The "Species" of White Sands

Ecotone Sceloporus undulatus

Dear attentive blog followers. As may be obvious, I have been slacking a bit lately. As such, this blog will focus on last-last week's reading on species concepts. I think that the three species in White Sands provide interesting examples of how we might define species. I'll start by posing the question:

Could any of the three White Sands lizard populations be considered distinct species from their dark-soils counterparts? If so, which ones?

Here I'll consider each of the three White Sands lizards in the light of the major species concepts illustrated on page 27 of SPECIATION. I will focus mainly on:

1. The Biological Species Concept (BSC)

2. The Genetic/Phenotypic Cohesion Species Concept (RSC, CSC)

3. The Evolutionary Cohesion Species Concept (EcSC, EvSC)

4. The Evolutionary History/Phylogenetic Species Concept (PSC).

Now... to the species:

Sceloporus undulatus

The strongest line of evidence that Sceloporus form good species is based on the fact that local White Sands Sceloporus do not recognize 'invading' males in their territory as members of their own species. Indeed, instead of engaging in antagonistic behaviour, the local males seem to get confused... trying out courtship, exploratory and other confused displays (Robertson and Rosenblum in prep). We don't know yet what confuses these males, but this does indicate that some degree of speciation has occurred. A further confounding factor is that Sceloporus are continuously distributed across the ecotone separating dark soils and White Sands. This discourages allopatric speciation between the two habitats as the presence of a 'hybrid-zone' allows gene flow.

Thus, we generally can conclude that White Sands Sceloporus forms a pretty poor species. The presence of 'hybrid-like' individuals at the ecotone rules out the group as good species according to the Genetic cluster species concept (GCSC). It is difficult to rule out the possibility of multiple invasions of White Sands from dark soils, such that we cannotconclude that White Sands Sceloporus are monophyletic (largely ruling out the Evolutionary and phylogenetic species concepts).

Furthermore, Kayla will be looking at the further possibilities of mate choice and recognition in Sceloporus, thereby in a way, examining elements of the Recognition species concept. I personally am looking at the ecological roles of White Sands and dark soils Sceloporus... thereby determining if either makes good Ecological species.

Aspidoscelis inornata

Less work has been completed on this species, but it's useful to consider if the White Sands populations make a good species compared to the other two. At the ecotone, unlike Sceloporus, Aspidoscelis individuals are actually dark. Again, it's likely that multiple colonizations of White Sands occurred. We have not performed recognition and mate choice studies, so it is difficult to quantify the degree of behavioural isolation between dark and white lizards. As such, it seems as though Aspidoscelis do not form very good species by any criteria.

Holbrookia maculata

Of the three White Sands colonist lizards, Holbrookia seems the most likely candidate for a new species, distinct from dark soils populations. Following are two main lines of evidence in support of this statement.

a. White Sands Holbrookia males more often choose local females, and both sexes recognize local mates over non-local mates (see Rosenblum 2008). This suggests behavioural (pre-mating) isolating mechanisms in effect.

b. White Sands Holbrookia populations are disjunct from ancestral dark soils populations. That is, there is no 'hybrid' zone at the ecotone (individuals at the ecotone are indistinguishable from inter-dune individuals). This limits gene flow into the White Sands populations from dark soils populations.

But are White Sands Holbrookia monophyletic?

Were there multiple invasions from dark soils?

How does this compare to the other species?

A new member of our lab, Tyler Hether will be looking at landscape genetics and perhaps we can begin to uncover some of these questions...

Again, Kayla and I will be looking at subjects related to the recognition and ecological species concepts (respectively)... hopefully we can find out more about the nature of species formation at White Sands.