to linkage disequilibrium; this is the case under more standard concepts of reinforcement”

Under “Alternative hypotheses: Fusion”:

Changed “Those” to “Those species pairs”

Under “Alternative hypotheses: Sexual selection”:

Changed “In a Fisherian runaway process, selection against the low-fitness hybrids favors assortive mating, increasing mate discrimination rapidly” to “A Fisherian runaway process can occur during the process of reinforcement, increasing mate discrimination rapidly”

Under “Arguments against reinforcement”

Changed “These objections have since been largely contradicted by evidence from nature.” To “These objections have since been largely contradicted by evidence from nature and additional models”

Changed “In addition, specific alleles that have a selective advantage within the overlapped sympatric populations are only useful within that population.” To “In addition, specific alleles that have a selective advantage within the overlapped populations, such as alleles causing prezygotic isolation, are only useful within that population.”

Changed “However, if they are selectively advantageous, gene flow should allow the alleles to spread throughout both populations.” To “However, if they are selectively advantageous, gene flow should allow the alleles to spread throughout both the sympatric and allopatric populations.”

Changed “To prevent this, the alleles would have to be deleterious or neutral” To “To prevent this, the alleles would have to be deleterious in the other population.” (if they were neutral they will still spread).

Review 2

Review by Roger Butlin , University of Sheffield
add this statement to wikidata
These assessment comments were submitted on 23 Feb 2021, and refer to this previous version of the article

Reinforcement is a very important topic in speciation research. It has also been controversial historically, although that controversy is less evident currently. Some aspects of reinforcement are quite difficult, both conceptually and in terms of the data needed to make a strong case for the process. For these reasons, I think a well-founded and up-to-date review could contribute a lot to the field. There has not been such a review for some time – perhaps Servedio and Noor (2003) and the review within Coyne and Orr’s (2004) book come closest to comprehensive treatments but quite a lot has happened since. While a good review would be valuable, a poorly conceived review could be damaging, perpetuating existing miss-understandings or introducing new confusion.

This review is based quite closely on the Wikipedia entry for Reinforcement. I had not read this entry previous. I have to say that I hope it is not used by my students! I regret to say that the entry and this review often provide weak and incomplete explanations of key concepts. They are sometimes positively misleading. I provide detailed reactions below. Part of the problem lies in the structure, which separates ‘models’ from ‘arguments against reinforcement’ and ‘alternative hypotheses’ from ‘evidence’. Models are intended to investigate the circumstances under which reinforcement might enhance reproductive isolation (and how far this enhancement might go – a topic barely acknowledged in this review). Therefore, they necessarily treat ‘arguments against reinforcement’. Similarly, ‘evidence’ is hard to assess unless one considers ‘alternative hypotheses’.

There are then some missing topics. The magic/multiple-effect trait idea, the one–allele mechanism, the issue of matching for signal-response traits and the issue of reinforcement of traits other than mating behaviour are inter-related topics that are absolutely crucial to understanding reinforcement but receive little or no attention and certainly not clear exposition. They all relate to the key problem of recombination and the weak effect of indirect selection, which it, itself, poorly treated in this review.

A recent topic that is not even mentioned is the possibility of “cascade reinforcement”.

In conclusion, my view is that this article needs a root-and-branch rethink of its structure and then a much more careful explanation of key concepts. Evidence sections need to be updated, with a focus on the most informative studies, rather than lists of old studies repeated from previous surveys. In the absence of line numbers, I use quotations to locate my specific comments below.

“The secondary contact of originally separated incipient species (the initial stage of speciation)” – this implies that speciation always follows a route that begins with spatial isolation which is not the current view.

“for example after an allopatric population comes into secondary contact” – a single population cannot come into secondary contact, it has to be at least two populations.

“In the 1980s, many evolutionary biologists began to doubt the plausibility of the idea” – my 1987 paper in Trends Ecol. Evol. Would be considered by many to be a key reference at this point.

