Divergence Along Genetic Lines of Least Resistance & Final Conclusions

Last week Kayla and I finished "The Ecology of Adaptive Radiation"- the first novel in our reading list this semester.

The final (non-concluding) chapter focused on Divergence Along Genetic Lines of Least Resistance and was perhaps the only title that I had no clue what would contain. I hadn't really heard of the concept of genetic lines of least resistance and perhaps that's why I had such a hard time ploughing through the forty or so pages in this chapter. So here's a definition as far as I can manage:

Phenotypic evolution by natural selection is primarily determined by genetic variance and covariance that bias evolution away from greater fitness.

I suppose I have a feeble hold on the concepts of quantitative genetics so this definition scared me. This was further complicated by my tenuous grasp of the idea of 'additive genetic variance'... which is evidently the variation we are talking about in this case. I've come to understand additive genetic variance as the variation in offspring that is a direct result NON-interacting genes from the parents. That is, the sum of genetic variance from the two parents, where there is no epistasis or other interactions.

We have another interesting definition here, that I have some trouble wrapping my mind around.

I find that thinking about lines of least resistance in terms of examples is most useful. The common one is that if you had two traits, beak broadness and beak length, these traits would always be tied to each other such that you could never get a really narrow long beak, even if this was the 'fittest' phenotype. Hence, the 'adaptive hillside' on which the population rests would be a combination of these traits that was genetically feasible, but not necessarily the 'fittest' given the landscape.

I suppose that white sands lizards could also have traits like this. Say colour. Say dorsal (cryptic) colour and ventral (signaling) colour vary in such a way that you can never get to be completely matched to your dark soil surroundings, unless you sacrifice some ventral colour. On the other hand, if you lose pigmentation on your dorsal surface, your ventral colour is enhanced (this appears to be the case in white sands, but the genetic correlation between ventral and dorsal colour, if it exists, is unknown). So if we go into dark soils, even though the best situation would be a bright ventral colour for signaling and a dark dorsal colour for camouflage, this combination can never be achieve... and essentially you have the lizards sitting on an adaptive peak that is below that which is optimum, fitness-wise.

