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All those types of speciation

Posted by RPM on December 16, 2008
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Over at Wilkins’ cabana, there’s a post (Some new work on speciation and species) on a paper by Nitin Phadnis and Allen Orr (doi:10.1126/science.1163934). Phadnis and Orr isolated a gene responsible for both reproductive isolation and sex-ratio distortion between two populations of Drosophila pseudoobscura. Wilkins doesn’t like speciation genes, and he’s rails on the concept in his post.

What I’m interested in are the comments on Wilkins’ post — primarily the confusion over what the speciation gene hunters mean when they talk about different flavors of speciation. Most population geneticists talk about three types of speciation: allopatric, sympatric, and parapatric. Wilkins’ commenters get caught up in the traditional, geographic definitions of those terms. Specifically, allopatry refers to populations with non-overlapping ranges, sympatry refers to populations with completely overlapping ranges, and parapatry refers to populations with adjacent ranges.

However, when population geneticists use those terms, they do not refer to geography. Instead, they refer to measures of gene flow, using the parameter ‘m’. That parameter is the probability that an individual from one population mates with an individual from another population. If m=0, there is no interbreeding between the two populations (these are allopatric populations). If m=0.5, there is random mating between the populations (these are sympatric populations). And 0

Larry Moran was also hanging out in the comments of Wilkins’ post, downplaying the importance of natural selection in speciation. Orr (and Jerry Coyne) have previously argued (Speciation) that natural selection is important for all flavors of speciation (ie, all values of m). I hope it’s obvious how selection against hybrids is important for m=0.5 (or close to it). However, many people think that sympatric speciation is rare, if not impossible. Therefore, natural selection may not be important when averaged over all speciation events.

But Coyne and Orr also argued that if m=0 (allopatry), natural selection may still be important for speciation. They point to laboratory experiments where populations of Drosophila were reared in separate vials for many generations. The vials subjected to the same environment did not evolve reproductive barriers, while those reared in different environments become more reproductively isolated. This goes against the tradition view of allopatric speciation (that of Ernst Mayr), in which it was driven purely by genetic drift.

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Comments

  1. #1 Jim Thomerson
    December 16, 2008

    Well, population geneticists didn’t talk about allopatric, sympatric, etc. in that sense back when I took population genetics in 1964. As a fish taxonomist this is the first I have heard of that use. So those terms now have to be defined when used because different people use them with different meaning. Do population geneticists use the term “syntopic”, or is its original meaning still intact?

  2. #2 John S. Wilkins
    December 16, 2008

    I do not rail against speciation genes in individual cases. I am quite sure that there is a genetic component to speciation in each case. What I rail against is the generalization of one particular group to another. Just because changes in gene X causes speciation in case A doesn’t mean X is the cause in case B, as seems to be the implication. In other words, there are no genes which form the class of “speciation genes” in every case, even just for animals.

  3. #3 MattK
    December 16, 2008

    I guess I’m one of those confused commenters from Evolving Thoughts even though I did allude to the existence of more than one view of speciation modes. Anyway, now I’m going to be confused over here. Specifically, speciation by definition (assuming something like Mayr’s definition) involves reductions in gene flow unless gene flow has already ceased due to vicariance etc. So if there is some sort of incipient speciation going on then m should decline towards 0. So if speciation modes are defined by m then wouldn’t all speciation become allopatric? Or is there some sort of provision for defining the mode based on m at a particular point in the process? Alternatively, if the mode depends on some conception of potential m then what is the difference between the population geneticists’ version and the original (as far as I know) geographical configuration version (which I would guess is an expression of the exact same thing)?

  4. #4 RPM
    December 17, 2008

    Matt, that’s a good point. It’s unclear at which point of reproductive isolation (post-zygotic, extrinsic pre-zygotic, intrinsic pre-zygotic, etc) to draw the line for speciation. One big problem is that the line is drawn in different places in different taxa (and often within the same taxon).