“The most recent theoretical work on speciation has come from several studies using highly complex computer simulations” – there is rather heavy reliance on Coyne and Orr’s book in this paragraph. It is also important that analytical theory advanced, not just simulation, particularly through the work of Kirkpatrick and Servedio (e.g. Genetics 151:865–84).

“The term secondary contact has also been used” – this is not a distinct use, but the same as for Gastrophryne.

“Reinforcement, under his definition, included pre-zygotic divergence and complete post-zygotic isolation” – needs to be reworded: Howard included pre-zygotic divergence that evolved in response to complete post-zygotic isolation.

“Maria R. Servedio and Mohamed A.F. Noor include any detected increase in pre-zygotic isolation as reinforcement, as long as it is a response to selection against mating between two different species” – it is important to clarify that this formulation includes direct costs of mating, not just indirect costs via reduced hybrid fitness.

“true reinforcement is restricted to cases in which isolation is enhanced between taxa that can still exchange genes” – note that this corresponds to my distinction, mentioned previously (but first made in the 1987 TREE article. This paragraph should also note that the term ‘reproductive interference’ is now commonly used for what was called ‘reproductive character displacement’ (RCD) in my 1987 review, with RCD retained to describe the pattern that might result from either reinforcement or reproductive interference. To complete the history, the paragraph should also mention Butlin and Smadja’s (2018) discussion of the set of processes that might be considered forms of ‘reinforcement’.

“it can happen under any mode of speciation[3]:355 (e.g. allopatry, parapatry, sympatry, ecological, etc.)” – I agree. This should be mentioned, perhaps, in the paragraph about terminology because some have considered reinforcement following secondary contact to be a different process from the enhancement of reproductive isolation during speciation with continuous gene flow.

“regardless if individual preferences have no effect on survival and reproduction” – should be ‘no direct effect’. Indeed, the distinction between direct and indirect selection is absolutely crucial to any discussion of reinforcement and should be made explicitly.

“Gene flow acts as the primary opposing force against reinforcement, as the exchange of genes between individuals leading to hybrids cause the genotypes to homogenize across the incipient species” – recombination needs to be considered as well as gene flow. Citation of Felsenstein (1981) is critical here, in order to distinguish the contributions of gene flow and recombination.

“are a form of reproductive character displacement” – this deviates from the current usage noted above (the use of RCD in the next sentence, where it clearly describes the pattern, not a process).

“ecological character displacement can result in the same patterns.[22]” – Noor (1999) would be a better reference here.

“the number of alleles altered by mutations affecting mate choice” – this refers to the one-allele vs.

two-allele distinction of Felsenstein but this fundamental division needs a more complete exposition.

“In sympatry, patterns of strong mating discrimination are often observed—being attributed to reinforcement” – this seems to be an empirical point that does not belong in this section.

“Reinforcement is thought to be the agent of gametic isolation” – this also seems to be an empirical point. It certainly needs further elaboration since ‘gametic isolation’ has not been mentioned previously and it is unclear why it should be a special case in terms of reinforcement. The sentence implies that gametic isolation does not originate in other ways, which is clearly going further than intended.

“If selection at B and C cause changes in the frequency of allele A, assortive mating is promoted, resulting in reinforcement. Both selection and assortive mating are necessary, that is, that matings of A × A and a × a are more common than matings of a × A and A × a” – should be ‘assortative’ not ‘assortive’. This is not an adequate description of the Felsenstein result, or the process by which indirect selection on A/a can increase overall reproductive isolation.

“as it decreases the probability of the differing genotypes to exchange” – meaning is unclear.

“An alternative model exists to address the antagonism of recombination, as it can reduce the association between the alleles that involve fitness and the assortive mating alleles that do not” – the critical role of recombination has not yet been described and so this makes little sense. The ‘alternative model’, presumably the ‘one-allele’ mechanism, is not described. This mechanism is widely misunderstood, despite its importance, so a careful exposition here is crucial. Servedio and Noor (2003), Bank et al. (2012) and Butlin and Smadja (2018), as well as Felsenstein (1981) itself, all try to explain the distinction. See also Kopp et al (2018) which is also relevant here.

