Evolutionary Biology Online Journal Club

Food for thought: geography as (just another) isolating mechanism

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Hey everyone, I thought I’d post this here as it might be interesting to us. I mentioned this paper briefly in the last discussion, and though it might be too theoretical for our group (though I am open to have that questioned), I thought some thoughts in here would be helpful to us.

So even though I think we have mostly agreed that while interesting, focusing too much in geographical aspects of speciation (sympatry/parapatry/allopatry) as very crucial can be distracting, and confusing if treated as an alternative hypothesis to other processes (such as ecology) driving diversification. Nonetheless, we keep on coming back to this topic, and I started wondering if it could be just because we lacked a more formal framework that we could have to substitute the one we’ve been exposed to so far.

I was reading a paper by Mark Kirkpatrick and Virginie Ravigné (Speciation by Natural and Sexual Selection, The American Naturalist 2002 159:S22-S35) and I think they might offer just that. In the paper, they list five conditions necessary to speciation via selection (natural or sexual), of which the second one, “A Prezygotic Isolation Mechanism”, deals with the element of geography. From the paper (emphasis are mine):

Here again there is the opportunity to unify models of speciation. Assortment and mating preferences can be treated as a single form of prezygotic isolation by regarding assortment as the special case where a mating preference acts on itself.

The Geographical Setting

One of Mayr’s great legacies for our understanding of speciation was his emphasis on the importance of biogeography (Mayr 1963; see Coyne 1994). As a result, the geographical context of speciation—the allopatry versus parapatry versus sympatry continuum—is traditionally viewed as the most important factor in speciation (table 1, sec. II.C).

Our list of fundamental elements for speciation can be simplified, however, if we view geography as simply another form of assortative mating. Consider geography as a genetic locus. Each geographical location then represents a different allele. Migration is replaced by a form of frequency‐dependent mutation at the geography locus; movement between populations is equivalent to mutation of both alleles at the geography locus. That locus enforces the most extreme form of assortative mating since only individuals that carry the same allele at the geography locus are allowed to mate.

The evolutionary dynamics of a population with this form of assortative mating are exactly equivalent to those of a geographically structured population. Although this equivalence may at first seem obscure, it is useful for three reasons. First, it shows that we can simplify thinking about speciation by treating geography as just one more type of assortative mating; geography does not need to be modeled separately.Second, allopatry and sympatry do not need to be treated as qualitatively distinct situations. Third, the correspondence between geography and assortment helps make clear why allopatric speciation is so powerful and so prevalent: it is an exceedingly accurate form of assortative mating. Very few other types of assortative mating guarantee that only individuals carrying the same allele at a single locus will mate together. Combining this with ubiquitous spatial variation in selection produces a potent engine for generating new species. It may well be critical to the great majority of speciation events, as argued by Mayr (1963).

Although this might seem like just a “modeling trick”, I think the conceptual basis for it can be very helpful: we can view allopatry as a mechanism of prezygotic isolation with a behavior very similar to other possible mechanisms, with the important notion that it can (quite commonly) be a very strong one.

To me, this makes it very clear that geography can be one of many mechanisms of isolation, that it can act in concert with other occurring mechanisms, and most importantly, it needn’t be treated as an “alternative hypothesis” to ecological speciation. Ecology can play a role in many of the five conditions listed by Kirkpatrick and Ravigné; in some cases it might reinforce other prezygotic barriers (including geographical ones), in others it might weaken them, and in other cases it might just affect other of the conditions to speciation, that do not relate to prezygotic barriers. I think this can help deal with much confusion in the topic.

Well, these are my thoughts, would love to hear what you have to say!

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Author: Rafael Maia

Postdoctoral fellow at the University of Idaho, studying the development and evolution of iridescent colors in birds.

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