Author: Yoel Stuart Page 1 of 6

Hey, that’s not an anole! American Rubyspot Damselfly (Hetaerina americana). Copyright Steve Collins

My colleagues and I recently published a paper documenting character displacement in Anolis carolinensis following the invasion of A. sagrei into Florida. The former moved up into the trees and evolved larger toepads. We did a lot of work in that paper to show with a high degree of certainty that the interaction between the two species is what led to character displacement in A. carolinensis. However, an open question remains as to exactly what kind of interaction, or interactions, they share. Most likely, the two species are competing for food (i.e. exploitative competition). They may also be interacting indirectly through a shared predator or parasite (i.e., apparent competition), and they are  known to eat each other’s hatchlings (i.e., intraguild predation).

Today, I’d like to explore another possible interaction in depth: perhaps the two species have diverged to lessen aggressive interspecific interactions for space and territory (i.e., interference competition). For more, let’s turn to the anoles of the Odonata world (provocative statement, I know!): rubyspot damselflies (Hetaerina spp).

In a recent issue of the Proceedings of the Royal Society B, Jonathan Drury and Greg Grether investigated the role of aggressive (or agonistic) interactions in driving divergence between two species of rubyspot damselflies.

Previous work [1,2,3] in Grether’s group had shown that male competitor recognition in rubyspot damselflies depends on hindwing coloration, and that cross-species recognition and male wing coloration diverges between species living in the same area. This suggests that aggressive interactions between males of different species have driven divergence in wing color to reduce the frequency of energy-intensive, aggressive interactions between species. This divergence is consistent with a type of character displacement called Agonistic Character Displacement (ACD), which is the divergence between species in some sort of species recognition trait to lessen the negative effects of aggressive encounters.

However, another type of character displacement, Reproductive Character Displacement (RCD) is also consistent with these previous findings. RCD is divergence, usually in some sort of mate recognition trait, between two species. By diverging in such a trait (think anole dewlaps), males and females of different species are less likely to spend precious time courting or mating in wasteful, failed cross-species reproductive efforts.

By this point, you, the astute reader, may have noticed that both ACD and RCD predict changes in signaling traits–the former species recognition traits, and the latter mate recognition traits.

Whenever the same trait functions as a signal for both species and mate recognition, and that does happen often, telling apart the action of these two distinct processes (i.e., selection to reduce wasted aggressive effort versus selection to reduce wasted reproductive effort ) can be very difficult*.

Drury and Grether designed a very nice test for successfully discerning between these two hypotheses.

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In a recent paper in The American Naturalist, Martha Muñoz, Johanna Wegener, and Adam Algar noted an interesting pattern in two clades of Caribbean anoles evolving independently on Cuba and Hispaniola: high elevation species tended to have smaller body sizes than lower elevation species*.

Figure 1: Body size-elevation relationships in Hispaniolan (cybotes clade) and Cuban (sagrei clade) anoles. The x-axis is elevation in meters (on the log scale). The y-axis is SVL, or snout-vent length, a measure of size. The colors represent individual species within each clade; grey represents A. cybotes and A. sagrei on Hispaniola and Cuba, respectively.

Having found that the two groups converged independently on a similar evolutionary pattern, the authors wanted to know: was the underlying evolutionary progression also the same?

To answer this question, the authors took advantage of the fact that the two clades harbored multiple species. By measuring body size-elevation patterns within each species, and then asking how those patterns combined with interspecific patterns to create the overall body size-elevation cline (SEC) observed across all species, Muñoz & Co. could discern subtle differences between clades in the evolutionary trajectory towards convergence. For example, one clade might build its overall size-elevation cline by having the same SEC relationship present in each species, with species also sorting themselves by elevation and size (Model H1). Whereas another clade might build its size-elevation cline just through interspecific differences in size and elevation, without an SEC relationship within species (Model H2).

Figure 2: Two models, of eight that the authors proposed, which might explain how body size-elevation clines evolve. Within-species clines are represented by different colored/dashed lines. Across-species clines are best visualized by drawing an imaginary line through average size and elevation of each species . In H1, each species has the same size-elevation relationship (i.e., the negative slope) and is found at different elevations. This creates a size-elevation relationship that depends on both intra- and interspecific patterns. In H2, each species has no size-elevation relationship (i.e., the flat slope) but is found at different elevations. Here, the size-elevation relationship is driven purely by interspecific differences in elevation and size.

The authors developed eight models for how elevation and size might be related within species and across species. They tested which of those eight models best explained variation in the relationship between size and elevation within species and clades, while accounting for spatial autocorrelation among collection localities and differences in elevational range among species . They then compared best models across clades to see whether convergence was reached by similar or different evolutionary pathways.

What did they find?

On Hispaniola, each species tended not to show any significant intraspecific SEC relationships: note the flat slopes of the dashed lines in the left panel of Figure 3. Instead, much of the overall SEC comes from interspecific differences in size and elevation, consistent with Model H2 (Figure 2).

On Cuba, in contrast, the authors found some significant within-species SEC relationships–the solid lines in the right panel of Figure 3–but found that interspecific differences in size and elevation explained very little of the clade’s overall SEC (Figure 4)**.

The model most consistent with the Cuban data.

Thus the authors answered their question: “Although the precise mechanisms underlying inverse size[-elevation] clines remains  unknown, it is clear that they were constructed in different ways on Cuba and Hispaniola.” In other words, the two clades show a pattern of convergence to small size, but they took different routes of intra- and interspecific evolution to get there. It reminds me of Yogi Berra’s response when asked directions to his house: “When you get to the fork in the road, take it!”

CITATION: M.M. Munoz, J.E. Wegener, and A.C. Algar. 2014. Untangling Intra- and Interspecific Effects on Body Size Clines Reveals Divergent Processes Structuring Convergent Patterns in Anolis lizards. The American Naturalist 184: 636-646.

* This pattern was measured from 16 Anolis species: nine in the sagrei clade (Cuba) and seven in the cybotes clade (Hispaniola). The finding of small body size at high elevations is the inverse expectation of Bergmann’s rule. Bergmann’s rule, as originally conceived, states that endothermic species living in colder climates should be larger (or have a larger surface area to volume ratio), all else equal, to conserve heat. As lizards are ectothermic, one would expect an inverse Bergmann. Perhaps we could call the inverse cline Nnamgerb’s rule? It does have a certain charm to it, no?

** I wonder if the authors might chime in in the comments section. What does it mean that a size-elevation cline wasn’t found on Cuba when using the mean size and elevation of each population (Fig 5 below), but it was found when species identity was ignored (Fig 1 above)? Is this an example of Simpson’s paradox?