“Genetic models often differ in terms of the number of traits associated with loci;[27] with some relying on one locus per trait[24][28][29] and others on polygenic traits.[21][20][30]” – as it stands, this is not very helpful – it tells us that there are some effects, but it says nothing about their direction or strength.

“The structure and migration patterns of a population can affect the process of speciation by reinforcement. It has been shown to occur under an island model, harboring conditions with infrequent migrations occurring in one direction,[20] and in symmetric migration models where species migrate evenly back and forth between populations.[24][28] Reinforcement can also occur in single populations,[27][21] mosaic hybrid zones,[29] and in parapatric populations with narrow contact zones.[31]” – so, there are conditions in all of these scenarios where reinforcement can contribute to reproductive isolation. However, it is critical to give some information on how likely this is in each case, or at least on how large a portion of parameter space is associated with reinforcement. There is an underlying tension that needs to come out: high rates of contact result in strong selection but they also provide the greatest opposition through gene flow and (where relevant) recombination.

“Population densities are an important factor in reinforcement, often in conjunction with extinction.[21] It is possible that, when two species come into secondary contact, one population can become extinct—primarily due to low hybrid fitness accompanied by high population growth rates.[21] Extinction is less likely if the hybrids are inviable instead of infertile, as fertile individuals can

still survive long enough to reproduce” – here it is important to bring out the idea that reinforcement and extinction can be alternative outcomes.

“Speciation by reinforcement relies directly on selection” – this section is a bit of a jumble of ideas. The direct vs indirect selection distinction should have been introduced earlier. Linkage disequilibrium can be built be selection or by other forces (such as gene flow). Indirect selection is a result of LD. The sentences about direct selection should refer to ‘magic’ (Servedio et al 2011) or ‘multiple-effect’ traits (Smadja and Butlin 2011). There should be mention of the decline in selection as reinforcement proceeds, calling into question whether it can complete speciation. There should also be mention of the role of sexual selection (using the Liou and Price model cited earlier). The core point, that reinforcement is more likely when selection against hybrids is strong, is missing. Similarly, the critical point that the strength of indirect selection depends on recombination is also missing.

“Some initial divergence in mate preference must be present for reinforcement to occur” – first, reinforcement does not operate only on mate preference but also on mating signals and any other trait contributing to pre-zygotic isolation (as the author goes on to say). Second, to be more general, the key issue is a requirement for some association (LD) between isolating traits and those under direct selection. Third, this ignores one-allele mechanisms.

I don’t see text references to the figures. Fig. 1 describes only one possible scenario for reinforcement and so could be a bit misleading. Fig. 2 is helpful but it needs to be clear that this relates to one model of reinforcement specifically, even if some general features apply more broadly.

“Evidence for speciation by reinforcement has been gathered since the 1990s” – this is an odd statement since clearly evidence was gathered earlier and at least one really key paper came out in the 1980s, Coyne and Orr’s (1989) survey of Drosophila.

“along with data from comparative studies and laboratory experiments” – implying that these are somehow not ‘evidence’!

“in a sympatric population in comparison to an allopatric population” – comparing two populations is never enough. The pattern must be shown to be replicated across populations (or perhaps clinally). This is an important point because it is a common weakness in studies of reinforcement: inadequate replication at the population level.

“Reinforcement's prevalence is unknown” – bit at least one estimate exists in Yukilevich (2012)

“Plants are thought to provide suitable conditions for reinforcement to occur” – I see no conceptual basis for singling out plants in this way, nor do I think there is any good reason to suggest that there is more or less evidence for reinforcement in plants than in other taxa.

“Studies of reinforcement in nature often prove difficult, as alternative explanations for the detected patterns can be asserted.” – This is also critical and deserves some elaboration, with respect to both comparative studies and case studies.

The caption for the table needs to say two things: first that this is a selection of examples, it is not intended to be an exhaustive survey, and second that studies that did not find evidence for reinforcement in cases where it might have been expected are not included (and often will not have been published). There is, therefore, a strong bias in favour of reinforcement in this compilation.