    My point is, m!=0 for speciation. And, even if m=0 between real populations, they may not be good species if they are just allopatric in the geographic sense. That is, they may have the potential for gene flow if they were brought in contact. So, there’s two things going on: 1. The actual gene flow (measured in m) between real populations, and 2. the potential for gene were the populations brought into contact.

  5. #5 Jim Thomerson
    December 17, 2008

    I know of one instance where an intergeneric hybrid swarm between two minnow species has maintained itself for at least 40 years. It is an unusual situation, and, so far as I know, there is little or no introgression occuring (could be out of date in thinking that).

    If we taxonomists are faced with similar, but different, syntopic populations, our task of recognizing and identifiying species is usually straightforward. The real problem is with allopatric populations. The usual method in the past was comparative. How do the differences/similarities of these allopatric populations compare with differences/similarities among related syntopic species where we are confident of our recognition of the situation?

    If one can successfuly breed and raise offspring from both allopatric populations, but cannot successfullly produce hybrids between the allopatric populations, I would take that as support that two species were involved. On the other hand if they hybridize readily, one still has a niggling doubt about what they might do if they came in contact in nature.

    I had several fish populations, all allopatric, with some striking differences in color patterns. So I made some crosses. As it turned out all heterolocality crosses produced more eggs and more viable fry than any of the homolocality control crosses. No loss in fertility in F1, F2, or any other cross. I think it is all one species.

  6. #6 MattK
    December 17, 2008

    Jim Thomerson

    I know of one instance where an intergeneric hybrid swarm between two minnow species has maintained itself for at least 40 years

    What species are these?

    I know of another odd cyprinid situation involving Phoxinus eos (Northern Redbelly Dace) and P. neogaeus (Finescale Dace). I pasted in some URLs below that lead to abstracts related to this wonderfully screwy situation. It’s almost as crazy as the Ambystoma complex weirdness. In that case there are ancient lines of all female gynogenic hybrids with verious ploidies (OK I just wanted to use that word) ranging up to hexaploid (I think) and containing chromosomes from several species. There are at least four species that have donated chromosomes to this complex. The two common ‘hybrids’ with A. laterale + A. jeffersonanium chromosomes used to be referred to by names like A. tremblayi and A. platineum before the complexity of the situation was appreciated. Some sources still do this because these are associated with common names (Tremblay’s Salamander and Silvery Salamander) but now I think it is more usual to just say Ambystoma complex if one doesn’t know what a particular beast is, and something like LLJ if one knows, for example, that a particular individual has two sets of laterale and one set of jeff. chromosomes. I think I’m going to have to take some time to read over that stuff again, it’s just so cool. Too bad I don’t have access to an institutional subscription right now.

    Anyway, some Phoxinus abstracts:

    http://www.ingentaconnect.com/content/bsc/mecol/2007/00000016/00000021/art00013

    http://jhered.oxfordjournals.org/cgi/content/abstract/89/2/151

    http://www.jstor.org/pss/1445762

    http://www.ncbi.nlm.nih.gov/pubmed/15266975?dopt=Abstract

  7. #7 MattK
    December 17, 2008

    RPM, thanks for the response.

    It’s unclear at which point of reproductive isolation (post-zygotic, extrinsic pre-zygotic, intrinsic pre-zygotic, etc) to draw the line for speciation.

    That isn’t quite what I meant. I’m pretty comfortable drawing the line based on prezygotic mechanisms. I think that species concepts should be intuitive wherever possible since, at least originally, they were meant to formalize an intuitive concept about kinds of organisms. When I think about organisms that I’m somewhat familiar with, I know of cases where isolation is mostly prezygotic yet, to my mind at least, they are clearly separate species. I think that in most cases a prezygotic definition adequately satisfies the condition of separate evolutionary trajectories re. the evolutionary species concept.

    What I was trying to ask had more to do with temporal changes in m. I was wondering, if during speciation m decreases non-abruptly then how is m used to define or describe the type of speciation?