Just a disclaimer here- I think this is a bad example because it is quite probable that selection on dorsal and ventral colour is mostly unrelated. Selection on ventral colour likely has more to do with the environment in which signaling takes place (bright white sands versus ancestral dark heterogeneous habitat).

~~~

Well that's about it for now. I wanted to conclude by saying that I tried to read this book in undergrad and couldn't make it through; however, I am very happy that I picked it up again in my second year of my PhD. This was a much more appropriate time to read it and Schluter has definitely enhanced my understanding of many topics as well as filled in large gaps in my understanding of evolutionary ecology and adaptive radiation, specifically.

... on to SPECIATION!

Reproductive isolation and ecological speciation

In chapter 8 of The Ecology of Adaptive Radiation, Schluter addresses the role of reproductive isolation in adaptive radiation, focusing on different ways that reproductive isolation between populations may arise. Mayr and Dobzhansky initially formulated the idea that pre- and postmating isolation could arise between populations evolving in distinct selection environments, with complete reproductive isolation resulting ultimately as a consequence of divergent natural selection.

There are two models of how divergent selection causes reproductive isolation and ecological speciation:

By-product speciation is the first model. In the case of by-product speciation, divergent natural selection on phenotypes causes reproductive isolation to evolve incidentally between populations in different environments. Reproductive isolation is considered a “by-product” because it is not directly favored by selection. Both postmating and premating isolation can occur through by-product speciation. Postmating isolation evolves when genetic changes favored in different populations in different environments are incompatible when recombined in hybrids (this is known as hybrid inviability). Premating isolation can occur if mating preferences are genetically correlated with morphological or physiological traits that are the targets of divergent selection.

The second model Schluter mentions is competitive speciation. When intermediate genotypes in a population are at a fitness disadvantage selection may favor a population split leading to reproductive isolation. Competitive speciation is considered to be reinforcement if a period of allopatry leads to incomplete pre- and postmating isolation, and then when the populations come back together hybrids are selected against because they are at a fitness disadvantage. The process is known as sympatric speciation when there is no allopatric phase, and speciation is initiated by disruptive selection in a population. These two versions of competitive speciation are distinct from the by-product model because reproductive isolation is directly favored by natural selection in the sympatric phase.

I am interested in investigating whether premating isolation has occurred in white sands and dark soils lizards. In the by-product model, premating isolation between populations can develop when populations are in different selection environments, and mate preferences are linked to traits that are affected by natural selection in these differing environments.

What would have to occur for premating isolation to evolve between white sands and dark soils populations? First of all, the populations would have to be diverging in different selection environments. Check. White sands populations differ dramatically from dark soils populations in skin color, among other things. Second of all, divergent natural selection would have to affect traits linked to mate preferences or recognition. Here’s where things get a little more ambiguous. All three lizard species in the area have social signaling patches that are used in inter- and intrasexual interactions. The colors of these patches are different between white sands and dark soils populations. We know that this difference in skin color is linked to differences in the Mc1r gene. Mutation of Mc1r in white sands lizards has led to adaptive blanched dorsal skin coloration, and this change has affected the coloration of the social signaling patches as well.

So yes, social signaling patch color, which is involved in social interactions, is diverging due to natural selection in different environments. But is social signaling patch color related to mate choice or recognition? Maybe mate choice is linked to chemical secretions or social interactions that are divergent between populations by chance, not because of divergent natural selection? Or, for that matter, is there even mate choice occurring in these lizards? Maybe they would just be game for mating with any conspecific they run into. These are questions I’m hoping to begin answering during my first field season this spring. Okay, this is getting long, so I’ll stop…but if anyone has any input on my ideas I would love to hear it!

Ecological Opportunity and White Sands

Simone here.

Before I start, I want to provide some links to my website and my supervisors' websites.

Mine

Luke Harmon

Erica Bree Rosenblum

Today I'm going to write about Ecological Opportunity and how this concept relates to the three lizard colonists of White Sands. This topic has been the focus of my first field season so forgive me if I go on too long!

Let's start with a definition. I'm going to pull my definition from all over the place... and primarily from my head.

An ecological opportunity is basically anything that leads to the increased niche breadth of a species. For example, the evolution of a key innovation (that allows access to a new adaptive zone), the colonization of a novel environment, the extinction of antagonists (competitors, predators)... or some combination of the above. I should note that originally ecological opportunity was defined mostly in the context of release from competition pressures. And really, we can think of all of the above as relating to this in some context. A population might colonize a new environment without competitors, in the case of White Sands, for example.

I think this is a good opportunity (ha) to talk about my own research a bit. I am looking at White Sands as a potential setting for ecological release for the three species of lizard colonists. During my last field season I surveyed inside and outside White Sands to see if there was a significant reduction in the number of potential predators and competitors in the latter. Furthermore, I began to look for evidence of ecological release in one of the three colonist species: Sceloporus undulatus (see picture of male from White Sands above). I looked at whether sceloporus use of their microhabitat (perches) was more variable in White Sands compared to their use in ancestral dark populations.

My results have so far indicated that

1. the abundance of the total lizard population in White Sands is comparable to that in ancestral dark soils. This is cool because the former only has three species, whereas the latter has several- up to 10 or more. We use the term 'density compensation' to describe this increase in population size.

2. bird abundance also was higher outside White Sands- although I haven't yet classified this birds as potential competitors or predators (or neither)... and it will be interesting to see how this influences my results.

3. my results from quantifying sceloporus perches were provocative. Both sceloporus in dark soils and White Sands use their perches selectively... that is, not proportional to their availabilities in the habitats. However, White Sands lizards used a greater variety of their available* perches than did dark soils lizards. In other words, the DIVERSITY of perch use by White Sands sceloporus was higher than that of dark soils sceloporus.... cool!

This could suggest that there is some sort of alleviation from selective pressures in White Sands sceloporus. Perhaps the absence of interspecific competitors in White Sands allows the sceloporus there to expand their perch use. Perhaps research into diet and food resource use will demonstrate a similar trend... but that is for another field season.

*I say and mean available. This is an important point because, as Schluter says, resource heterogeneity may exist; that is, the 'depauperate communities' that are colonized not only have fewer competitors, but also may contain fewer available resources. This is likely the case in White Sands, where perches are far scarcer than in surrounding dark soils.

Thanks Kayla- nice coverage of character displacement.

I'd like to add one thing to that too:
Schluter focuses a lot on character displacement between closely related taxa...
