Wednesday, April 14, 2010
Reinforcement is distinct from reproductive character displacement. This term was coined by Butlin to describe situations in which taxa come into secondary contact and produce hybrids with are completely unfit. In this case gene flow is not possible, as the hybrid will not be able to produce surviving offspring. This means the two populations are already good species, and so any further enhancement of prezygotic isolation would have nothing to do with speciation and should not be considered reinforcement. In contrast, in the case of reinforcement the hybrids are only partially unfit, so gene flow is possible, and the speciation process is not complete. Enhancement of prezygotic isolation in this case is relevant to speciation.
As Felsentein (1981) explained, it should be theoretically difficult for reinforcement to evolve. Reinforcement requires that individuals assortatively mate with others that share similar phenotypic traits. This means that there must be strong linkage disequilibrium between alleles underlying the characters that have undergone divergent evolution between the two populations, and the alleles underlying mating preferences. Felsenstein pointed out that although this linkage may be generated by selection, recombination often destroys it. Unless the two loci are tightly linked, it should be difficult to overcome recombination.
One way to look for evidence of reinforcement is to quantify the level of prezygotic isolation between populations in sympatry and in allopatry. The prediction would be that prezygotic isolation would be higher between the populations in sympatry. This is because there would only by hybridization between the two populations they come into contact, and so in sympatry there would be an opportunity for natural selection to increase the strength of adaptations leading to prezygotic isolation, where in allopatry there would not.
Could you use this kind of a prediction to look for reinforcement in white sands lizards? Or is reinforcement even a relevant concept to apply to this system? Upon first examination I thought it would make sense that reinforcement could be going on at the ecotone. After all, you have white populations of lizards in white sands, dark populations in the various surrounding desert habitats, and intermediately colored lizards at the ecotone. So it seemed to me that these intermediately colored individuals could be hybrids of dark and light populations that had diverged in allopatry and come back into secondary contact. Turns out it is likely that this isn’t the case. For one thing, there really aren’t any light populations and dark populations located close enough together to suggest that frequent hybridization of parental populations at the ecotone is common. In addition, the genetic basis for dark and blanched coloration in Aspidoscelis and Sceloporus is a simple dominant recessive relationship. In Aspidoscelis the mutation in Mc1r causing blanched coloration is recessive, while in Sceloporus it is dominant (Rosenblum et al 2009). So for example, if you had a mating between a dark (heterozygous or homozygous dominant) Aspidoscelis and light (homozygous recessive) Aspidoscelis, you might get either a light colored hybrid or a dark colored hybrid, but you shouldn’t see an intermediately colored individual. So, though it is interesting to contemplate where these intermediately colored individuals are coming from, their existence may not be relevant in the case of our discussion of reinforcement. It seems from the evidence we have currently that reinforcement of prezygotic isolation is not an important mechanism of speciation in the white sands system.
Tuesday, March 9, 2010
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.
Wednesday, March 3, 2010
Thursday, February 25, 2010
Sunday, February 21, 2010
Parapatric speciation refers to the divergence of populations between which there is some gene flow. One model of parapatric speciation is the clinal model. In this model a species is distributed continuously across a variable environment. Subpopulations adapt to local habitats, but adaptation is hindered by gene flow from nearby ecologically distinct populations. When populations have become differentiated enough, reinforcement may play a role in generating reproductive isolation. In the stepping stone model of parapatric speciation, there are discrete populations with restricted gene exchange. Since populations are discrete it is easier for selection or drift to result in reproductive isolation than in the clinal model.
Though white sands and dark soils lizards have not speciated, we can still think of which of these models is most applicable to divergence of these populations. The first question we should answer is how much gene flow is occuring between white sands and dark soils lizards. Previous research has shown that there is some gene flow, though the amount differs between the different species of lizards (Rosenblum 2006). This means that the parapatric models are more applicable than the allopatric models. Next question: are populations distributed continuously over a variable environment, or are there discrete populations? While the environment is definitely variable from white sands to dark soils, the amount of connectivity between populations is also different between different species of lizards. For example, Holbrookia maculata show the most genetic differentiation between populations, and also the largest color difference between white sands and dark soils populations. Sceloporus undulatus shows an intermediate level of phenotypic and genetic difference between populations, and Aspidoscelis inornata shows the least of both (Rosenblum 2006).