“The zone of overlap sometimes forms hybrids” – I may not be fully up to date but I think there is only evidence for F1 hybrids, not for gene flow. It is probably not possible to consider the strength of evidence for every example in this table. Therefore, it might be better just to summarise the main issues concerning case studies, which relate to the criteria for reinforcement mentioned earlier and the difficult of distinguishing reinforcement from other processes, mentioned at the start of the Evidence section.

Hyla cinerea is now Dryophytes cinereus. It is worth checking for taxonomic updates in other cases too.

“has reinforced the observed character displacement of breeding times” – such confident expressions of cause should be avoided.

Pseudacris chorus frogs might also be included.

“collard flycatcher” – should be ‘collared’. Recent progress in this important study system is not included.

“sea urchins” – recent work is not included here either. In some ways, this table would be most useful if it focused on examples since the compilation in Coyne and Orr’s (2004) book, rather than replicating information from that widely-available resource.

“due to divergence of the alleles that code for bindin proteins: an example of post-zygotic isolation” – not, this is an example of pre-zygotic (gametic) isolation.

“This example of reproductive character displacement is highly suggestive of being the result of— and has been cited as strong evidence for—reinforcement” – and yet, with no hybrids found, it appears to fail one of the tests outlined at the start of this article.

“Female benthic morphs in sympatric populations actively discriminate against limnetic males, resulting in low rates of crossing (some gene flow has occurred between the morphs) and low fitness hybrids” – poorly constructed sentence because it implies that female discrimination results in low fitness of hybrids, which is the reverse of the intended meaning. Actually, this does not make clear the evidence for reinforcement either – the issue is the cause of the discrimination.

Mammals – the Mus hybrid zone should be discussed. This is one of the most powerful examples available and should not be omitted.

“Female mate discrimination is increased with intermediate migration rates between allopatric populations” – it would be good to give more emphasis to patterns like this, which are more specific predictions of reinforcement than things like RCD.

“Drosophila is one of the most studied species in speciation research.” – it is, of course, a genus.

More recent work than the classic Noor paper should be cited for D. pseudoobscura and D. persimilis, especially evidence for one-allele reinforcement.

Insects – evidence for reinforcement in Heliconius should be included.

“A unique case of post-zygotic” – should this be ‘a unique case of reinforcement of post-zygotic isolation’?

Anthoxanthum and Agrostis – selfing is also contributing to isolation in these cases.

The ‘Laboratory experiments’ table is odd. It does not include the classic Thoday and Gibson experiments, or the experiments by Higgie et al mentioned in the previous table. On the other hand, it lists a whole series of ‘throw away the hybrids’ experiments that are not relevant to reinforcement, established long ago.

Fig. 3 makes little sense as plotted, because the dependent variable (isolation) has been placed on the x-axis. The red and blue background shading has no obvious meaning.

The Coyne and Orr study (1989, updated in 1997) is pivotal in reinforcement research and needs much better explanation. First, effects of genetic distance must be explained. Second, phylogenetic correction must be explained. Finally, Coyne and Orr’s arguments to favour reinforcement over reproductive interference or the Templeton effect must be summarised. Then, it is critical to consider the more recent analysis of these data by Yukilevich (2012).

“another later study by Howard” – as the author noted earlier, Howard did not distinguish between reinforcement and reproductive interference. His comparative study should be seen in this light.

“A survey of the rates of speciation in fish and their associated hybrid zones found similar patterns in sympatry, supporting the occurrence of reinforcement.[111] One study in the plants Glycine and Silene; however, did not find enhanced isolation.[112]” – a survey in littorinid snails found evidence for RCD but could not exclude effects other than reinforcement (Hollander et al 2013). I think there are probably other studies too.

“There is no single, overarching signature of reinforcement” – see Yukilevich (2012) for a discussion and a suggestion that is not noted here. Also note the pattern described for Timema by Nosil that was mentioned in the evidence table.

“Though one experiment in stickleback fish that explicitly tested this hypotheses found no evidence” – this is not a complete sentence. Also, the point is unclear – there is plenty of evidence for ecological character displacement itself, but not for its impact on pre-zygotic isolation.