However, character displacement can occur between distantly related taxa as well, it just seems to be less studied. For example, certainly the presence of other competing lizards NOT closely related to Sceloporus undulatus in New Mexico will cause them to use a smaller portion of their available resources.

Consider S. undulatus in surrounding dark soils habitat; there are several other competing lizards (such as Uta stansburiana pictured above), that may reduce the available niche space for Sceloporus. There are several potentially competing lizard species in the dark soils surrounding White Sands... but in White Sands itself, there are only three lizard species. Does this then mean that Sceloporus (and other White Sands inhabitants) are able to use a greater proportion of their habitat than members of their ancestral populations in dark soils? In other words, is something... the opposite... of character displacement occurring in White Sands?

We'll talk more about ecological opportunity and ecological release next week.

And I'm burnt out so... perhaps a little more later.

Mechanisms of divergence and character displacement

In chapter six Schluter addresses competition and how it may lead to divergence through character displacement. Character displacement is defined as “the process of phenotypic divergence caused or maintained by interspecific resource competition.”

Schluter states that there are three main patterns found in nature that indicate character displacement has occurred. The first is exaggerated divergence in sympatry, where species found together are more different phenotypically than in areas where they do not coexist. The second is trait over-dispersion. This occurs when the mean phenotypes found within a species are more evenly spaced along a trait axis than would be expected from a random sample. The final pattern, species-for-species matching, is defined as replicated guild structure between independently evolving sets of species. There are six criteria for assessing whether character displacement has occurred between different species:

1. Phenotypic differences between species should have a genetic basis.
2. Chance should be ruled out as an explanation of the pattern.
3. Differences must represent evolutionary shifts rather than species sorting.
4. Shifts in resource use should match changes in morphology.
5. Environmental differences between sympatric and allopatric sites must be controlled for.
6. Independent evidence should be gained that similar phenotypes compete for resources.

So what’s the relevance of character displacement to the lizards of white sands? Is character displacement likely to occur between these three species sometime in the future? For lizards in the white sands area there are fewer competitors, so the interspecific competition for resources may not be strong enough to lead to the evolution of character displacement. However, there are also fewer resources in white sands than in the rest of the desert, which should increase interspecific competiton. Looks like it’s time for some field work examining the overlap in the niches of the different species of white sands lizards!

Schluter also listed some other mechanisms besides competition that may promote divergence. The one that seemed most relevant to white sands is the process of the evolution of food webs. Through this mechanism divergence occurs in newly colonized areas where there were originally few inhabitants, and a new food web is essentially generated from scratch. When this happens many species and populations undergo trophic level transitions, where they begin to take advantage of resources from other trophic levels than they did previously. White sands has few inhabitants, and so colonists may diverge through the process of exploiting new resources and through the generation of a new food web.

Thanks for reading…take it away Simone!

Diversity begets diversity...in white sands?

Thanks for the introduction Simone!

This is Kayla signing on, and I’d like to address some of the questions Simone brought up at the end of her last entry. Specifically, I want to talk about the idea that diversity begets diversity. Schluter introduces this idea, which was first suggested by Whittaker, in chapter four of his book. Basically Whittaker proposed the idea that the addition of new species to an area, through colonization or evolution, constitutes an addition of new resources to an area which other species in higher trophic levels can take advantage of. The increase in resources permits new predator species to evolve or colonize the area, which in turn decrease the numbers of prey species and allows for the introduction of new prey species…and so on.

This idea could be relevant to an increase in species diversity in white sands. Currently there are three lizard species inhabiting the area, and we are only aware of a few avian predators having much of an effect on their populations. The presence of the lizards could encourage the introduction of new predator species to the area; these species could reduce the population sizes of the three lizard species and create additional ecological opportunity for colonization or evolution of new lizards in the area.

But could this kind of predator-prey interaction lead to an adaptive radiation? Simone mentioned the fact that some classic adaptive radiations seem to have come to be through multiple colonization events. This type of interaction seems like it could increase the chances of multiple colonization events, or even encourage the evolution of new species in white sands.

Though the idea that diversity begets diversity may apply to white sands, there is still the problem of gene flow to think about. As Simone mentioned, the environment is dynamic with few physical barriers. The predator-prey interaction envisioned by Whittaker would not decrease the chances of gene flow in this system. Gene flow is likely a major obstacle to speciation of white sands lizards, but as we continue to read Schluter, maybe we can to gain more insight into this and the other questions Simone posed at the end of her entry. Tune in next week to find out!

ADAPTIVE RADIATION- how the heck does this relate to White Sands lizards.

This image of White Sands, NM is from google maps.

Simone- signing on.

So Schluter focused a lot on the definitions of Adaptive Radiation and other concepts surrounding the term. Let's see what he says.
On the first few pages of chapter 3, Schluter outlines the four main features of an adaptive radiation. Here they are in abbreviated form:
1. common ancestry of component species
2. phenotype-environment correlation
3. trait utility (certain traits have a fitness advantage in respective environments)
4. rapid speciation.

We're not by any means suggesting that adaptive radiation has or will occur in White Sands lizards. But the parallel colonization by the three lineages is interesting in this context because many believe (Lister, Wilson, Simpson etc.) that this invasion of open and new territory is a key component of the initial stages of an AR. We'll talk about this concept of Ecological Release in more detail later.

Why don't we think that AR wouldn't occur in White Sands lizards?
We discussed this a little when Kayla and I met last time. Well first of all, when we look at adaptive radiations on islands... well, see, that's the thing. Adaptive radiations are on islandS! Not just one! Hawaiian silverswords, fruitflies and honey creepers, Galapagos finches and Caribbean anoles- they all inhabit archipelagos. Even if we consider White Sands as a relatively isolated and recently formed island... it is just one. Even the subdivision of interdune areas by high uninhabitable dunes is only ever temporary. The dunes are always moving, changing the configuration of habitable, vegetated space. One might imagine that the populations of lizards in White Sands are quite dynamic. And an adaptive radiation in the face of that kind of gene flow? I think not!

But some adaptive radiations seem to be born from multiple colonizations of the same lineage in one 'island' (stickleback, cichlids, anoles...):

Could we get an adaptive radiation of lizards in White Sands due to several invasions from surrounding dark soils habitats?
What do population genetics tell us about the structure of populations in White Sands?
Would this be more common in certain lizard species? (i.e. Holbrookia populations outside White Sands are far more disjunct and they have no continuous distribution over the ecotone from dark soils to White Sands).
Is there enough distinctive niche space for lizards in White Sands? Or is niche space indeterminate, with diversity begetting diversity (Whittaker)?

Over to you, Kayla!