This evidence shows that Holbrookia populations are probably the most discrete, with Sceloporus populations occurring in a less discrete distribution, and Aspidoscelis distributed almost continuously. So maybe the stepping stone model is most likely for Holbrookia, clinal for Aspidoscelis, and some combination for Sceloporus? And if the models are different, does this mean that different types of reproductive isolation could be important for different species? Reinforcement is thought to be important for the clinal model, but in the stepping stone model premating isolation might evolve more easily as a byproduct of selection or drift. Hmmmmmm…
Monday, February 15, 2010
1. Premating isolation barriers- barriers that impede gene flow before transfer of gametes to members of other species.
a. Behavioral isolation: Members of different species do not initiate courtship or copulation with each other due to a lack of “cross-attraction”.
b. Ecological isolation: Barriers to reproduction are direct byproducts of adaptation to the local environment. This category includes isolation because of differences in habitat preference, timing of breeding, and pollinator interactions.
c. Mechanical isolation: Members of different species do not reproduce because they have incompatible reproductive structures.
d. Mating system isolation: This barrier refers to the evolution of partial or complete self fertilization or the asexual production of offspring, which can result in the formation of new species.
2. Postmating, prezygotic isolation barriers- barriers that act after transfer of gametes but before fertilization takes place.
a. Copulatory behavioral isolation: The behaviors of individuals during copulation do not allow fertilization to occur.
b. Gametic isolation: The transferred gametes cannot effectively cause fertilization.
3. Postzygotic isolation barriers- barriers resulting in hybrid sterility and inviability.
a. Extrinsic barriers: Sterility and inviability are results of the biotic or abiotic environment. This category includes ecological inviability, where hybrids have a lower fitness because of a failure to find an appropriate ecological niche. Behavioral sterility, where hybrids cannot obtain mates, is also included.
b. Intrinsic barriers: Sterility and inviability are results of developmental problems. Hybrid inviability is a result of developmental problems causing full or partial lethality. Hybrid sterility may be physiological or behavioral; physiological sterility leads to problems in the development of gametes or reproductive organs, while behavioral leads to developmental problems causing hybrids to be incapable of successful courtship.
Which of these potential barriers could be acting currently among populations of lizards in white sands and dark soils? Of the premating isolation category, behavioral and ecological isolation both seem possible. In fact, it seems like a combination of these two types would be most likely. As discussed previously, behavioral isolation could be a result of either a preference for local mates that are phenotypically similar, or a lack of recognition of mates from populations adapted to living in different environments. Ecological isolation is certainly important in terms of habitat preference; if white lizards and dark lizards don’t encounter each other because they live in different areas, this will result in reproductive isolation. I think ecological isolation would be important over most of the habitat, while behavioral isolation may be important at the ecotone.
In terms of postzygotic isolation, extrinsic barriers seem most relevant to the white sands system. Depending on the genetic basis of dorsal coloration (which is different among different lizard species in white sands) hybrids could suffer from ecological inviability. The mutation that causes blanched coloration is dominant in one white sands species and recessive in another. Either way, if a hybrid finds itself not well matched to its substrate and can’t effectively choose a better niche, it won’t do as well (it will probably end up as food for someone else).
From reading about all these different isolation barriers I have come to the realization that as I continue on with my plans to investigate behavioral premating reproductive isolation in these lizards, I can’t overlook the possibility that other reproductive barriers could be playing an important role as well, and I need to be aware that many factors have likely shaped the system as it is today.
Tuesday, February 9, 2010
I just wanted to add a few comments on the subject. Schluter mentions that when traits covary (in the ways Simone described earlier) the amount that one trait covaries with another will affect how far evolution will be pulled away from the direction in which selection is at its strongest. Take the finch beak example that Simone gave, and imagine that selection is strongest in the direction favoring an extremely long, narrow beak. If beak width and length depend on each other to a great extent, so that selection on one automatically changes the other dramatically (selection for a longer beak leads to a wider beak too), then this interaction would lead to evolution in a direction that is greatly skewed off of the path of strongest directional selection. If the traits did not affect each other as much, the direction of evolution would deviate from the direction of strongest selection to a smaller extent.
Schluter mentions that over time this effect of genetic constraint can be overcome and a population may make it to the top of an adaptive peak. But the stronger the covariance between the traits is, the stronger the skew will be away from the direction of strongest selection, and the longer it will take to make it up the peak.
One cool result of this whole covariance thing is that the degree of covariance between traits can help you more accurately predict the direction of evolution. If you realize that selection is strongest in a certain direction on an adaptive landscape, you can examine phenotypic traits that would be affected by selection, and based on how these traits covary you could make predictions about the direction of evolution along the landscape. Neat!
Now that’s really it for the Ecology of Adaptive Radiation…Speciation here we come!