“mating trait divergence results in complete or near complete sexual isolation among populations” – this point from Hoskin and Higgie deserves a comment because of the argument that reinforcement can be self-limiting. There may be good evidence that reinforcement has increased pre-zygotic isolation in a system without the isolation being anywhere near complete. In other words, one must distinguish between the operation of reinforcement and the completion of speciation by reinforcement.

Noor (1999) should be mentioned in this section on ecological divergence.

Fusion – this section on the fusion hypothesis (aka Templeton Effect) is important but the explanation is really hard to follow. I think a substantial re-write is needed to make the proposal clear and also to show why it seems unlikely to explain the Coyne and Orr Drosophila data.

Sympatry – this section is unhelpful as it stands. First, it does not allow for ‘parapatry’ (arguably the norm). Second, the Fig. 4 caption is hard to follow. Third, the underlying distinction is not clear – as argued above, the modern view is that speciation with high levels of gene flow involves a process indistinguishable from reinforcement. It also certainly involves post-zygotic isolation, although typically ‘extrinsic’ rather than ‘intrinsic’ (although that distinct is far from absolute).

“Post-zygotic isolation is not needed, initiated simply by the fact that unfit hybrids cannot get mates” – if hybrids cannot get mates, there is post-zygotic isolation. In general, this section on sexual selection is quite confused. It should really start with the Liou and Price model.

The ‘alternative process’ of reproductive interference is not mentioned, even though it is arguably the hardest to distinguish from reinforcement in field data and in comparative analyses. Even in the Coyne and Orr data it is hard to exclude because measures of hybrid fitness in field conditions, or direct estimates of gene flow, are missing for many species pairs.

“A number of objections were put forth, mainly during the 1980s, arguing that reinforcement is implausible.[7][18][3]:369 Most rely on theoretical work which suggested that the antagonism between the forces of natural selection and gene flow were the largest barriers to its feasibility.[3]:369–372 These objections have since been largely contradicted by evidence from nature and additional models.[17][3]:372” – I am not sure that this is a very helpful summary. It remains true that there are strong genetic and other constraints on reinforcement and alternative explanations for many observable patterns. Evidence has gradually accumulated to suggest that reinforcement can happen but it remains unclear how commonly it contributes to speciation. Recombination is a major constraint but magic/multiple-effect traits or one-allele mechanisms can avoid this constraint (known since 1981).

“selection cannot drive the fixation of alleles for pre-zygotic isolation” – this is not what Felsenstein said.

“Somehow, the populations must be maintained.” – this is a distinct problem. It is not an issue in Felsenstein’s models where populations are independently regulated. It is also not a problem in hybrid zones. There is now abundant theory to show how frequency-dependent selection can overcome this problem in many other situations (see Gavrilets 2004, for example).

“For reinforcement to work, gene flow must be present, but very limited” – there is more to say here too. Mating traits will often be under stabilising selection which may oppose reinforcement if gene flow from populations not experiencing costs of hybridisation is common. Alternatively, the RCD signature may be lost if reinforced traits are not opposed in other populations. Then there is the issue of self-limitation, mentioned above.

“Recent studies suggest reinforcement can occur under a wider range of conditions” – the most recent citation attached to this statement is Coyne and Orr’s 2004 book. Things have happened since then!

“This natural setting was reproduced in the laboratory” – but this study is not included in the earlier table of experimental studies.

“The production of unfit hybrids is effectively the same as a heterozygote disadvantage; whereby a deviation from genetic equilibrium causes the loss of the unfit allele” – this is the point about population maintenance that I commented on above.

Editorial decision

Latest comment: 2 years ago1 comment1 person in discussion

Comments by Andrew Leung ,
These editorial comments were submitted on 17 Sep 2022, and refer to this previous version of the article

Author did not respond to the reviewers' comments nor editorial board's follow-up inquiries despite multiple attempts. Editorial board has decided that this submission to be declined.

OhanaUnited^{Talk page} 01:16, 18 September 2022 (UTC)Reply

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