Frankly, it is late in the night and I have no desire to frustrate myself talking about the former. so it'll be Ecological Isolation I discuss.
Well, what is Ecological Isolation? In my words, adapted from Coyne and Orr (citing Stebbins 1950), ecological isolation is the confinement of groups of individuals to different habitats and the selection against hybrids in either of these. So my question is, if we are saying that these groups of individuals are restricted to different habitats, but we make a point of defining habitats as different (but not mutually exclusive) from niches, then can we say that these two groups are sitting on different adaptive peaks? And, that hybrids fall somewhere in the valley between these peaks?
Habitat Isolation on the other hand refers to a similar spacial confinement of individuals to different habitats, but instead of the selection against hybrids that have already formed (post-zygotic isolation), it is unlikely that members of each parental group will breed to form these hybrids in the first place (pre-zygotic isolation). The formation of hybrids is unlikely because parental individuals are unable to live in the other habitat- gene flow is thus reduced, and speciation is possible between the parental lines.
So on to White Sands species. I feel like it is a dangerous area to get into by arguing that any of the White Sands lizards formed by some form of habitat or even ecological isolation. Yes, we can see today that White Sands and dark soils (and even the ecotone) form distinctive habitats more or less spatially separated from others. But it is difficult to know the qualities of initial selection after colonization of White Sands. Not to mention the whole 'hybrid' thing is a bit of a mess. What is a White Sands x dark soils 'hybrid' anyway? Are they selected against in either habitat? Are they found at the ecotone (i.e. is the ecotone a hybrid zone?)
If I may draw your attention to Rosenblum 2006, where on page 13 we can see the qualities of members of each species across the ecotone:
Holbrookia: ecotone lizards are phenotypically indistinguishable from White Sands individuals. Furthermore, population structure is high and there is no evidence of gene flow or changes in population size.
Sceloporus: ecotone lizards are intermediate between dark and White Sands individuals. Population structure is moderate, gene flow is evident, but population size is constant.
Aspidoscelis: ecotone lizards are indistinguishable from dark soils lizards. Population structure is weak, there is no evidence of gene flow but there are indications of increasing population size.
This might tell us something about what we may call 'hybrids' in each species. Both Holbrookia and Aspidoscelis show little or no evidence of gene flow between dark and White Sands populations; however, ecotone individuals are composed of individuals from opposite areas in either species. This suggests that populations are not presently continuous across the ecotone: hybrids between both dark and White Sands populations do not (often) form.
But what about Sceloporus? Evidence suggests that there is gene flow across the ecotone. Furthermore, individuals at the ecotone are actually intermediate phenotypically (in terms of colour at least)! But is this local adaptation to ecotone conditions? The ecotone generally has white gypsum sand, but is characterized by denser vegetation than the heart of the dunes (see above pictures). But what does this matter anyway if most Sceloporus are found basking on yucca stalks (which, consequently, are the same colour everywhere). So are these hybrids between dark soil and White Sands populations? If their population size is constant, they aren't being selected against, are they?
This summer we plan on initiating a mark recapture study. I think that it is especially important to sample individuals across the ecotone. First, because populations of each species are structured so differently here, and second, because we might be able to resolve some questions regarding selection at this crucial spot.
Thanks for tuning in this week! There was something else I was going to say about ecology, but I forget it now. Maybe it'll come to me in my sleep.
~ Simone
Sorry, been meaning to respond to this. A few things:
ReplyDeleteYou say "we make a point of defining habitats as different (but not mutually exclusive) from niches, then can we say that these two groups are sitting on different adaptive peaks? And, that hybrids fall somewhere in the valley between these peaks?"
I think that this depends on your view of adaptive landscapes. Your description could certainly be the case, but only if landscapes are fairly simple and stable through time. If landscapes are complex or change through time, then all bets are off!
I like your discussion of the status of ecotone individuals across the three species. I don't know if we should think about these as hybrids, though, unless we know that they really are!