Behavioral Isolation

This week we read chapter 6 or Speciation, titled “Behavioral and Nonecological Isolation.” Coyne and Orr describe 4 main categories of “evolutionary forces” that lead to behavioral isolation. These include 1) selection on mate preference, 2) selection on certain traits, 3) genetic drift, and 4) nongenetic mechanisms.

1) In the first category, initial selection pressures act on mate preferences of individuals either directly or indirectly. Direct selection on mate preferences increases the immediate fitness of the chooser by improving the chooser’s ability to acquire resources, or allowing the chooser to avoid deleterious features associated with mating. Indirect selection on preference includes selection that does not increase the chooser’s immediate fitness. For example, in the runaway model of selection preference for a certain trait is genetically correlated with the gene conferring that trait.

2) In the second category there is selection on traits that improve the attractiveness of the bearer to the opposite sex, improve the ability of the bearer to overcome competition from members of the same sex, or that facilitate species recognition. This last category includes trait changes that evolve through natural selection with preferences coevolving. It also includes the evolution of reinforcement, which improves the ability of individuals to discriminate conspecifics from heterospecifics in order to reduce maladaptive hybridization.

3) Genetic drift may affect behavioral isolation if nonselective changes in allele frequencies affect a signal or a preference.

4) Nongenetic mechanisms of isolation include cultural drift, and host parasitism.

It seems to me that direct selection on traits would be most important for behavioral isolation of populations of white sands lizards and dark soils lizards, especially through species recognition. The two subcategories for the species recognition section seem quite similar to me. In the first scenario, natural selection causes a trait to change, and preferences for that trait coevolve because mating with individuals with that trait will improve the fitness of offspring. In the second scenario, evolution favors discrimination between individuals with different traits to reduce the chance of producing less fit offspring. It almost seems like two sides of the same coin; recognize a trait and mate with that individual to create more fit offspring, or recognize a trait and don’t mate with that individual to avoid making less fit offspring.

Coyne and Orr distinguish the two by emphasizing that in the first scenario there need not be a closely related sympatric species present. Divergent selection in different habitats could cause changes in traits and preferences for those traits, which could lead to behavioral isolation between locally adapted populations. The book cites the example of divergent selection on body size for resource acquisition in benthic and limnetic morphs of sticklebacks .

In the case of reinforcement, there must be a related sympatric species present, and the fact that hybrids are less fit leads to selection favoring the avoidance of hybridization through increased ability to recognize conspecifics. So I guess if the difference between these two ideas is just whether there is divergence of locally adapted populations or of sympatric sister taxa, then the lizards in white sands would fall under the first scenario. This distinction is still a little fuzzy to me so if anyone has any comments please feel free to let me know where I’m going wrong.

Ecological Isolation

We read two chapters this last week: Sympatric Speciation and Ecological Isolation.

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